BACKGROUND ART Two types of bonding method are most widely used: 1) solid phase bonding and 2) liquid phase bonding. Solid phase bonding usually occurs by means of applying high pressure and/or high temperature to bonding articles or to additional bonding layer (s) to facilitate and/or accelerate diffusion between two or more materials which all are in solid state. Several approaches have been proposed, namely, high pressure bonding, bonding with heating of bonding materials or additional layers to a temperature near the melting point (s) thereof, explosive bonding and the like. The second type of bonding method,

i. e. liquid phase bonding, involves formation of a liquid phase made of one or more bonding materials or an additional bonding layer (s) between them and subsequent cooling of the resulting structure to transform said liquid phase into solid state. Namely, these are welding, soldering, fusion bonding and the like. In accordance with this classification the present invention may be regarded as a new kind of fusion bonding method.

Many methods of article or wafer bonding or article- to-substrate bonding which allow the formation of all-in- one microstructures such as pressure sensors, accelerometer, microsensors and the like have been developed in recent decades. In most cases, hermetic sealing, high interface bonding strength and low bonding temperature are of major concern. In addition, a rigidity of bonding layer (s) is essential where a piezoelectric material and a substrate are bonded to facilitate vibration transfer from the piezoelectric material to the substrate. However, the general trend in the technology of lowering the bonding temperature often results in a lowering of the rigidity of the bonding layer (s).

Furthermore, it is well known that materials with low melting points generally have a low rigidity due to softening affect which occurs when the temperature approaches the melting point. Thus, it is desirable to combine low bonding temperature and improved rigidity of the bonding layer (s) between the piezoelectric material and the substrate.

In addition, there are many cases which need bonding materials with a melting point which is higher than the bonding temperature, for example, to assemble a structure wherein previous bonding layer between structure elements should be in solid state during the subsequent bonding of

other structure elements. The present invention provides an improved bonding method which meets both of said requirements.

Several technologies utilizing fusion bonding are known in art. Namely, silicon fusion bonding at 1000°C and anodic bonding at 450°C are known from A. R. Mirza et al., Silicon wafer bonding : key to MEMS high-volume manufacturing, Sensors 15 (12) (1998) 24-33. However, the process temperature is relatively high in this case. The anodic bonding can be performed at 180°C by using high alkaline content glass [see S. Shoji et al., Low- temperature anodic bonding using lithium aluminosilicate- quartz glass ceramic, Sens. Actuators, A 64 (1998) 95- 100], nevertheless, it is not a commercially available process. The intermediate bonding by using Au-Si eutectic bonding material with eutectic temperature of 363°C has been proposed in R. F. Wolffenbuttel, Low-temperature intermediate Au-Si wafer bonding ; eutectic or silicide bond, Sens. Actuators, A 62 (1997) 680-686. Au/Sn to Ni/Au solder bonding, and Sn/Pb to Ni/Au solder bonding were used for making a sealed cavity of capacitive pressure sensor, while the respective treatment temperatures were 300°C, and 250°C for fluxless soldering in a vacuum oven with infrared light [see B. Rogge et al., Solder-bonded micromachined capacitive pressure sensors, Proc. SPIE 3514 (1998) 307-315]. A low-temperature wafer bonding process based on using In-Sn alloy as the intermediate layer for bonding was described in C. Lee et al., Wafer bonding by low-temperature soldering, Sensors and Actuators 85 (2000) 330-334.

Actually, all materials that provide enough bonding force to the adhesive can be used at the bonding interface.

Epoxy and thermoplastic polymers have been applied to the

wafer level packaging with heat treatment temperatures of 150°C and 280°C, respectively [see G. Klink et al., Wafer bonding with an adhesive coating, Proc. SPIE 3514 (1998) 50-61].

The following Patent documents may be regarded as being relevant: US 6,419, 147; 6,404, 063; 6,173, 886; 6,059, 175; 6,024, 276; 5,994, 666; 5,686, 318; 5,655, 000; 5,318, 217; 5,242, 102; 4,988, 035.

