Patent Application
20200304095
The present invention relates to a bonded body of a piezoelectric material substrate and supporting substrate, a method of producing the same and an acoustic wave device.
SOI substrates have been widely used for realizing improved rates and lower electric power consumption of a CMOS device. However, it is known that high frequency characteristics of the CMOS devices are deteriorated due to fixed charges of SiO2 (Non-patent document 1). Specifically, even in the case that a high-resistance silicon is used as an underlying substrate, the effective resistivity may be lowered, electric field may be leaked and parasitic capacitance may be generated.
For preventing these problems, it is proposed an SOI substrate having the structure that a layer including many carrier trapping levels (so-called trap rich layer) is introduced direct under the SiO2 film. Specifically, the trapping levels are formed by forming a polycrystalline silicon layer (Non-patent document 2). Further, It is reported that the sizes of the microcrystals of the polycrystalline silicon layer are made smaller to improve the level density and to improve the effect of prevention (Non-patent document 2).
Further, it is realized a high-performance acoustic wave filter using an adhered substrate composed of a piezoelectric material substrate, an SiO2 film and silicon substrate (Non-patent document 4). However, according to an acoustic wave filter for applying a high-frequency signal, it is speculated the deterioration of the performance due to the fixed charges of the SiO2 film as the CMOS device.
Thus, according to patent document 1, it is disclosed the structure of forming an amorphous Si film or polycrystalline Si film on the surface of the silicon substrate, as the method of the suppression.
Amorphous Si film or polycrystalline Si film are film-formed by CVD method at a temperature of 400 to 1000° C. Thus, a large stress remains in the film after the film-formation. According to adhered substrates composed of a piezoelectric material body, an SiO2 film and a silicon substrate, the respective thermal expansion coefficients are considerably different from each other, so that the fracture of the bonded body occurs during a process accompanied with heating, which is problematic. It is further proved that the problems or cracks or fracture becomes considerable in the case that the stress is present in the film.
An object of the present invention is, in bonding a piezoelectric material substrate and silicon substrate through a bonding layer composed of silicon oxide, to prevent the fracture of cracks of the bonded body and, at the same time, to improve effective resistivity of the bonded body over a wide frequency range.
The present invention provides a method of producing a bonded body, the method comprising:
a silicon film-forming step of forming a silicon film on a supporting substrate comprising silicon by physical vapor deposition method;
a heat treatment step of subjecting the silicon film to heat treatment at a temperature of 400° C. or higher and 600° C. or lower to generate an intermediate layer; and
a bonding step of bonding a piezoelectric material substrate to the supporting substrate through a bonding layer comprising silicon oxide and the intermediate layer.
The present invention further provides a bonded body comprising:
a bonding substrate comprising silicon;
a piezoelectric material substrate;
an intermediate layer formed by heat treatment of a silicon layer at a temperature of 400° C. or higher and 600° C. or lower, the silicon layer being provided on the supporting substrate by physical vapor deposition method; and
a bonding layer provided between said intermediate layer and a piezoelectric material substrate, the bonding layer comprising silicon oxide.
The present invention further provides an acoustic wave device comprising:
the bonded body; and
an electrode provided on the piezoelectric material substrate.
When a piezoelectric material substrate and silicon substrate are bonded through a bonding layer composed of silicon oxide, the inventors have tried to provide a silicon film on the supporting substrate of silicon by sputtering and to bond the silicon film onto the piezoelectric material substrate through the bonding layer composed of silicon oxide. As physical vapor deposition method is a kind of a low temperature process, different from the case that the polycrystalline silicon film or amorphous silicon film is provided as the production method described in the prior art, it is considered that a residual stress in the silicon film is small resulting in the suppression of cracks or fracture of the bonded body.
As the silicon film is actually provided on the silicon substrate by physical vapor deposition method and the silicon film is bonded to the piezoelectric material substrate through the bonding layer of silicon oxide, the fracture or cracks of the bonded body could be suppressed. It is, however, proved that there is still a room of improvement of the frequency characteristics of the effective resistivity of the bonded body.
