Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2002-04-01
2003-04-01
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C029S025350, C310S335000
Reexamination Certificate
active
06542054
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to an acoustic mirror that reflects acoustic waves within a defined frequency range.
An acoustic mirror of this type is described, for example, in the reference by B. Olutade et al., titled “Sensitivity Analysis of Thin Film Bulk Acoustic Resonator Ladder Filter”, IEEE International Frequency Control Symposium 1997, 737. The acoustic mirror contains a stack of thin films with alternately high and low acoustic impedances. The thickness of the thin films is, for example, a quarter of the wavelength of the acoustic waves that are to be reflected in the respective thin film. It is preferable for the production of the acoustic mirror to be as compatible as possible with silicon process techniques, so that the acoustic mirror can be integrated with other components on a chip.
It is known from the reference by Y. Hayashi et al., titled “Nitride Masked Polishing (NMP) Technique for Surface Planarization of Interlayer-Dielectric Films”, Ext. Abstr. of the 1992 Int. Conf. on Solid State Devices and Materials, Tsukuba (1992), 533, that in order to produce a surface which is as planar as possible for a layer of borophospho-silicate glass (BPSG), which covers an elevated cell array and a periphery of a memory cell configuration, to apply a stop layer of silicone nitride to a part of the BPSG layer above the periphery and then to planarize the layer by chemical mechanical methods. Prior to the planarization, part of the BPSG layer above the cell array is elevated. On account of the stop layer, the part is removed selectively with respect to the part of the BPSG layer that lies above the periphery during planarization.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an acoustic mirror and a method for producing the acoustic mirror which overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, whose production is compatible with silicon process techniques and which can have a particularly high reflectivity compared to the prior art.
With the foregoing and other objects in view there is provided, in accordance with the invention, an acoustic mirror. The acoustic mirror contains a substrate, and a layered sequence formed of at least two insulating layers and at least two metal layers disposed alternately on top of one another on the substrate. The insulating layers have substantially equal thicknesses and the metal layers have substantially equal thicknesses. The thicknesses of the insulating layers and of the metal layers are such that a reflection condition is satisfied. An auxiliary layer having at least one recess formed therein, is provided, and at least the metal layers are disposed inside the at least one recess.
The greater the difference between the acoustic impedances of the layers of the acoustic mirror, the higher the reflectivity becomes. Metal layers have a particularly high impedance. The reflectivity of the acoustic mirror can be particularly high on account of the use of metal layers.
Acoustic waves are reflected by a stack that contains the insulating layers and the metal layers. The reflection condition which is satisfied by the thicknesses of the layers consists, for example, in the fact that an acoustic wave which has a defined frequency and impinges on the substrate at a defined angle is reflected particularly well by the layers.
The thicknesses of the insulating layers and of the metal layers are preferably in each case substantially &lgr;/
4
, where &lgr; is the wavelength of an acoustic wave in the respective layer which is reflected particularly well if it impinges on the substrate at right angles. Given the same frequency of the wave, the wavelength is dependent on the impedance and on the specific density of the material passing the wave.
The insulating layers have a low acoustic impedance, while the metal layers have a high acoustic impedance. The thicknesses of the metal layers are therefore preferably identical. The same applies to the insulating layers. However, the insulating layers and the metal layers are generally of different thicknesses.
The lowermost layer in the stack is, for example, one of the insulating layers.
The uppermost layer in the stack is preferably one of the metal layers.
The layers of the stack are applied substantially conformally.
Adjacent layers of the stack preferably bear against one another. However, it is also possible for interlayers, for example diffusion barriers, which are very thin in relation to the other layers, to be disposed between the adjacent layers.
Since the metal layers are produced in at least one recess, the metal layers have smaller horizontal areas than the substrate. The capacitance of the acoustic mirror, which is formed by the metal layers, is smaller than an acoustic mirror in which the metal layers cover the entire substrate.
A metal layer that covers the entire substrate, on account of a high layer stress, causes bending of the substrate, which increases as the thickness of the metal layer rises. In the acoustic mirror according to the invention, bending of the substrate is avoided, since the metal layers do not cover the entire substrate, but rather are only disposed in one or more recesses.
The metal layers are in each case initially deposited over the entire surface and then structured by chemical mechanical polishing in such a way that they are disposed inside the respectively associated recess. The metal can be structured more effectively by chemical mechanical polishing than by etching with the aid of a photolithography process. The reason for this is the bending of the substrate after deposition of the metal layer over the entire surface, which leads to problems with photolithographic processes. By way of example, in the event of substantial bending it is impossible to find alignment marks in order to align masks. Furthermore, in the event of bending of the substrate, sharp focusing during exposure to produce the masks from photoresist is no longer possible. In the method according to the invention, the recess can be produced with the aid of a photolithographic method prior to deposition of the metal layer, i.e. with an unbent substrate. The shape of the recess defines the shape of the metal layer.
The method allows the simultaneous production of contacts or lines, which are likewise produced on the substrate, by corresponding structuring of the auxiliary layer and by filling the recesses or contact holes produced by the deposition and the chemical mechanical polishing of the metal layer. In this case too, the required lithography takes place with an unbent substrate.
As a result of the provision of an auxiliary layer which surrounds the metal layer, the difference in height between a region in which the metal layer is disposed and a region that is disposed next to the metal layer is nonexistent or lower than if the metal layer had been structured by etching. In a method according to the invention, therefore, no topology problems are encountered. In contrast, when metal is etched with the aid of a photolithography method to produce the metal layer without the use of the auxiliary layer and of the recess, a step is formed, which causes problems during the deposition of further layers or during photolithographic methods, in which both a region of the structured metal layer and an adjoining region are exposed. Considerable differences in height cause problems in particular for photolithography process steps, since even when the photoresist is being applied by centrifuging it is impossible to produce a homogeneous layer without fluctuations in thickness. Photolithographic steps of this type are required, for example, if electrodes, for example, are to be applied to the acoustic mirror.
One of the metal layers may be disposed in a recess in which there is otherwise no further metal layer. In this case, after the recess has been produced, the metal layer is deposited and is planarized by chemical mechanical methods. If a further metal layer is to be produced a
Aigner Robert
Elbrecht Lüder
Marksteiner Stephan
Sänger Annette
Timme Hans-Jorg
Greenberg Laurence A.
Infineon - Technologies AG
Locher Ralph E.
Stemer Werner H.
Summons Barbara
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