A composite substrate of the present disclosure is a composite substrate in which a piezoelectric substrate and a sapphire substrate are directly bonded, and a bonding surface of the sapphire substrate has a step bunch structure. A piezoelectric device of the present disclosure includes the composite substrate. A method for manufacturing a composite substrate includes the steps of: preparing a piezoelectric substrate and a sapphire substrate including a surface having a predetermined off-angle to a specific crystal plane, heat treating the sapphire substrate in an oxidizing atmosphere to form a step bunch on the surface of the sapphire substrate, and bonding the piezoelectric substrate and the surface of the sapphire substrate directly.

A bonded body includes a piezoelectric single crystal substrate; a supporting substrate composed of a polycrystalline ceramic material or a single crystal material; a bonding layer provided on the piezoelectric single crystal substrate and having a composition of Si.sub.(1-x)O.sub.x (0.008≤x≤0.408); and an amorphous layer provided between the supporting substrate and bonding layer, the amorphous layer containing oxygen atoms and argon atoms. A concentration of the oxygen atoms at a central part of the amorphous layer is higher than a concentration of the oxygen atoms at a peripheral part of the amorphous layer.

A method of manufacturing a monocrystalline layer comprises the following successive steps: providing a donor substrate comprising a piezoelectric material of composition ABO.sub.3, where A consists of at least one element from among Li, Na, K, H, Ca; and B consists of at least one element from among Nb, Ta, Sb, V; providing a receiver substrate, transferring a layer called the “seed layer” from the donor substrate on to the receiver substrate, such that the seed layer is at the bonding interface, followed by thinning of the donor substrate layer; and growing a monocrystalline layer of composition A′B′O.sub.3 on piezoelectric material ABO.sub.3 of the seed layer, where A′ consists of a least one of the following elements Li, Na, K, H; B′ consists of a least one of the following elements Nb, Ta, Sb, V; and A′ is different from A or B′ is different from B.

A multi-layer piezoelectric microactuator assembly has at least one poled and active piezoelectric layer and one poled but inactive piezoelectric layer. The poled but inactive layer acts as a constraining layer in resisting expansion or contract of the first piezoelectric layer thereby reducing or eliminating bending of the assembly as installed in an environment, thereby increasing the effective stroke length of the assembly. Poling only a single layer would induce stresses into the device; hence, polling both piezoelectric layers even though only one layer will be active in use reduces stresses in the device and therefore increases reliability.

A bonded body includes a supporting substrate, a silicon oxide layer provided on the supporting substrate, and a piezoelectric material substrate provided on the silicon oxide layer and composed of a material selected from the group consisting of lithium niobate, lithium tantalate and lithium niobate-lithium tantalate. The surface resistivity of the piezoelectric material substrate on the side of the silicon oxide layer is 1.7×10.sup.1 5Ω/□ or higher.

A bonded body includes: a piezoelectric single crystal substrate; a supporting substrate composed of a single crystal silicon; a bonding layer—provided between the supporting substrate and piezoelectric single crystal substrate and having a composition of Si.sub.(1-x)O.sub.x (0.008≤x≤0.408); and an amorphous layer provided between the supporting substrate and bonding layer and containing silicon atoms, oxygen atoms, and argon atoms. The concentration of the oxygen atoms in an end part of the amorphous layer on a side of the bonding layer is higher than the average concentration of the oxygen atoms in the bonding layer.

A method of manufacturing a monocrystalline layer, comprises the following successive steps: providing a donor substrate comprising a piezoelectric material of composition ABO.sub.3, where A consists of at least one element from among Li, Na, K, H, Ca; and B consists of at least one element from among Nb, Ta, Sb, V; providing a receiver substrate, transferring a layer called the “seed layer” from the donor substrate on to the receiver substrate, such that the seed layer is at the bonding interface, followed by thinning of the donor substrate layer; and growing a monocrystalline layer of composition A′B′O.sub.3 on piezoelectric material ABO.sub.3 of the seed layer where A′ consists of a least one of the following elements Li, Na, K, H; B′ consists of a least one of the following elements Nb, Ta, Sb, V; and A′ is different from A or B′ is different from B.

A strain sensor unit and a skin sensor module comprising the same are provided. The strain sensor unit according to an embodiment of the present disclosure includes a substrate having a through-hole, and including a first electrode and a second electrode formed at one side and the other side of the through-hole on one surface of the substrate, a piezoelectric device drawn from the first electrode and extending inward the through-hole, and a piezoresistor drawn from the second electrode and extending inward the through-hole, wherein the piezoresistor overlaps with a whole or part of the piezoelectric device.

A bonded body includes a supporting substrate; a piezoelectric material substrate composed of a material selected from the group consisting of lithium niobate, lithium tantalate and lithium niobate-lithium tantalate; and a bonding layer bonding the supporting substrate and the piezoelectric material substrate and contacting a main surface of the piezoelectric material substrate. The bonding layer includes a void extending from the piezoelectric material substrate to the supporting substrate.

A bonded body includes a supporting substrate; a piezoelectric material substrate composed of a material selected from the group consisting of lithium niobate, lithium tantalate and lithium niobate-lithium tantalate; and a bonding layer bonding the supporting substrate and the piezoelectric material substrate and contacting a main surface of the piezoelectric material substrate. The bonding layer includes a void extending from the piezoelectric material substrate toward the supporting substrate. A ratio (t2/t1) of a width t2 at an end of the void on a side of the supporting substrate with respect to a width t1 at an end of the void on a side of the piezoelectric material substrate is 0.8 or lower.