Thus, the inventors have tried to form a silicon film on a silicon substrate by physical vapor deposition method and to subject the silicon film to heat treatment at a temperature of 400° C. or higher and 600° C. or lower. As the silicon film after the heat treatment is bonded with the piezoelectric material substrate through the bonding layer of silicon oxide, it is found that the fracture or cracks are hardly generated in the bonded body. Moreover, in this case, it is found that the effective resistivity is maintained at a high value over a wide temperature range, and the present invention is thus made.
Further, the present inventors tried to observe the microstructures of the silicon films formed on the silicon substrate by physical vapor deposition method before and after the heat treatment. However, it could not be found clear difference in the microstructures before and after the heat treatment. On the other hand, in the case that the silicon film is subjected to the heat treatment, the effective resistivity of the bonded body can be maintained high over a wide temperature range. It is thus clear that the microstructure of the silicon film or the microstructure along the interface of the silicon film and silicon substrate is changed. However, at present, it is unclear the procedure of clarifying the change of the microstructure by a physical means so that its clarification as a product is considered to be difficult and non-practical.
The present invention will be described further in detail, appropriately referring to drawings.
As shown in
Further, as shown in
According to a preferred embodiment, plasma is irradiated onto surfaces of the first silicon oxide layer 4 and second silicon oxide layer 6 to perform the surface activation to form activated bonding surfaces.
Then, as shown in
At this stage, an electrode may be provided on the piezoelectric material substrate 5. However, preferably, as shown in
The respective constituents of the present invention will be described further in detail below.
According to the method of the present invention, the silicon film is provided on a supporting substrate of silicon by physical vapor deposition method.
Although the kind of silicon forming the supporting substrate is not particularly limited, silicon single crystal is preferred, and phosphorus or boron may be doped into silicon. Further, silicon forming the supporting substrate may preferably be a high-resistance silicon having a volume resistivity of 1000 Ω·cm or higher.
The silicon film is film-formed on the supporting substrate by physical vapor deposition method. At this time, on the viewpoint of the present invention, the physical vapor deposition may preferably be performed at a temperature of 200° C. or lower, more preferably be performed at a temperature of 150° C. or lower, and particularly preferably be performed at a temperature of 100° C. or lower.
The physical vapor deposition method includes sputtering and vapor deposition. The sputtering method may preferably be reactive sputtering method on the viewpoint of stability of film quality and film-forming rate.
Specifically, a target of Si metal is sputtered with Ar+ ions and then subjected to reaction with oxygen plasma to form a silicon oxide film. Further, as the vapor deposition method, it is preferred ion beam-assisted vapor deposition method for improving film density and surface smoothness. According to each of the film-forming methods, the temperature elevation during the film-formation can be suppressed at 150° C. or lower.
Then, according to the present invention, the silicon film is subjected to heat treatment at a temperature of 400° C. or higher and 600° C. or lower to generate an intermediate layer. It is thus possible to prevent the cracks and fracture of the bonded body and, at the same time, to improve the effective resistivity of the bonded body over a wide frequency range.
The thickness of the intermediate layer may preferably be 50 nm or larger and more preferably be 100 nm or larger, on the viewpoint of the present invention. Further, the thickness of the intermediate layer may preferably be 2 μm or smaller and more preferably be 1 μm or smaller. The time duration of the heat treatment may preferably be 2 to 10 hours, and the atmosphere during the heat treatment may preferably be inert gas atmosphere such as nitrogen or argon or vacuum environment.
The piezoelectric material substrate is then bonded with the supporting substrate through the bonding layer of silicon oxide and intermediate layer. In this case, the silicon oxide layer may be provided on the intermediate layer, and the silicon oxide layer may be directly bonded with the piezoelectric material substrate. Alternatively, the first silicon oxide layer may be provided on the intermediate layer on the supporting substrate, the second silicon oxide layer may be provided on the piezoelectric material substrate, and the first silicon oxide layer and second silicon oxide layer may be directly bonded with each other to generate the bonding layer.