The nearest prior art is disclosed in the JP Patent application 2000-335867 filed 02.11. 2000 and relating US Patent application No 09/822,636 filed 30.03. 2001 in the name of FUJITSU LIMITED and published 20.06. 2002 under No 2002/0074902, IPC7 HO1L 41/04. Namely, the last document discloses a method of bonding a piezoelectric element and an electrode, including the steps of forming a first coating of a material selected from the group consisting of Au, Al, Zn, Cu, and Sn on a bonding surface of the piezoelectric element, and forming a second coating of a material selected from the group consisting of Au, Al, Zn, Cu, and Sn on a bonding surface of the electrode. The combination of the materials of the first and second coatings is preferably Au/Au, Au/Al, Zn/Cu, or Sn/Cu. The method further includes the step of bringing the first and second coatings into close contact with each other and heating them under pressure to form a metallic bond or intermetallic compound between the first and second coatings, thereby bonding the piezoelectric element and the electrode.

The method disclosed in the US 09/822,636 (see claim 1) relates to the solid phase diffusion bonding type. Though there is no reference on this fact in claim 1, but according to paragraphs 0040 and 0044 of the specification "... the temperature of the laminate was about 200°C"and

"... the bonding portion between the base electrode 24 and the piezoelectric element 28a was heated to 200°C without heating the head 56". The temperature of 200°C is lower than the lowest liquidus temperature in the preferable Au/Au, Au/Al, Zn/Cu, or Sn/Cu systems. Thus, there is no liquid phase occurred during the bonding process.

In addition, the method has several disadvantages. For the first, the method discloses the bonding between piezoelectric element and electrode, while the electrode is usually made of metal or metal alloys. Thus, there is no reference to the bonding between piezoelectric material and substrate made of materials different from metal or metal alloys. For the second, the method discloses heating under pressure, i. e. requires applying additional pressure to form a bond. For the third, the method does not disclose the formation of bond having a melting point which is higher than bonding temperature, what makes it impossible to assemble a structure wherein previous bonding layer between structure elements should be in solid state during the subsequent bonding of other structure elements. For the fourth, two preferable systems include Au component which is very expensive, and other components are pure metal which are relatively expensive too.

The method disclosed in claim 7 of the US 09/822,636 relates to conventional soldering methods and, consequently, has all corresponding disadvantages thereof.

DISCLOSURE OF INVENTION It is therefore an object of the present invention to provide a method of bonding a piezoelectric material and a substrate, which makes it possible to assemble a structure wherein previous bonding layer between structure elements

should be in solid state during the subsequent bonding of other structure elements.

It is another object of the present invention to provide a method of bonding a piezoelectric material and a substrate, wherein a resulting bonding layer has relatively high rigidity to facilitate vibrations transfer from the piezoelectric material to the substrate.

It is still another object of the present invention to provide a method of bonding a piezoelectric material and a substrate, which is relatively easy and not expensive.

It is still another object of the present invention to provide a method of bonding articles made of any materials, which makes it possible to assemble a structure wherein previous bonding layer between structure elements should be in solid state during the subsequent bonding of other structure elements.

In accordance with an aspect of the present invention, there is provided a method of bonding a piezoelectric material and a substrate having a melting point TSUBI the method comprising the steps of: a) depositing a layer of a first metallic material (M1) having a melting point T1 on a bonding surface of the piezoelectric material; b) depositing a layer of the material M1 or a layer of a second metallic material (M2) having a melting point T2 which is lower than Ti, on a bonding surface of said substrate, wherein the material M2 being melted is capable to interact by diffusion with the material MI to form a metallic bond having a melting point T12 which is higher than T2 ; c) depositing at least one layer of the material M2 on the bonding surface of said piezoelectric material and/or substrate ;

d) bringing said bonding surfaces of the piezoelectric material and the substrate into close contact; and e) heating a place of said contact to a temperature which is higher than T2 but lower than any of the temperatures T12, T1 and TSUB to form the metallic bond.

