Density-modulated phononic membranes
The invention relates to a mechanical oscillator device comprising an unsupported membrane with a multitude of discrete mass elements distributed to form Phononic crystal cells in the form of regions of additional mass each comprising a plurality of mass elements. The phononic crystal structure has a defect for confining a mechanical oscillation mode having a resonance frequency, f, with the mass elements have a smallest lateral dimension of less than 1/10 of a wavelength of the mechanical oscillation mode. The invention is based on a distribution of tiny additional mass elements providing a periodic density contrast pattern to create the bandgap. This approach keeps the tensile stress uniform which ensures perfect overlap between the tensile stress distribution and mode-shape. This again reduces the damping and thus allows for very high quality factors, Q.
1 . A mechanical oscillator device comprising:
a thin film on a supporting substrate, wherein the supporting substrate is shaped to expose an area of the thin film to form an unsupported membrane; wherein the membrane comprises periodic pattern of regions of additional mass, wherein:
the periodic pattern of the regions of additional mass provides a phononic crystal structure on the membrane wherein each region of additional mass is a unit cell in the phononic crystal structure and
the phononic crystal structure exhibits a defect for confining a mechanical oscillation mode having a resonance frequency, f, and corresponding wavelength λ and
wherein the unit cell is formed by a distribution of discrete mass elements; and a minimum lateral dimension, d, of the mass elements is less than 1/10 of the wavelength of the mechanical oscillation mode.
2 . The mechanical oscillator device according to claim 1 , wherein the defect and/or the regions of additional mass adjacent to the defect are configured for the resonance frequency of the mechanical oscillation mode to fall within a frequency range of a bandgap of the phononic crystal structure.
3 . The mechanical oscillator device according to claim 1 , wherein the mass elements are pillars.
4 . The mechanical oscillator device according to claim 1 , wherein the mass elements have lateral dimension, d, and a mean distance, a, and 0,2a<d<0,8a.
5 . The mechanical oscillator device according to claim 1 , wherein the mass elements have a mean distance, a, with a<5 micron.
6 . The mechanical oscillator device according to claim 1 , wherein a majority of the mass elements are distributed periodically within regions of additional mass.
7 . The mechanical oscillator device according to claim 6 , wherein the periodical distribution is a hexagonal pattern.
8 . The mechanical oscillator device according to claim 1 , wherein a majority of the regions of additional mass are formed by at least substantially uniform distributions of at least substantially identical mass elements.
9 . The mechanical oscillator device according to claim 1 , wherein a majority of the regions of additional mass is formed by at least substantially identical mass elements whose distribution have a density that decreases towards a boundary of the regions of additional mass.
10 . The mechanical oscillator device according to claim 1 , wherein a majority of the regions of additional mass is formed by an at least substantially uniform distribution of mass elements whose dimensions decreases towards a boundary of the regions of additional mass.
11 . The mechanical oscillator device according to claim 1 , wherein the membrane is a silicon nitride or silicon carbide membrane.
12 . The mechanical oscillator device according to claim 1 , wherein a uniform tensile stress along at least one direction in the plane of the membrane is provided.
13 . The mechanical oscillator device according to claim 1 , wherein the minimum lateral dimension, d, of the mass elements is less than 1/20 of the wavelength of the mechanical oscillation mode.
14 . A sensor comprising the mechanical oscillator device according to claim 1 , wherein the sensor is configured to detect an oscillation characteristic of the membrane.
15 . A method for providing a mechanical oscillator device, comprising:
providing a thin film on a supporting substrate;
providing on a first section of the thin film, a periodic pattern of regions of additional mass wherein the periodic pattern of the regions of additional mass provides a phononic crystal structure on the first section of the thin film wherein each region of additional mass is a unit cell in the phononic crystal structure; and
shaping the supporting substrate to expose the first section of the thin film to form an unsupported membrane;
wherein the phononic crystal structure exhibits a defect for confining a mechanical oscillation mode of the membrane having a resonance frequency, f, and corresponding wavelength, λ and wherein each region of additional mass is formed by a distribution of discrete mass elements and a minimum lateral dimension, d, of the mass elements is less than 1/10 of the wavelength of the mechanical oscillation mode.
16 . The method according to claim 15 , wherein:
providing the thin film on the supporting substrate comprises depositing the thin film on the substrate by a chemical vapor deposition technique;
providing a multitude of discrete mass elements on a first section of the thin film comprises patterning the multitude of discrete mass elements on the thin film by a reactive ion etching technique;
providing a multitude of discrete mass elements on a first section of the thin film comprises introducing a defect by removing one or more discrete mass elements; and
shaping the supporting substrate to expose the first section of the thin film to form an unsupported membrane comprises etching of the supporting substrate with potassium hydroxide, from a side opposite a side holding the thin film to form the unsupported membrane.