JPH11163668A - Laminated piezo-electric single crystal substrate and piezo-electric device using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a junction wafer which easily discriminates a junction boundary by joining without a sticking layer, and as a result to produce a wafer for a piezo-electric element which does not have so many errors at the time of working and has high manufacturing yield by utilizing the junction wafer and to obtain a high frequency piezo-electric resonator, for instance, by producing a thin, but high precision junction wafer that has a polarized inverted structure. SOLUTION: A 1st single crystal piezo-electric substrate (colored wafer) 2 that is colored and a 2nd single crystal piezo-electric substrate (non-colored wafer) 1 with a different color tone are directly connected, and the surface and rear or the boundary are easily discriminated as a polarized inverted structure junction wafer (junction wafer) 3. Also, by detecting a different color boundary with an optical method, it is possible to measure layer thickness by utilizing laser beams before and after working or during working.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for bonding a single crystal of lithium niobate or lithium tantalate used for a piezoelectric element such as an oscillator, a filter or an actuator, and more particularly to a method for bonding a bonded body after bonding. The present invention relates to a single-crystal joined body that is easy to identify, hardly causes an error in handling, and contributes to improvement in processing accuracy, a piezoelectric device using the same, and a method of manufacturing the same.

[0002]

2. Description of the Related Art For example, as a joined body in which lithium tantalate single crystals are joined, there is known a joined body directly joined by arbitrarily inclining the polarization directions of each other as shown in Japanese Patent Application Laid-Open No. 7-206600. ing. Such a joined body is obtained by using a method called a direct joining method. As materials that can be directly bonded, lithium niobate, quartz, and the like are known in addition to lithium tantalate. The joined body joined by the direct joining method has no adhesive layer at the joining interface and the joining is performed at the atomic level, so the joining strength is strong, the joining state does not deteriorate even at high temperatures, and The change is small.

[0003] Further, the bonded body can endure various processing methods such as grinding, polishing, cutting, and etching. This conjugate is
It is possible to solve various problems as described in Japanese Patent Application Laid-Open No. 1-71207, which are problems when manufacturing a device using a lithium niobate substrate having a domain-inverted region. For example, since high-temperature heating or the like, which is indispensable for manufacturing an inversion layer, is not required, complicated control of manufacturing conditions is eliminated. In addition, it is possible to solve the problem that the swell of the inversion layer boundary occurs and it is difficult to control the depth. Further, it is effective as a method for solving problems such as the applicable cut angle and limitation of the material. Further, in a single crystal, two or more layers can be stacked, and the application range is wide.

Devices using these domain-inverted regions include a thickness vibrator, a bending or longitudinal vibration actuator, and the like. In particular, the above device comprising a bonded body obtained by the direct bonding method has no undulation at the boundary of the inversion layer, and can accurately control the thickness of each device. Can be formed.

As described above, the characteristics of these junction devices are mainly determined by the shape and thickness of the boundary between the substrates bonded together. In the joined body, the joining interface is joined after being mirror-polished, so that it naturally becomes almost mirror-like, and the former problem can be almost ideally obtained. However, regarding the latter problem of the thickness of each layer,
Since the thickness is determined only by the thickness of the substrate to be joined, the thickness management of the substrate before joining determines its accuracy. For relatively thick substrates of 200 μm or more, handling is easy,
Thickness accuracy is sufficient. However, in a device in which one layer has a thickness smaller than that, generally, a thicker substrate with good thickness accuracy is laminated, and then one or both of them are thinned to a desired thickness. By this step, a joined body having a thickness corresponding to the characteristics is obtained.

As an example of applying this structure to an actual device, there is a piezoelectric resonator capable of exciting a second harmonic. For example,
Conventionally, among the piezoelectric resonators used in the MHz band, the piezoelectric resonator that vibrates in the thickness vibration mode has to reduce the thickness of the piezoelectric body. However, a piezoelectric body that has been directly bonded by reversing the polarization can double the resonance frequency as compared with a single-plate vibrator of the same thickness that is not bonded, so the upper limit of the frequency, which was limited by the processing limit, was increased. There is an advantage that can be raised.