When the silicon oxide layer is formed on the intermediate layer or piezoelectric material substrate, although the film-formation method of the silicon oxide layer is not limited, sputtering, chemical vapor deposition (CVD) and vapor deposition may be listed. When the silicon oxide layer is formed on the intermediate layer, the silicon oxide layer can be formed by sputtering or ion injection of oxygen into the intermediate layer, or by heating under oxidizing atmosphere.
The thickness of the bonding layer composed of silicon oxide may preferably be 0.05 μm or larger, more preferably be 0.1 μm or larger and particularly preferably be 0.2 μm or larger, on the viewpoint of the present invention. Further, the thickness of the bonding layer may preferably be 3 μm or smaller, more preferably be 2.5 μm or smaller and further preferably be 2.0 μm or smaller.
The piezoelectric material substrate is made single crystals of lithium tantalate (LT), lithium niobate (LN) or lithium niobate-lithium tantalate solid solution. As the materials have high propagation speeds of a surface acoustic wave and large electro-mechanical coupling factors, it is preferred for use in a piezoelectric surface wave device for high frequency and wide-band frequency applications.
Further, the normal directions of the main surface 5a and 5b of the piezoelectric material substrate 5 are not particularly limited. For example, in the case that the piezoelectric material substrate is made of lithium nitride, it is preferred to use the substrate rotated from Y-axis toward Z-axis by 32 to 55° (180°, 58° to 35°, 180° on Eulerian angle representation) around X-axis, which is a direction of propagation of a surface acoustic wave, because of a low propagation loss. In the case that the piezoelectric material substrate is made of lithium niobate, (i) it is preferred to use the substrate rotated from Z-axis toward −Y-axis by 37.8° (0°, 37.8°, 0° on Eulerian angle representation) around X-axis, which is a direction of propagation of a surface acoustic wave, because of a large electro-mechanical coupling factor. Alternatively, (ii) it is preferred to use the substrate rotated from Y-axis toward Z-axis by 40 to 65° (180°, 50 to 25°, 180° on Eulerian angle representation) around X-axis, which is a direction of propagation of a surface acoustic wave, because a high acoustic speed can be obtained. Further, although the size of the piezoelectric material substrate is not particularly limited, for example, the diameter may be 100 to 200 mm and thickness may be 0.15 to 1 μm.
Oxygen plasma may preferably be irradiated onto the piezoelectric material substrate and the respective silicon oxide layers at a temperature of 150° C. or lower to activate the respective surfaces, before the surface of the piezoelectric material substrate and the silicon oxide layer on the intermediate layer are directly bonded with each other, or before the first silicon oxide layer and second silicon oxide layer are directly bonded with each other.
The pressure during the surface activation may preferably be 100 Pa or lower and more preferably be 80 Pa or lower. Further, the atmosphere may be composed of oxygen only, or nitrogen gas in addition to oxygen.
The temperature during the irradiation of the oxygen plasma is made 150° C. or lower. It is thereby possible to obtain the bonded body having a high bonding strength and without deterioration of the piezoelectric material. On the viewpoint, the temperature during the oxygen plasma irradiation is made 150° C. or lower and may more preferably be 100° C. or lower.
The energy of the oxygen plasma irradiated onto the surface of the piezoelectric material substrate may preferably be 100 to 150 W. Further, the product of the energy and time duration of irradiation during the irradiation of the oxygen plasma may preferably be 20 to 50 Wh. Further, the time duration of irradiation of the oxygen plasma may preferably be 30 minutes or longer.
Further, the pressure during the plasma irradiation onto the surface of the silicon oxide layer may preferably be 100 Pa or lower and more preferably be 80 Pa or lower. The energy at this time may preferably be 30 to 120 W. Further, the product of the energy of the plasma irradiation and irradiation time duration may preferably be 1 Wh or lower.
