Selective etching of thin films

Details Selective etching of thin films Selective etching of thin films

2000-02-04

2001-10-23

Stinson, Frankie L. (Department: 1746)

Etching a substrate: processes

Gas phase etching of substrate

Application of energy to the gaseous etchant or to the...

C216S077000, C438S734000

Reexamination Certificate

active

06306313

ABSTRACT:

REFERENCE TO RELATED APPLICATIONS
This application is related to a copending application entitled “Method of Isolation for Acoustic Resonator Devices” filed on the same day. The aforementioned application is hereby incorporated herein by reference.
FIELD OF THE INVENTION
The invention pertains to the field of electronic device fabrication. More particularly, the invention pertains to methods of etching thin films, and products made by such methods.
BACKGROUND OF THE INVENTION
Thin film resonators (TFRs) are thin film acoustic devices which can resonate in the radio frequency to microwave range, for example, 0.5 to 5 Gigahertz (GHz), in response to an electrical signal. A typical TFR contains a piezoelectric film between a first electrode and a second electrode which supports the electric field across the piezoelectric film when a signal is applied to the device. The piezoelectric film is made of a piezoelectric crystalline material, such as zinc oxide, aluminum nitride (AlN) and other piezoelectric crystalline material, which exhibits a piezoelectric effect. The piezoelectric effect occurs when the piezoelectric material induces a strain in response to an electric field applied across the piezoelectric material, or produces an electric field in response to mechanical stress applied to the piezoelectric material. If the electric field across the piezoelectric film is an alternating electric field having frequency components corresponding to resonant frequencies of the piezoelectric film, the film vibrates at the resonant frequencies (e.g. a fundamental frequency and harmonics), the fundamental frequency of which is approximately defined for a film of uniform thickness as the acoustic velocity (v) in the film divided by two (2) times the thickness (t) of the film or f
r
=v/2t. The film mechanically vibrates at the resonant frequencies which in turn produces an alternating electric field at the resonant frequencies.
The physical size of a resonant structure in an electrical circuit is dictated by its operational frequency. For example, the length (L) of a side of a resonant cube is given by L=&lgr;/2=v/f, where &lgr; is the wavelength for the wave of frequency f, and v is the wave's velocity in a particular medium. In the microwave region, these wavelengths are on the order of a millimeter, and it is in this region of the electromagnetic spectrum where application of resonant cavities to electrical circuits becomes practical. However, on the scale of today's microelectronics, a single device of millimeter dimensions is comparatively large.
In electronic circuit applications, such materials can be used to form resonant structures, that is, structures which affect a circuit's impedance as a function of a signal's frequency. For example, when embodied as an electrical filter, such a resonant structure will offer low impedance to a desired range of frequencies (the pass band) and high impedance to frequencies outside that range (the stop band). In this way, the device selects the frequencies which pass through the circuit and ideally rejects all others.
Use of piezoelectric materials in the fabrication of microwave resonators offers a potential for tremendous size reduction. The action of a piezoelectric is such that it converts a wave traveling at the speed of light to one traveling at the speed of sound, and vice versa. In most media, the velocity (v) of sound is 1/10,000
th
that of light. Thus, referring to the above expression, the wavelength (&lgr;)—and therefore the device dimension—is reduced accordingly.
However, due to the imperfections in the piezoelectric material (polycrystalline structure, grain boundaries, etc.) of the device, waves are also launched parallel to the surface (perpendicular to the bulk mode), producing lateral, or shear, acoustic waves in addition to the acoustic waves propagating perpendicularly. These lateral waves lower the efficiency of these devices. The extra piezoelectric material not directly between the electrodes allows propagation of these shear waves.
Therefore, a method for etching the superfluous portions of a material that permits patterned removal of the material without affecting a surface immediately below it is desired.
SUMMARY OF THE INVENTION
The invention involves removing a portion(s) of a material of interest, while leaving an adjacent or underlying electrode(s), for example an aluminum electrode, intact. For example, a plasma containing at least two halogen-containing gases is used to etch the exposed portion(s) of the material layer such as a piezoelectric layer of aluminum nitride (AlN) adjacent to at least one electrode layer, such as an aluminum electrode layer. While each halogen-containing gas used independently will etch the piezoelectric layer, the combination of two or more halogen-containing gases enables a useful etch rate and a stable plasma. Removal of at least one of the halogen-containing gases near the end of the etch cycle completes the etch while preserving an adjacent electrode. Using this method, the superfluous portion(s) of the piezoelectric material is removed without affecting the electrode.


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Suto, S., et al.; Highly Selective Etching of Si3N4to SiO2Employing Fluorine and Chlorine

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