Patent Publication Number: US-2021167749-A1

Title: Composite substrate

Description:
FIELD 
     The present disclosure relates to a composite substrate. 
     BACKGROUND 
     In recent years, thinning of a piezoelectric element such as an SAW (surface acoustic wave) element that is used for a communication device such as a mobile phone is demanded. 
     Accordingly, for example, Patent Literature 1 proposes an SAW device that positions an LT (LiTaO 3 ) substrate on a surface of a sapphire substrate and includes a via conductor that is positioned at a hole that is communicated with such a sapphire substrate and a LT substrate. 
     Thus, Patent Literature 1 does not provide an LT substrate as a single body but also uses a sapphire substrate with a mechanical strength that is higher than that of the LT substrate, so that it is possible to maintain an overall mechanical strength and thin the LT substrate, and thinning of a SAW device is realized. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: International Publication No. 2016/084936 
       
    
     SUMMARY 
     A composite substrate according to the present disclosure is a plate-shaped body where a first substrate and a second substrate overlap. Furthermore, it has a hole in the first substrate and the second substrate in a thickness direction of the plate-shaped body. Furthermore, a ratio A/B of an average thickness A of the first substrate to an average thickness B of the second substrate is 1/5 or less. Furthermore, an interface between the first substrate and the second substrate on an inner wall of the plate-shaped body that is positioned at the hole has a part that is covered by a covering layer that contains a component that composes the second substrate. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional diagram that illustrates an example of a composite substrate according to the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A composite substrate according to the present disclosure will be explained with reference to the drawings. 
     As illustrated in  FIG. 1 , a composite substrate  10  according to the present disclosure has a first substrate  1  and a second substrate  2 . The composite substrate  10  is a plate-shaped body. 
     As illustrated in  FIG. 1 , in the composite substrate  10  according to the present disclosure, the first substrate  1  and the second substrate  2  overlap. 
     Herein, the first substrate  1  is a substrate that has a piezoelectric performance, and is, for example, a lithium tantalate (LiTaO 3  that will be described as LT below) substrate, a lithium niobate (LiNbO 3  that will be described as LN below) substrate, or the like. On the other hand, the second substrate  2  is a silicon substrate or a sapphire substrate if it is of a single crystal or an aluminum oxide substrate, an aluminum nitride substrate, or a silicon nitride substrate if it is of a poly crystal, or the like, and has a mechanical strength that is higher than that of the first substrate  1 . 
     Additionally, it is possible to specify a material that composes the first substrate  1  and the second substrate  2  by using an X-ray diffractometer (XRD) so as to execute measurement thereof and identifying a value of an obtained  2 θ (where  2 θ is an diffraction angle) in a JCPDS card. 
     Then, as illustrated in  FIG. 1 , the composite substrate  10  according to the present disclosure has a hole  3  in the first substrate  1  and the second substrate  2  in a thickness direction of a plate-shaped body thereof, and a ratio A/B of an average thickness A of the first substrate  1  to an average thickness B of the second substrate  2  is 1/5 or less. Moreover, an interface between the first substrate  1  and the second substrate  2  on an inner wall of a plate-shaped body that is positioned at the hole  3  has a part that is covered by a covering layer  4  that contains a component that composes the second substrate  2 . 
     As such a configuration is satisfied, an interface that is positioned at the hole  3  is covered by the covering layer  4  that contains a component that composes the second substrate  2  that is excellent in a mechanical strength even if a thickness of the composite substrate  10  is small, so that a crack is prevented from being readily generated from an interface that is positioned at the hole  3  in the composite substrate  10  according to the present disclosure. 
     Herein, from a viewpoint of improving of a mechanical strength of a composite substrate, a ratio A/B of an average thickness A of the first substrate  1  to an average thickness B of the second substrate  2  may be 1/50 or greater. Additionally, an average thickness A of the first substrate  1  may be, for example, 1 μm or greater and 40 μm or less. Furthermore, an average thickness B of the second substrate  2  may be, for example, 50 μm or greater and 200 μm or less. 
     Additionally, it is sufficient that an average thickness A of the first substrate  1  and an average thickness B of the second substrate are measured and calculated by a following method. First, the composite substrate  10  is cut so as to provide a cross section as illustrated in  FIG. 1  and is polished by a cross section polisher (CP). Then, a photograph of such a polished cross section is taken by using a scanning electron microscope (SEM). Then, it is sufficient that thicknesses of each of the first substrate  1  and the second substrate  2  at, at least, 5 or more places in a taken photograph are measured and an average value thereof is calculated. 
     Furthermore, in the covering layer  4 , a content of a component that composes the second substrate  2  is 90% by mass or greater of 100% by mass of all components that compose the covering layer  4 . Herein, it is sufficient that a component that composes the covering layer  4  and a content thereof is confirmed by a following method. First, the composite substrate  10  is cut so as to provide a cross section as illustrated in  FIG. 1  and is polished by a CP. Then, it is sufficient that the covering layer  4  in such a polished cross section is measured by using an energy dispersive X-ray analyzer (EDS) equipped with an SEM or a transmission electron microscope (TEM) and a component that composes the covering layer  4  and a content thereof are calculated. 