In accordance with another aspect of the present invention, there is provided a method of bonding a piezoelectric material and a substrate having a melting point Ts, the method comprising the steps of: a) depositing a layer of a first metallic material (Ml) having a melting point T1 on a bonding surface of the piezoelectric material; b) depositing a layer of the material M1 or a layer of a second metallic material (M2) having a melting point T2 which is lower than T1, on a bonding surface of said substrate, wherein the material M2 being melted is capable to interact by diffusion with the material MI to form a metallic bond which comprises an alloy and/or intermetallic compound and/or solid solution; c) depositing at least one layer of the material M2 on the bonding surface of said piezoelectric material and/or said substrate; d) bringing said bonding surfaces of the piezoelectric material and the substrate into close contact; and e) heating a place of said contact to a temperature which is higher than T2 but lower than any of the temperatures T1 and TsuB to form the metallic bond.

In accordance with a further aspect of the present invention, there is provided a method of articles bonding, the method comprising the steps of: a) depositing a layer of a first material (M1) having a

melting point T1 on a bonding surface of a first article having a melting point TAR1 ; b) depositing a layer of a second material (M2) having a melting point T2 which is lower than T1 on a bonding surface of a second article having a melting point TAR2, wherein the material M2 being melted is capable to interact by diffusion with the material M1 to form a bond having a melting point Tis which is higher than T2 ; c) bringing said bonding surfaces of the first and second articles into close contact; d) heating a place of said contact to a temperature which is higher than T2 but lower than any of the temperatures T12, T1, TAR, and Tm2 to form said bond.

The main idea of bonding method according to the present invention is in formation of at least two layers of metallic materials (i. e. metal or metallic alloy) consisted of one or more components on bonding surfaces of the piezoelectric material and the substrate to be bonded, wherein one of said materials has a low melting point.

Diffusion mixing of the metallic materials after melting the material with low melting point results in formation of intermetallic compounds or solid solutions with required melting temperatures.

According to one of the preferred embodiments of the present invention the materials M1 or M2 are chosen so that they can be well adhered to the bonding surfaces of the piezoelectric material and the substrate.

The substrate (or bonding surface thereof) may be of any material to which the materials M1 and M2 may be adhered, preferable, of plastic materials (for example, polycarbonates), dielectrics (for example, glasses and ceramics), semiconductors (for example, Si wafers), metals

(steel, stainless steel, nickel, brass) and the like. A piezoelectric material may be also used as the substrate, and the piezoelectric materials which forms both bonding articles may be the same material with different polarization directions.

However, the invention is not limited by bonding the piezoelectric material and the substrate and is adapted for bonding any articles made of the materials to which the materials Mi and M2 may be adhered.

The M1 and M2 layers may be deposited on the piezoelectric material and the substrate, respectively; or the M1 and M2 layers may be sequentially deposited on the piezoelectric material while the M2 layer is deposited on the substrate; or the M1 and M2 layers may be sequentially deposited on the substrate while the M1 layer is deposited on the piezoelectric material; preferably, both MI and M2 layers are sequentially deposited both on the piezoelectric material and the substrate. According to preferred embodiment of the present invention, the Ml and M2 layers deposited on the piezoelectric material may have different compositions comparing the M1 and M2 layers deposited on the substrate.

According to another preferred embodiment of the present invention the step of heating comprises two substeps, wherein the first substep is carried out to form a liquid phase of the material M2, and the heating temperature on the second substep is increased to activate mutual diffusion of the materials M1 and M2 and to facilitate/accelerate formation of the metallic bond.

According to another preferred embodiment of the present invention, in case the materials MI and M2 are the following metals or alloys based on the following metals: Ni and In, Ni and Sn or Cu and Sn, respectively, the

metallic bond comprises at least one intermetallic compound or at least one solid solution based on said intermetallic compound. In case the materials Ml and M2 are the following metals or alloys based on the following metals: Cu and Bi, Zn and Bi, Al and Bi, respectively, the metallic bond may comprise a solid solution based on the materials Ml and M2.

According to still another preferred embodiment of the present invention, the piezoelectric material is a Pb (ZrxTil-x) 03 based material (PZT). However, the piezoelectric material may have other chemical composition as well as may be a composite. Two types of low- temperature bonding may be used for formation of bi-and multi-morph structures comprising the piezoelectric material. The first type has a bonding temperature lower than temperature of depolarization (a Curie temperature Tc). Second one has a higher temperature than temperature of depolarization. The polarization process for piezoelectric material is usually carried out by applying high voltage (for example, 1-5 kV per one cm of piezoelectric material thickness). Preferably, the piezoelectric material is previously poled before bonding and has a Curie temperature Tc which is higher than T2, and the heating temperature is lower than Tc.