[0007]

However, when this frequency band exceeds 20 MHz, it is necessary to bond the substrates together and to further polish them so that the polished plate thickness is less than 200 μm. In the conventional method, starting from a plate having a total thickness of 400 μm in which a 200 μm plate is bonded, further thinning after bonding is necessary.

In order to obtain a substrate having such a thickness, for example, in order to obtain a substrate having a total thickness of 200 μm, 1
Two 200 μm substrates are bonded together, and one side is 100 μm
Polishing is performed alternately by m, or both sides are simultaneously polished by double-side polishing.

[0009] However, since these bonded bodies have completely identical crystals, it is difficult to identify the position of the bonded interface. Although it is possible to determine the plate thickness and the front and back sides by cross-sectional etching, this method requires polishing of the end face, and is basically a destructive inspection. Therefore, when it is necessary to inspect the substrate many times during processing, this is almost impossible. In addition, the product inspection and quality control after the device is made can be determined only by the characteristics, and there is a problem that much time is required for the inspection.

Further, it is difficult to completely eliminate the mistake even if the front and back sides are strictly controlled during a work such as polishing. For example, in a 400 μm plate in which a 200 μm back surface roughened plate is bonded, 100
Polished single sided by μm, total thickness 200μm, layer 10
Even when obtaining a 0 μm plate, after polishing one side,
Without polishing the back surface, the same surface was polished again,
A 400 μm laminated plate is completed.
There has been a problem that it becomes a single plate of μm.

Further, a 40 μm plate of 40 μm is bonded.
On a 0 μm plate, both sides are polished to a total thickness of 200 μm.
m, even when obtaining a 100 μm thick plate, since the polishing amount of the upper surface and the lower surface during polishing differs due to the difference in the amount of wraparound of the abrasive grains, the upper surface and the lower surface are being worked to equalize the polishing amount. However, there is a problem that it is difficult to determine the upper and lower surfaces, errors are likely to occur during this operation, and the thickness of each layer of the finished plate is unbalanced.

[0012] The unbalance of the thickness of each layer at the time of polishing is not a problem because the bonding boundary hardly deviates from the center when the total polishing amount of the plate is small, but the thickness of the plate is further reduced, and When the amount is large, the imbalance tends to be large due to a wrong front and rear, which affects the characteristics. In particular, the thinner the substrate, the larger the ratio to the total plate thickness. For example,
In the thickness resonator, when the balance of each layer is lost, a fundamental wave to be suppressed appears and becomes spurious.

[0013]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention makes it easy to discriminate wafers by intentionally joining wafers of different colors. That is, the present invention is based on the results of various studies on a method of discriminating front and back sides prior to the processing of a device involving thinning alternately from both sides or one side of a bonded wafer. By bonding a comparable wafer to a normal, generally transparent wafer, there is no degradation in the quality of the bonded wafer, and it is possible to obtain a bonded wafer that can easily distinguish the front and back of the wafer from the difference in color. It is due to the finding. According to this, it is possible to easily identify the front and back of a wafer without spending time and money, and it is also possible to control the quality of the front and back. The quality control of the element can be easily performed. Further, a laminated piezoelectric device having a thinner layer than the conventional one can be manufactured with high accuracy.

[0014]

(Embodiment 1) Next, an embodiment of the present invention will be described. First, a 2000 mm wafer was prepared by slicing a 76.2 mm diameter xcut wafer from a 77 mm diameter lithium tantalate single crystal ingot. The ingot has an orientation flat perpendicular to the z-axis and a sub-orientation flat so that the front and back of the crystal can be distinguished. In addition,
This ingot is pulled up by a general Czochralski method, but is colored pale brown because it uses a platinum-rosium alloy as a crucible. This coloring is considered to be caused by a small amount of rhodium mixed into the crystal. This coloring is problematic when lithium tantalate is used for optical purposes, for example, as a second harmonic generation material because it has absorptivity to laser light of 400 to 500 nm. There is no inferiority.