     Furthermore, the covering layer  4  in the composite substrate  10  according to present disclosure may be positioned so as to surround an interface between the first substrate  1  and the second substrate  2 . Herein, the covering layer  4  being positioned so as to surround an interface means that a whole of an interface is covered by the covering layer  4  and a place with an exposed interface is absent at the hole  3 . If such a configuration is satisfied, a whole of an interface is covered by the covering layer  4 , so that a crack is further prevented from being readily generated from an interface that is positioned at the hole  3  in the composite substrate  10  according to the present disclosure. Furthermore, in a case where the composite substrate  10  is cleaned by a cleaning agent such as an alkaline solution before a via conductor is formed at the hole  3 , the cleaning agent does not penetrate into an interface and it is possible to maintain adhesion between the first substrate  1  and the second substrate  2 , so that it is excellent in reliability. 
     Furthermore, in the composite substrate  10  according to the present disclosure, the covering layer  4  may be positioned at 90% or greater of a thickness from an interface to an opening of the first substrate  1 , on an inner wall of the first substrate  1  that is positioned at the hole  3 . Herein, the covering layer  4  being positioned at 90% or greater of a thickness from an interface to an opening of the first substrate  1  means that, when A is a distance from an interface to an opening of the first substrate  1 , the covering layer  4  covers an interface to a part at 0.9A or greater toward a side of the opening, on an inner wall of the first substrate  1  that is positioned at the hole  3 . 
     Then, if such a configuration is satisfied, the covering layer  4  protects the first substrate  1  with a less mechanical strength from thermal stress that is caused by a via conductor when the via conductor is formed at the hole  3 , so that a crack is prevented from being readily generated in the first substrate  1 . Additionally, from a viewpoint of preventing of a crack from being readily generated by the first substrate  1 , the covering layer  4  may cover a whole of an inner wall of the first substrate  1  that is positioned at the hole  3 . 
     Furthermore, the covering layer  4  in the composite substrate  10  according to the present disclosure may have a part with a thickness that gradually increases from a side of an opening to a side of an interface of the first substrate  1 . If such a configuration is satisfied, it is possible for a place that covers an interface on the covering layer  4  to efficiently relax thermal stress that is caused by a via conductor, so that a crack is further prevented from being readily generated from an interface that is positioned at the hole  3  in the composite substrate  10  according to the present disclosure. 
     Furthermore, the covering layer  4  in the composite substrate  10  according to the present disclosure may have a part with a thickness that gradually increases from a side of an opening to a side of an interface of the second substrate  2 . If such a configuration is satisfied, it is possible for a place that covers an interface on the covering layer  4  to efficiently relax thermal stress that is caused by a via conductor, so that a crack is further prevented from being readily generated from an interface that is positioned at the hole  3  in the composite substrate  10  according to the present disclosure. 
     Herein, a maximum thickness of the covering layer  4  may be, for example, 0.5 μm or greater and 2.5 μm or less. If the covering layer  4  is of a maximum thickness as described above, formation of a via conductor at the hole  3  is facilitated and it is possible for the covering layer  4  to protect an interface efficiently. 
     Furthermore, as illustrated in  FIG. 1 , for the hole  3  in the composite substrate  10  according to the present disclosure, a size of an opening of the first substrate  1  may be greater than a size of an opening of the second substrate  2 . If such a configuration is satisfied, it is possible to cause a current to flow through the first substrate  1  via a via conductor at the hole  3  efficiently, so that an electrical characteristic of the composite substrate  10  according to the present disclosure is improved. 
     Furthermore, a size of an opening of the first substrate  1  at the hole  3  may be, for example, of a circular shape with a diameter of 40 μm or greater and 90 μm or less. On the other hand, a size of an opening of the second substrate  2  at the hole  3  may be, for example, of a circular shape with a diameter of 30 μm or greater and 70 μm or less. 
     Furthermore, a surface roughness of an inner wall of the first substrate  1  may be greater than a surface roughness of an inner wall of the second substrate  2 . In other words, a surface roughness of an inner wall of the second substrate  2  may be less than a surface roughness of an inner wall of the first substrate  1 . 
     In a case where a surface roughness of an inner wall of the first substrate  1  is greater than a surface roughness of an inner wall of the second substrate  2 , the covering layer  4  readily tightly adheres to the inner wall of the first substrate  1 . Hence, it is possible for the covering layer  4  to cover an interface stably. 
     On the other hand, in a case where a surface roughness of an inner wall of the second substrate  2  is less than a surface roughness of an inner wall of the first substrate  1 , a metal member is readily formed on a whole of an inner wall of the second substrate  2  in a step of forming a metal member that is represented by a via conductor on an inner wall of the first substrate  1  and the inner wall of the second substrate  2  by vapor deposition or sputtering. In particular, a metal member is also readily formed at a part that is distant from an interface on the second substrate  2 . 
     A surface roughness of an inner wall of the first substrate  1  and a surface roughness of an inner wall of the second substrate  2  are parameters of a ten-point average roughness in conformity with JISB0601:2013 appendix JA. 
