Integrated BST microwave tunable devices using buffer layer transfer method

A BST microwave device includes a single crystal oxide wafer. A silicon dioxide layer is formed on the single crystal oxide layer. A silicon substrate is bonded on the silicon dioxide layer. A BST layer is formed on the single crystal oxide layer.

BACKGROUND OF THE INVENTION

The invention relates to the field of BST microwave tunable devices, and in particular to an integrated BST microwave tunable device using a buffer layer transfer method.

(Ba,Sr)TiO3(BST), (Ba,Zr)TiO3(BZT) (Ba,Hf)TiO3(BHT), SrTiO3(ST), Bi1.5Zn1.0Nb1.5O7(BZN series, B:Bi, Ba) and related thin films are promising materials for tunable microwave devices application such as electronically tunable mixers, oscillators, and phase shifters and filters. It will be appreciated by those of skill in the art that BST is representative of one or more related perovskite-like tunable dielectric materials.

An objective of the invention is to integrate tunable components into monolithic microwave integrated circuits (MMICs). Although microstrip planes are the most common transmission line component for microwave frequencies, the ground-plane is difficult to access for shunt connections necessary for active devices, when used in MMICs. The CPW (Coplanar Waveguide) is an attractive alternative, especially due to the ease of monolithic integration, as the ground plane runs adjacent to the transmission line. The possibility of creating BST microwave tunable devices on oxide substrates has been demonstrated in recent years. There is a great incentive to replicate these achievements on silicon-based wafers for integrated microwave device applications.

Much work has been done to obtain epitaxially grown ferroelectric thin films on Si substrates. Currently, chemical vapor deposition methods such as MBE (Molecular Beam Epitaxy) and ALD (Atomic Layer Deposition) and as well as physical vapor deposition methods such as pulsed laser deposition have been used. However, it has not been easy to obtain high quality buffer films without residual stress and defects resulting from a dimensional misfit between the crystal lattices.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a BST microwave device. The BST microwave device includes a single crystal oxide wafer. A silicon dioxide layer is formed on the single crystal oxide layer. A silicon substrate is bonded on the silicon dioxide layer. A BST layer is formed on the single crystal oxide layer.

According to another aspect of the invention, there is provided a method of forming BST microwave device. The method includes providing a single crystal oxide wafer and forming a silicon dioxide layer on the single crystal oxide layer. A silicon substrate is bonded on the silicon dioxide layer. Also, the method includes forming a BST layer on the single crystal oxide layer.

DETAILED DESCRIPTION OF THE INVENTION

Single crystal (MgO, SrTiO3, LaAlO3, Al2O3, MgAl2O4, YSZ, CeO2) buffer layers, with low loss, are first separated from the single crystal oxide wafers by a hydrogen or helium induced cutting method and then transferred to a Si wafer using wafer bonding technology. The basic structure for BST or Bi1.5Zn1.0Nb1.5O7(BZN series, B:Bi, Ba) microwave devices are developed in one of the two following methods: (1) wafer bonding and ion cutting of SiO2covered single crystals, with ion implantation or (2) wafer bonding of SiO2covered double side polished single crystals and Si-micromachining (with/without ion implantation).

FIGS. 1A-1Eare schematic diagrams illustrating wafer bonding and ion cutting of SiO2covered single crystals.FIG. 1Ashows a SiO2thin or thick layer2that is deposited on a single crystal oxide wafer4(e.g. MgO, SrTiO3, LaAlO3, Al2O3, MgAl2O4, YSZ, CeO2) using PECVD, LPCVD, and/or ALD for purposes of subsequent wafer bonding to the Si wafer8. Note Si—SiO2or SiO2—SiO2bonding is much easier, if one achieves very clean surfaces.FIG. 1Bshows hydrogen being implanted into the SiO2covered single crystal5to a desired depth6(the hydrogen stopping range) at which cleavage is desired. Note that He can also be used for implantation.FIG. 1Cshows a receiver Si (or SiO2/Si) substrate8being bonded to the SiO2deposited single crystal5through direct wafer bonding technology.FIG. 1Dshows the bonded wafers12being separated along the hydrogen implantation stopping region6by heat treatment, resulting in the transfer of a thin pure single crystal oxide10to the Si substrate8.FIG. 1Eshows high quality BST thin films13being deposited onto the single crystal oxide layer10bonded to the Si substrate8. This film13is suitable for the fabrication of microwave tunable components such as resonator, phase shifter, tunable bandpass filter, or the like.

FIGS. 2A-2Care schematic diagrams illustrating wafer bonding of SiO2covered double side polished single crystals and Si-micromachining with/without ion implantation.FIG. 2Ashows a thin or thick SiO2layer14being deposited onto various single crystal oxide wafers16(e.g. MgO, SrTiO3, LaAlO3, Al2O3, MgAl2O4, YSZ, CeO2) using PECVD, LPCVD, and/or ALD.FIG. 2Bshows a receiver substrate18, such as Si or thick SiO2/Si, being bonded to the SiO2deposited single crystal14through direct wafer bonding technology. If a thin buffer layer is needed, one can polish back the oxide single crystal14.FIG. 2Cshows high quality BST thin films20being deposited on the single crystal oxide layer16. This film20is suitable for the fabrication of microwave components such as resonator, phase shifter and tunable bandpass filters, or the like. To reduce loss through the Si substrate18, a portion of the Si substrate18below the microwave device is removed by micro-machining methods, for example, anisotropic KOH chemical etch. Note that Au electrodes22are formed on the BST layer20to complete microwave circuits.

The invention is an alternative to the buffer layer transfer technique by wafer bonding and ion cutting method. The advantage of the invention is the provision of the same environment as with single crystal substrate growth, for example, low surface roughness and high quality material.