The depolarization process for piezoelectric material occurs when the temperature of piezoelectric material approaches or overcomes the Curie temperature Tc, and the rate of depolarization process is increased with rising the temperature. When the piezoelectric material is the Pb (ZrxTil-x) 03 based material (the Curie temperature Tc is about 200-250°C), in both cases maximum temperature is preferably lower than 400°C to prevent Pb loses from PZT. If the piezoelectric material is the previously poled PZT,

the preferable materials MI and M2 are Ni or Ni-based alloy and In or In-based alloy, respectively; most preferably, the materials M1 and M2 are Ni 70 wt. %-Cu 30 wt. % alloy and In, respectively, and the heating temperature on the is between about 160°C and about 200°C because the experimental data show that heating treatment of the piezoelectric material in the temperature range from 2/3 Tc to Tc leads to partial depolarization and respective decreasing piezoelectric coefficients of the piezoelectric material.

According to still another preferred embodiment of the present invention, additional layers of other materials are deposited on the bonding surfaces of the piezoelectric material and the substrate before depositing of said M1 or M2 layers.

According to still another preferred embodiment of the present invention, the material M2 is deposited in a such amount comparing to an amount of the material M1, and the step of heating is carried out during a such period of time so that the material M2 is completely spent on formation of the metallic bond. In this case the method is adapted for assembling a structure, wherein every previous bonding of structure elements should be in solid state during the subsequent bonding of structure elements.

The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF DRAWINGS FIGS la)-c) are schematic diagrams illustrating contacting, compressing and temperature-raising stages of the present invention respectively, magnified detail of the interface between the contacting layers being shown within the magnifying glass symbol.

FIGS. 2 to 7 shows a phase diagram of Ni-In, Ni-Sn, Cu-Sn, Bi-Cu, Bi-Zn, Al-Bi binary systems, respectively.

BEST MODE FOR CARRYING OUT THE INVENTION Some preferred embodiments of the present invention for the case of PZT piezoelectric material will now be described in detail with reference to the drawings.

The layer M1 and the layer M2 are sequentially formed on each of two PZT wafers 10,20. Material of the layer M2 is a metal (for example, In, Sn, Bi, Pb, Ga) or an alloy based on said metal (for example, In-Sn, Sn-Pb) having low melting temperature (lower 400°C). Material of the layer M1 is metal (for example, Ni, Co, Cu, Zn, Al, Ti, Zr, Au, Ag, Pt) having more high melting temperature (over 600°C), than material of layer M2. The selection of M1 and M2 materials is determined by the formation of solid solutions or compounds between them. The bonding of wafers is performed by an axial loading of more 0.5 kg/cm2 and a sequential annealing at the temperature higher than the melting point of the layer M2. After that the temperature may be raised up to 300-400°C to dissolve liquid phase and to form solid compound (s) or solid solution.

High roughness of PZT surface results in formation of many voids at the mechanical-connected layers M2 (Fig. la). At the same time the interface between the layer Mi and the layer M2 is much more uniform. As the result of this, the interaction between the layer M1 and the layer

M2 is preferable compared to interaction between the layers M2. This problem may be solved partially by compressing (if material of the layer M2 has high plasticity (Fig. lb).

The complete solution of this problem is in transformation of the contacting layers M2 in liquid phase that guarantees uniform wetting of surfaces, filling of voids and, finally, mutual dissolving of two layers M2 and the creation of an intermetallic compound or solid solution M3 (Fig. lc). The appearance of liquid phase results in increasing both the diffusion mobility and chemical activity of atoms of the layer M2.

Therefore, the general requirement to the material of layer M2 is the low melting temperature, at which interaction between layer M2 and layer M1 occurs.

Let's consider two different cases.