Next, the wafer was chamfered, wrapped, etched and polished, and then washed to obtain a wafer having both surfaces mirror-finished. Next, a wafer having the same shape as this wafer was manufactured using the same steps and the same equipment as described above. However, the ingot used
Since the crucible is made of iridium in an atmosphere-controlled state, it is almost transparent without coloring. By the above steps, 2,000 wafers having the same thickness and the same diameter can be obtained.

Next, as shown in FIG.
The uncolored wafer (transparent wafer) 1 and the colored wafer 2 are directly bonded one by one. At this time, the wafers are joined such that the back or front of the wafer is oriented in the same direction and only the z-axis is in the opposite direction. Since the z-axis is the polarization axis in lithium tantalate, by bonding in this way, a domain-inverted structure bonded wafer (single-crystal direct bonded body) 3 having a domain-inverted boundary at the center in the thickness direction of the wafer is obtained. can get. Next, the step of direct bonding used in this step will be described. Direct bonding is
The two surfaces facing each other for joining are mirror-polished and then cleaned, and the surfaces that have been subjected to a hydrophilic treatment are brought into contact with each other, and then subjected to a heat treatment. The heating temperature is conventionally performed. The temperature is sufficiently lower than the polarization inversion forming processing temperature, that is, 200 to 500 degrees. Therefore, complicated temperature management and atmosphere management are not required, and the production equipment can be simplified. In addition, the quality of the crystal does not change at high temperatures. With respect to the lithium tantalate wafer used in the first embodiment, no warping, breakage, or deterioration of the material due to direct bonding was observed.

Through the above steps, a domain-inverted structure bonded wafer integrated at the atomic level is obtained. This bonded wafer is
It is transparent when viewed from one side and has a pale brown appearance when viewed from the other side. Therefore, it is possible to determine not only the upper surface and the lower surface of the wafer but also the boundary surface using an optical method. The following is an example.

(Embodiment 2) As shown in FIG. 2, a predetermined portion of the bonded wafer 3 shown in Embodiment 1 is irradiated with a thickness-measuring laser beam 4, and of the reflected light, The thickness can be measured by reading the optical path difference between the reflected laser beam 5 and the beam 6 reflected at the bonding interface and returned. Similarly, by reading the optical path difference between the reflected lights 5 and 6 from the front surface or the bonding interface and the reflected light 7 from the back surface of the substrate, the thickness can be further optically detected.

When a normal transparent wafer is bonded, there is no difference in the refractive index at the bonding interface, so that the incident laser beam does not cause optical reflection at the bonding interface. Since this does not occur, only the total thickness of the plate can be measured by the optical path difference. However, in the bonded wafer of the present invention, the thickness of each layer can be determined, so that more accurate plate thickness control can be performed. Therefore, when used for a device, it is possible to further improve the processing accuracy and expect improvement in characteristics.
Next, in the method of manufacturing a device using the bonded wafer of the present invention, an example of a thickness resonator will be described in detail.

Embodiment 3 FIG. 3 is a side view showing a flow of a method of manufacturing a piezoelectric vibrator according to Embodiment 3 of the present invention. FIG. 3A is a side view of two piezoelectric single crystals having the same thickness. In the third embodiment, a Zcut lithium niobate substrate 1 having a thickness t = 200 μm and a colored substrate 2 having the same size, thickness and cut angle as the piezoelectric single crystal are used.

Since the polarization axis of the zcut wafer is oriented in the plate thickness direction, as shown in the drawing, the polarization axis is oriented vertically when viewed from the cross-sectional direction, and the polarization axis is inverted.
Note that in this embodiment, the lithium niobate substrate was annealed for several hours in a nitrogen atmosphere in order to obtain a colored wafer. Lithium niobate can be colored by this method because it is colored light blackish brown due to oxygen deficiency. Although the optical characteristics of the wafer colored by this method deteriorate, the deterioration of the piezoelectric characteristics is small. Therefore, deterioration of the piezoelectric characteristics of the bonded wafer can be minimized.