     Specifically, a sum of an average of a highest peak to a height of a 5th peak in order of a height and an average of a deepest valley to a depth of a 5th valley in order of a depth is represented. 
     A surface roughness of an inner wall of the first substrate  1  and a surface roughness of an inner wall of the second substrate  2  are not limited to particular values. For example, it is possible to set a surface roughness of an inner wall of the first substrate  1  at approximately 3 to 7 μm. Furthermore, it is possible to set a surface roughness of an inner wall of the second substrate  2  at approximately 1 to 5 μm. 
     A specific calculation method for a surface roughness of an inner wall of the first substrate  1  is as follows. The composite substrate  10  is cut so as to provide a cross section as illustrated in  FIG. 1  and the cross section is polished by a CP. Then, a photograph of a cross section is taken by using an SEM. Herein, an imaginary line is drawn so as to pass between a highest peak and a deepest valley of irregularities on a cross section while a reference length is provided as 10 μm. Then, a highest peak to 5th one in order of a height are selected and a height or distance of each peak from an imaginary line is measured. Then, an average value of heights or distances of 5 peaks is obtained. Similarly, a deepest valley to 5th one in order of a depth are selected and a depth or distance of each valley from an imaginary line is measured. An average value of depths or distances of 5 valleys is obtained. A sum of an average value of peak heights and an average value of valley depths is obtained. 
     A specific calculation method for a surface roughness of an inner wall of the second substrate  2  is as follows. The composite substrate  10  is cut so as to provide a cross section as illustrated in  FIG. 1  and the cross section is polished by a CP. Then, a photograph of a cross section is taken by using an SEM. Herein, an imaginary line is drawn so as to pass between a highest peak and a deepest valley of irregularities on a cross section while a reference length is provided as 10 μm. Then, a highest peak to 5th one in order of a height are selected and a height or distance of each peak from an imaginary line is measured. Then, an average value of heights or distances of 5 peaks is obtained. Similarly, a deepest valley to 5th one in order of a depth are selected and a depth or distance of each valley from an imaginary line is measured. An average value of depths or distances of 5 valleys is obtained. A sum of an average value of peak heights and an average value of valley depths is obtained. 
     Next, an example of a manufacturing method for a composite substrate  10  according to the present disclosure will be explained. 
     First, an LT substrate or an LN substrate is prepared as a first substrate  1 . Furthermore, a silicon substrate, a sapphire substrate, an aluminum oxide substrate, an aluminum nitride substrate, or a silicon nitride substrate is prepared as a second substrate  2 . Herein, the first substrate  1  is prepared so as to have a thickness in such a manner that a ratio A/B of an average thickness A of the first substrate  1  to an average thickness B of the second substrate  2  is 1/5 or less. 
     Then, the first substrate  1  and the second substrate  2  are bonded so as to obtain a plate-shaped body where the first substrate  1  and the second substrate  2  overlap. Such bonding is executed by direct joining that does not use an adhesive material or adhesion that uses an adhesive material. Herein, in a case of direct joining, after the second substrate  2  directly overlaps on the first substrate  1 , heating or pressurizing thereof is executed in a vacuum, in an air atmosphere, or in a predetermined atmosphere, so that the first substrate  1  and the second substrate  2  are bonded. 
     Then, a hole  3  that penetrates through the first substrate  1  and the second substrate  2  in a thickness direction of a plate-shaped body is formed by irradiating them with ultrashort pulsed laser that utilizes a third harmonic (355 nm) such as YAG laser from a side of the first substrate  1 . 
     Thus, if ultrashort pulsed laser is used, it is possible to form the hole  3  without generating a crack on the first substrate  1  and the second substrate  2 . Then, an output of ultrashort pulsed laser and a processing route are adjusted and a pulse fluence (an energy per unit area that is irradiated with  1  pulse of laser) is increased, so that a component that scatters from a processing region of the second substrate  2  adheres to an inner wall of a plate-shaped body that is positioned at the hole  3  so as to provide a covering layer  4 , and a value of the pulse fluence is controlled, so that it is possible to form the covering layer  4  with an any thickness at any place on the inner wall of a plate-shaped body that is positioned at the hole  3 . 
     Thus, an output of ultrashort pulsed laser and a processing route are adjusted so as to firm the hole  3 , so that it is possible to obtain the composite substrate  10  according to the present disclosure. 
     Furthermore, a control method for a surface roughness of an inner wall of a first substrate  1  and a surface roughness of an inner wall of a second substrate  2  will be described. A value of a pulse fluence of laser is adjusted, so that a surface roughness of an inner wall of the first substrate  1  and a surface roughness of an inner wall of the second substrate  2  are controlled. As a value of a pulse fluence of laser is set so as to be high, it is possible to increase a roughness thereof. 
     Although an embodiment(s) according to the present disclosure has/have been explained above, the present disclosure is not limited to an embodiment(s) as described above and various improvements and modifications thereof may be executed without departing from a spirit of the present disclosure. 
     REFERENCE SIGNS LIST 
     
         
           1 : first substrate 
           2 : second substrate 
           3 : hole 
           4 : covering layer 
           10 : composite substrate