The system, where the material of layer M1 is Ti, and the material of layer M2 is Al, can not be used for the bonding, nevertheless the significant difference between the melting temperature of these materials. The active interaction between Al and Ti thin films occurs at 400- 450°C and compounds Ti3Al and TiAl3 are formed (the melting temperature of Al is 660°C). Thus, the appearance of the Al liquid phase is not practically possible for the comparable amounts of Al and Ti, since the all Al will be consumed for the TiXAly compound formation.

The indium, the melting temperature of which is 156°C only, may be used as material of the layer M2. The nickel, the melting temperature of which is 1453°C, may be used as material of the layer Ml. As can be seen from the Ni-In phase diagram (see Fig. 2), the formation of few compounds Ni, Iny is possible in Ni-In system. These compounds have

the significantly higher melting temperature, than the pure indium. At temperatures below Curie point of PZT (about 200-250°C) the liquid phase is formed between two PZT plates, and the successful wafer bonding take place.

Such procedure can be applied for bonding of previously poled PZT plates.

If the PZT wafers are not previously poled or the piezoelectric material has higher Curie temperature, the two-step heat treatment may be performed. At the first stage the In is melting, and at the second stage the temperature is increased up to 300-400°C to activate diffusion of In and to form the NixIny compound. The formed compound has melting point higher than both the melting point of In and Curie point of PZT. Duration of the second stage is defined by full consumption of In melt.

Below several possible pairs of metals which can be used for bonding are given.

Ni-Sn pair (Fig. 3). This pair of metals can be used for bonding of non-polarized PZT plates. Low melting temperature of Sn (232°C) and high melting temperature of Ni (1453°C) allows to provide vacuum annealing. At first stage of heat treatment the Sn becomes liquid, and then during Ni-Sn interdiffusion, the melting point of NixSny compounds rises extremely from 232°C up to 1200°C with Sn content from 100 to 40 atom. %. The poling procedure could be performed after bonding.

Cu-Sn pair (Fig. 4). This pair of metals can be used for bonding of non-polarized PZT plates. Low melting temperature of Sn (232°C) and high melting temperature of Cu (1085°C) allows to provide vacuum annealing. At first stage of heat treatment the Sn becomes liquid, and then during Cu-Sn interdiffusion, the melting point of several phases forming in this system (for example,-, £-, 4-, 6-,

y-, P-phases) rises extremely from 232°C up to 750°C with Sn content from 100 to 30 atom. %. The poling procedure could be performed after bonding.

Cu-Bi pair (Fig. 5). This pair of metals can be used for bonding of non-polarized PZT plates. Low melting temperature of Bi (271°C) and high melting temperature of Cu (1085°C) allows to provide vacuum annealing. At first stage of heat treatment the Bi becomes liquid and then during Cu-Bi interdiffusion, the melting point of CuxBil-x solid solution rises extremely from 271°C up to 800 °C with Bi content from 100 to 20 wt. %. The poling procedure could be performed after bonding.

Zn-Bi pair (Fig. 6). This system may be used for bonding of unpoled PZT plates. Low melting temperature of Bi (271°C) and relatively high melting temperature of Zn (419, 5°C) allows to provide vacuum annealing. At first stage of heat treatment the Bi becomes liquid and then during Zn-Bi interdiffusion, the melting point of ZnxBil-x solid solution rises from 271°C up to 400°C. The poling procedure could be performed after bonding.

Al-Bi pair (Fig. 7) reveals similar behavior as the Bi-Zn pair. This system may be used for bonding of unpoled PZT plates. Low melting temperature of Bi (271°C) and relatively high melting temperature of Al (660°C) allows to provide vacuum annealing. At first stage of heat treatment the Bi becomes liquid and then during Al-Bi interdiffusion, the melting point of AlXBilx solid solution rises from 271°C up to 660°C. The poling procedure could be performed after bonding.

The following are examples of binary systems which are not suitable for bonding in accordance with the present invention. Sn-Pb system is not suitable for PZT bonding because of melting points of both components are higher

than the Curie temperature of PZT, and eutectic temperature is lower than temperature required for the subsequent poling. Bi-Sn system is not suitable for PZT bonding because of melting points both of components is higher than the Curie temperature, and eutectic temperature is lower than temperature required for the poling.