These are then directly bonded and integrated as shown in FIG. Thereafter, as shown in FIG. 3C, the uncolored wafer 1 on the upper surface is thinned to 50 μm. At this time, a hole 9 is made in a part of the polishing table 8 and a measuring laser beam 4 is applied to one part of the polished surface from one surface.
According to the principle shown in the second embodiment, polishing can be performed while monitoring the polishing amount. By doing so, the polishing amount can be easily controlled, and the thickness can be reduced to 50 μm with high accuracy.

Then, as shown in FIG. 3D, the colored wafer 2 side is also polished on one side to a thickness of 50 μm.
Polish to m. At this time, the thickness of the sheet can be monitored by a laser beam. Further, since one of the wafers is a colored wafer, even when the size of each wafer is small and the number of wafers is large, there is no error in setting the surface to be polished of the substrate, and the yield can be improved. The same effect can be obtained when the upper and lower sides are frequently switched by double-side polishing.

Finally, drive electrodes (excitation electrodes) 10 are formed on the upper and lower surfaces of the domain-inverted structure wafer 3, and a piezoelectric vibrator similar to that shown in FIG. 3E is completed. In this oscillator, the upper and lower surfaces are polished with high precision, and the boundary of polarization reversal is almost at the center. Therefore, the fundamental wave is effectively suppressed, and the second harmonic is an odd multiple, that is, the second, sixth, or tenth harmonic. Are the even-order mode vibrators showing thickness resonance of.

There is no restriction on the cut angle of the substrate, and if the bonding is performed with the direction of the polarization axis reversed, one-wavelength resonance can be obtained in a desired mode. Can be used. A crystal having no spontaneous polarization also has a reference axis called an electric axis, and a similar effect can be obtained by inverting and joining the reference axis. Also,
A temperature coefficient, a coupling coefficient, and the like required for a product can be appropriately selected.

There is no characteristic difference in the direction of the polarization axis between the positive and negative directions of the axes. It is also possible to make the etching process easy by using the direction with the higher rate as the substrate surface, or to finely adjust the final thickness with the direction with the lower etching rate as the substrate surface.

In either case, the polarity of the substrate surface is the same, and the upper and lower etch rates are the same.
By the optical method described above, the etching amount of one layer can be monitored from one side to accurately control the layer thickness of the plate. In this case, unlike the polishing process, the thickness of the substrate can be monitored while the substrate is stationary, so that more precise thickness control can be performed. Further, in the embodiment of FIG. 3, the process of manufacturing one piezoelectric vibrator is shown. However, after directly bonding a large-area wafer, a plurality of drive electrodes are formed and cut. It is likewise possible to produce many piezoelectric vibrators at once.

Further, in this embodiment, the example of the thickness vibrator has been described. However, the present invention can also be applied to the production of a bimorph bending sensor element or an actuator or a laminated surface acoustic wave element obtained by a domain-inverted structure. It is.
In this case, it goes without saying that the present invention can be similarly applied to a device having a laminated structure in which different crystals are laminated and joined, as well as the domain-inverted structure. The following is an example.

(Embodiment 4) FIG. 4 is a side view of a bimorph bent sensor element formed of a bonded wafer according to the present invention. As shown in the figure, two piezoelectric bodies of different colors are joined with their polarization axes inverted, and one end is fixed to the fixing part 11. When a shock in the direction of the arrow is applied to the element, the polarization axis is inverted and the element is joined, so that charges having different polarities are generated on the surface of the substrate, and the amount of the shock can be detected by voltage or current. These signals are extracted outside by the conductive resin 12.