According to a preferred embodiment, the surface of the piezoelectric material substrate and surfaces of the respective silicon oxide layers are subjected to flattening process before the plasma treatment. The method of flattening the respective surfaces includes lapping, chemical mechanical polishing (CMP) and the like. Further, the arithmetic surface roughness Ra of the flattened surface may preferably be 1.0 nm or lower and more preferably be 0.3 nm or lower.
The first silicon oxide layer and second silicon oxide layer may be then contacted with each other, or the silicon oxide layer and piezoelectric material substrate are then contacted with each other to perform the direct bonding. Thereafter, it is preferred to perform annealing treatment to improve the bonding strength. The temperature during the annealing treatment may preferably be 100° C. or higher and 300° C. or lower.
The bonded body of the present invention may preferably be applied as an acoustic wave device.
As the acoustic wave device, a surface acoustic wave device, Lamb wave-type device, thin film resonator (FBAR) or the like is known. For example, the surface acoustic wave device is produced by providing an input side IDT (Interdigital transducer) electrodes (also referred to as comb electrodes or interdigitated electrodes) for oscillating surface acoustic wave and IDT electrode on the output side for receiving the surface acoustic wave, on the surface of the piezoelectric single crystal substrate. By applying high frequency signal on the IDT electrode on the input side, electric field is generated between the electrodes, so that the surface acoustic wave is oscillated and propagated on the piezoelectric substrate. Then, the propagated surface acoustic wave is drawn as an electrical signal from the IDT electrodes on the output side provided in the direction of the propagation.
A material forming the electrodes (electrode pattern) of the piezoelectric material substrate may preferably be aluminum, an aluminum alloy, copper or gold, and more preferably be aluminum or the aluminum alloy. The aluminum alloy may preferably be Al with 0.3 to 5 weight % of Cu mixed therein. In this case, Ti, Mg, Ni, Mo or Ta may be used instead of Cu.
It was obtained a bonded body according to an embodiment of the present invention, as described referring to
Specifically, it was prepared an Si substrate (supporting substrate) 1 of a high resistance (≥2 kΩ·cm), a thickness of 0.23 mm and a diameter of 150 mm. The supporting substrate 1 was introduced into a sputtering system “RAS-1100BII” supplied by SYNCHRON Co. Ltd. to form a silicon film 2 having a thickness of about 500 nm. The film-forming conditions are as follows.
Bias voltage: 6000 W
Ar gas flow rate: 100 sccm
Electric power of microwave: 1500 W
Rate: 0.3 nm/sec
Pressure in chamber during film-formation: 0.1 Pa
The supporting substrate 1 with the silicon film formed thereon was drawn out of the chamber and then subjected to heat treatment in a clean oven at 500° C. for 10 hours to generate an intermediate layer 3. The atmosphere during the heat treatment was nitrogen atmosphere and the pressure of the atmosphere was 1 atm.
The supporting substrate 1 after the heat treatment was then introduced into the sputtering system again and a first silicon oxide layer 4 composed of SiO2 was then film-formed in a thickness of 600 nm. At the same time, it was introduced in the same chamber a piezoelectric material substrate 5 having a thickness of 0.25 mm, with both surfaces being mirror surfaces and composed of 42° Y-cut black lithium tantalate substrate, to form a second silicon oxide film 6. The film-forming conditions were as follows. Besides, when silicon is film-formed by sputtering, O2 gas was introduced at a flow rate of 200 sccm for oxidizing silicon.
Bias voltage: 6000 W
Ar gas flow rate: 100 sccm
Electric power of microwave: 1500 W
Rate: 0.3 nm/sec
Pressure inside of chamber during film-formation: 0.1 Pa
The supporting substrate 1 and piezoelectric material substrate 5 after the film-formation were drawn out of the chamber, and the first silicon oxide layer 4 and second silicon oxide layer 6 were subjected to CMP (chemical mechanical polishing) in a thickness of about 100 nm, respectively. Thereafter, Ra of each of the surfaces was about 0.2 nm, indicating that very smooth surfaces were obtained.