EXAMPLE 1 Both Ni and In films may be formed by vacuum deposition or by electrochemical deposition.

PZT surface is extremely rough always. In this reason application of thick (more than 5 JLm) metal films is required. Electrochemical metal deposition from aqueous solutions is an attractive technique for this purpose.

Compared with vacuum deposition, an electrochemical technique requires a cheaper equipment and shorter time of processing.

In our case In (indium) electrodeposition was used at Ni electrode, which was previously formed at PZT surface by vacuum deposition. Electrochemical deposition was performed in water solution of In2SO4 (50 g/1) under 10 mA/cm2 current density during 30 or higher minutes. Indium deposition rate was about 0.1 pm/min.

Before electrodeposition the PZT plate was cleaned in boiling propanol during 5 min to dissolve organic contaminations on the surface.

After In deposition, two PZT plates were mechanically bonded and annealed in vacuum (1x10-5 Tor) at 200°C during time from 30 to 60 min.

It was found that during annealing of Ni-In system, the indium dissolves in nickel film or nickel dissolves in

indium. Melting point of the formed peritectic NixIny alloy is higher than melting point of pure In and is higher than temperature of annealing (200°C). It was ensured that piezo properties of PZT didn't change after bonding procedure.

Measurements have shown, that the coefficient d31 was not less than 255x10-l2 Kl/N. The total thickness of the bimorph structure consisted of two PZT plates bonded was 0,34-0, 35 mm.

Electrolyte compositions for deposition of the metals mentioned above Applied for thickness increasing of vacuum evaporated Ni film. An additional Ni thickness is required for dissolving of low-temperature melted metal (In, Bi, Sn).

Electrolyte compositions are follows: Table 1. Conditions of Ni deposition. No Electrolyte composition Conditions of Notes Components Concentration electrolysis g/i 1 NiS04-7H20 200-240 jc=20-50 mA/cm Ni anodes Na2SO4#10H2 100-120 T=30-50°C Q 10-25 pH=5.2-5. 6 NaCl 30 2 NiCl2. 6H20 250-300 jc=20-30 mA/cm2 Ni anodes H3BO3 30 T=40-50°C pH=2 Table 2. Conditions of Bi deposition. No Electrolyte composition Conditions of Notes Components Concentration electrolysis g/l 1 BiCl3 100 jc=27 mA/cm2 Bi anodes NaCl 18.5 T=27°C HC1 180 ml/l pH=5.2-5. 6 2 Bi (N03) 2 75 jc=5 mA/cm2 Bi anodes KOH 65 T=75°C Vine acid 50 C4H606 125 ml/l

Table 3 Conditions of In deposition. No Electrolyte composition Conditions of Notes Components Concentration electrolysis g/l 1 In2 (S04) 3 50 jc=30 mA/cm2 Current Al2 (S04) 3 12 T=18-25°C efficiency Na2SO4 10 pH=2-2.7 is about 70% 2 Bi (N03) 2 75 jc=5 mA/cm2 Bi anodes KOH 65 T=75°C Vine acid 50 C4H606 125 ml/l Table 4. Conditions of Cu deposition No Electrolyte composition Conditions of Notes Component Concentration electrolysis g/l 1 CuSO4 200 jc=3-30 mA/cm2 Cu anodes; H2SO4 60-75 ml/l T=18-20°C After deposition immersion into SnCl2 water

Table 5. Conditions of Sn deposition No Electrolyte composition Conditions of Notes Component Concentration electrolysis g/l 1 SnS04 55 jc=1 0-20/Current H2S04 100 ml/l mA/cm2 efficiency 90% Table 6. Conditions of Pb deposition

No Electrolyte composition Conditions of Notes Components Concentration electrolysis , g/l 1 PbNO3 100 jo=10-20 mA/cm Current NaOH 150 ml/1 T=18-20°C efficiency glycerol 50 ml/1 100% ;

The foregoing specification was described in the various embodiments of present invention which has a number of advantages over the prior art. Various modifications and variations can be made without departing from the scope of the present invention, as set forth in the accompanying claims.