When the bonding substrate according to the present invention is used, the front and back of the substrate and the boundary position can be easily determined, so that the thickness of the layer can be accurately controlled and the thickness of the element portion can be reduced. As the element becomes thinner, the amount of deflection when the same impact is applied increases, so more charge is induced on the surface with the same amount of impact,
A device with higher sensitivity can be manufactured. In the present embodiment, the example using the bending displacement has been described.
A sensor and an actuator for sliding displacement can be manufactured from the same structure.

(Embodiment 5) In addition, when a piezoelectric body having a laminated structure with a very thin layer is formed, it is important to control the layer thickness. However, it is difficult to control the film thickness by the thin film technology. In addition, for the thin film, the selection range of the substrate material and the thin film material is narrow, and it is not easy to obtain a colored material. Therefore, it is difficult to achieve the configuration of the present invention. Therefore, it is difficult to obtain a piezoelectric layer having a controlled layer thickness. In particular, in the case of the surface acoustic wave element, the coupling coefficient and the temperature coefficient of the excited surface wave vibration can be controlled by using the laminated structure of the piezoelectric material.
Since the displacement is concentrated on the surface on which the comb-shaped electrode 13 is formed, it is necessary to make the thickness of the surface layer one wavelength or less with respect to the wavelength of the surface acoustic wave to be propagated (FIG. 5). Usually, the thickness is on the order of μm, and the accuracy is required to be sub-μm. The substrate and the manufacturing method of the present invention are useful in such a field.

In the embodiment of the present invention, lithium niobate and lithium tantalate are used as the piezoelectric single crystal. However, a colored crystal is obtained by any means, and lithium borate, niobate or potassium, Langasite, crystal,
It is also possible to apply to a single crystal piezoelectric material such as gallium phosphate. Further, the combination of wafers is not limited to the example of the colored wafer and the non-colored wafer, but may be a combination of wafers having different degrees of coloring.

[0033]

As is apparent from the above description, according to the present invention, a colored wafer and an uncolored wafer, or wafers having different degrees of coloring are joined by a direct joining method without an adhesive layer. It is possible to obtain a bonded wafer whose boundaries can be easily determined. As a result, it is possible to manufacture a piezoelectric element wafer having a small manufacturing error and a high production yield by using the bonded wafer. For example, even if it is thin, a bonded wafer having a highly accurate domain-inverted structure can be manufactured, so that a high-frequency piezoelectric vibrator can be obtained.

Further, a sensor and an actuator element having a laminated structure having a thinner layer can be easily obtained. Further, it is possible to manufacture a layered surface acoustic wave device having a thickness of the order of μm. Furthermore, since the laser beam can be reflected at the junction boundary, processing such as polishing and etching can be performed while monitoring the plate thickness. The production yield of the piezoelectric device can be improved.

[Brief description of the drawings]

FIG. 1 is a perspective view showing one embodiment of a method for manufacturing a bonded wafer according to the present invention.

FIG. 2 is a cross-sectional view for explaining the principle of measuring a plate thickness using a laser beam.

FIG. 3 is a side view showing a flow of a method for manufacturing a piezoelectric vibrator made of a bonded wafer according to the present invention.

FIG. 4 is a side view of a piezoelectric bimorph bending sensor element formed of a bonded wafer according to the present invention.

FIG. 5 is a side view of a laminated surface acoustic wave device including a bonded wafer according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Normal piezoelectric substrate 2 ... Colored piezoelectric substrate 3 ... Polarized structure bonded wafer 4 ... Measurement incident laser beam 5 ... Laser beam reflected on the surface 6 ...・ Laser beam reflected at the joint interface 7 ・ ・ ・ Laser beam reflected at the back surface 8 ・ ・ ・ Surface plate for polishing 9 ・ ・ ・ Through hole 10 ・ ・ ・ Exciting electrode 11 ・ ・ ・ Fixing member 12 ・ ・ ・ Conductivity Resin 13 ... comb-shaped electrode

Continued on the front page. (51) Int.Cl. ⁶ Identification code FI H01L 41/083 H01L 41/08 S (72) Inventor Tetsuro Oto 1006 Odakadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd.