The respective surfaces of the first silicon oxide layer 4 and second silicon oxide layer 6 were then cleaned to remove particles from the respective surfaces. The thus cleaned first and second silicon oxide layers were contacted with each other as shown in
For evaluating the high-frequency characteristics of the thus produced bonded body 8A, it was produced a coplanar type waveguide (CPW) shown in
L1: 2100 μm
L2: 2500 μm
L3: 3100 μm
W1: 60 μm
W2: 3000 μm
G1: 340 μm
High frequency probes (TP40-GSG-250-N-L) supplied by Techno Probe Co. Ltd. are contacted to both end parts of the CPW, respectively, and S parameter of the CPW was measured by a network analyzer “PNA-X” supplied by KEYSIGHT TECHNOLOGIES. The effective resistivity of the bonded body was calculated based on Shunt-Through method, from the thus measured data.
According to the present example, the silicon oxide layer 4 was formed on the supporting substrate, without forming the intermediate layer 3 functioning as a trap-rich layer. The other processes were made same as those of the inventive example 1 to obtain a bonded body. The change of the effective resistivity on frequency of the bonded body was measured according to the same procedure as the inventive example 1, and the results were shown in
As a result, according to the inventive example 1, the effective resistivity was considerably higher than that of the comparative example 1 without the trap-rich layer over the whole frequency range. For example, at 1 GHz, the effective resistivity of the comparative example 1 was 1×104 Ω·cm and that of the inventive example 1 was 3×104 Ω·cm, and the effective resistivity of the latter was higher by three times. That is, according to the inventive example 1, it was confirmed that the characteristics in a high frequency range was particularly improved.
According to the present example, a bonded body was produced according to the same processes as those of the inventive example 1, except that the heat treatment of the silicon film 2 formed by sputtering method was not performed. The change of the effective resistivity on frequency of the bonded body was measured according to the same procedure as the inventive example 1, and the results were shown in
As a result, according to the inventive example 1, the effective resistivity was considerably improved with respect to that of the comparative example 2 over the whole frequency range. For example, at 1 GHz, the effective resistivity of the comparative example 2 was 2×104 Ω·cm and the effective resistivity of the inventive example 1 was 3×104 Ω·cm, and that of the latter was higher by 1.5 times. It means that the properties of the silicon layer film-formed on the silicon substrate by sputtering were changed by the heat treatment to improve the effective resistivity.
It was produced a bonded body according to the same procedure as the inventive example 1. However, the silicon film 2 was not formed by sputtering on the supporting substrate. Instead, polycrystalline silicon was film-formed on the supporting substrate by LP-CVD method at 700° C. in a thickness of 500n m. The change of the effective resistivity on frequency of the bonded body was measured according to the same procedure as the inventive example 1, and the results were shown in
As a result, the change of the effective resistivity on frequency was substantially comparable with that of the comparative example 2.
(Heat Resistance Test)
The respective bonded bodies according to the inventive example 1 and comparative examples 1, 2 and 3 were charged into a clean oven at a temperature of 250° C., and the respective bonded bodies were drawn out after 20 hours. As a result, cracks or fracture was not observed and the effect of suppressing the residual stress due to low-temperature film formation was confirmed, according to the bonded bodies of the inventive example 1 and comparative examples 1 and 2. Contrary to this, the bonded body of the comparative example 3 was broken into two pieces.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2018-211338 | Nov 2018 | JP | national |
This application is a continuation application of PCT/JP2019/043392, filed Nov. 6, 2019, which claims priority to Japanese Application No. 2018-211338, filed Nov. 9, 2018, the entire contents all of which are incorporated hereby by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2019/043392 | Nov 2019 | US |
| Child | 16898925 | US |