Claims (11)

[Claims] 1. A colored first single-crystal piezoelectric substrate and a second single-crystal piezoelectric substrate having a different color tone from the first substrate are directly bonded to each other to have a desired color in a single bonded body. A laminated piezoelectric single crystal substrate characterized by having two or more layers of regions. 2. The method according to claim 1, wherein the colored first single-crystal piezoelectric substrate is lithium tantalate pulled up by a Czochralski method in a platinum-rhodium crucible, and wherein the second single-crystal piezoelectric substrate is 2. The multilayer piezoelectric single crystal substrate according to claim 1, wherein the substrate is lithium tantalate pulled up in an iridium crucible by the Czochralski method. 3. The method according to claim 1, wherein the colored first single-crystal piezoelectric substrate is lithium niobate annealed in a nitrogen atmosphere, and the second single-crystal piezoelectric substrate is an unannealed niobium. The laminated piezoelectric single crystal substrate according to claim 1, wherein the substrate is lithium oxide. 4. The method according to claim 1, wherein the first and second single-crystal piezoelectric substrates are selected from a single-crystal piezoelectric material of lithium tantalate, lithium niobate, lithium borate, quartz and langasite. 2. The laminated piezoelectric single crystal substrate according to 1. 5. A bimorph structure in which the first and second single-crystal piezoelectric substrates are directly joined so that their polarization axes or electric axes are substantially in opposite directions. The multilayer piezoelectric single crystal substrate according to claim 1. 6. The thickness of the colored first single-crystal piezoelectric substrate and the thickness of the second single-crystal piezoelectric substrate are accurately measured and processed using optical means, and the respective substrate dimensions,
2. The piezoelectric actuator using the laminated piezoelectric single crystal substrate according to claim 1, wherein the piezoelectric actuator generates a longitudinal displacement, a sliding displacement, and a bending displacement according to a configuration. 7. The thickness of the colored first single-crystal piezoelectric substrate and the thickness of the second single-crystal piezoelectric substrate are accurately measured and processed using optical means, and the respective substrate dimensions,
2. A piezoelectric resonator using the laminated piezoelectric single crystal substrate according to claim 1, wherein the piezoelectric resonator generates a longitudinal displacement, a sliding displacement, and a bending displacement according to a configuration. 8. The thickness of the colored first single-crystal piezoelectric substrate and the thickness of the second single-crystal piezoelectric substrate are accurately measured and processed using optical means, and the respective substrate dimensions,
2. The piezoelectric sensor using the laminated piezoelectric single crystal substrate according to claim 1, wherein the vertical displacement, the slip displacement, and the bending displacement according to the configuration are converted into electric signals using piezoelectricity and detected. 9. The thickness of the colored first single-crystal piezoelectric substrate and the thickness of the second single-crystal piezoelectric substrate are accurately measured and processed using optical means, and the respective substrate dimensions,
2. A wave according to a configuration is propagated.
A surface acoustic wave device using the laminated piezoelectric single crystal substrate described in 1 above. 10. A laser beam for thickness measurement is irradiated from one surface side of the laminated piezoelectric single crystal substrate according to claim 1,
Detecting a thickness-measuring laser beam reflected from the surface and the bonding interface at a predetermined position, and detecting the thickness of at least one layer of the laminated piezoelectric substrate by an optical path difference, and according to the detected thickness, Setting a polishing amount so that one layer has a desired thickness, polishing the set amount, and providing vibration electrodes on the front and back surfaces of the thinned laminated piezoelectric substrate. Characteristic method for manufacturing a piezoelectric device. 11. The step of detecting the thickness of one layer of the laminated piezoelectric body is performed by a laser length measuring machine installed in a polishing apparatus, and the thickness of the laminated single crystal piezoelectric substrate is removed by polishing without removing the substrate from the polishing apparatus. The method for manufacturing a piezoelectric device according to claim 10, wherein adjustment is performed.