Silicon microphone with softly constrained diaphragm
A microphone sensing element and a method for making the same are disclosed. The sensing element has a diaphragm and an attached electrical lead-out arm preferably made of polysilicon that are separated by an air gap from an underlying backplate region created on a conductive silicon substrate. The backplate region has acoustic holes created by removing an oxide filling in a continuous trench that surrounds hole edges and by removing oxide to form the air gap. The diaphragm is softly constrained along its edge by an elastic element that connects to a surrounding rigid polysilicon layer. The elastic element is typically a polymer such as parylene having a Young's modulus substantially less than that of the diaphragm. First and second electrodes are connected to the diaphragm through the lead-out arm and to the substrate through polysilicon via fillings, respectively, and thereby establish a variable capacitor circuit for acoustic sensing.
1. A silicon microphone sensing element with a softly constrained diaphragm, comprising:
(a) a diaphragm comprised of a semiconductor layer, said diaphragm has an edge, bottom surface, and a plurality of holes formed an equal distance from said edge and from a first trench that defines said edge wherein said diaphragm is formed over a backplate region with acoustic holes in a front side of a substrate and above a back hole with an opening in a back side of the substrate, said backplate region and diaphragm are separated by an air gap;
(b) an electrical lead-out arm that extends from the edge of said diaphragm and is comprised of the same semiconductor material as in the diaphragm and has a shape defined by a set of second trenches in the semiconductor layer;
(c) a rigid semiconductor layer having a plurality of horizontal sections disposed on a dielectric stack on said front side of the substrate, a plurality of holes formed an equal distance from the trench surrounding the diaphragm edge, and vertical sections that connect said horizontal sections and said front side of the substrate;
(d) a dielectric spacer stack disposed on a portion of said horizontal sections and having openings formed therein on said diaphragm and said electrical lead-out arm;
(e) a first electrode formed on said dielectric spacer stack and within an opening that contacts said electrical lead-out arm; and a second electrode formed on said dielectric spacer stack and within an opening that contacts a horizontal section of said rigid semiconductor layer;
(f) a soft constraint layer having an upper section formed above the edge of said diaphragm, above and within the first trench and plurality of holes, and above a portion of the rigid semiconductor layer, and having a lower section attached to the bottom surface of the diaphragm and rigid semiconductor layer adjacent to the first trench and plurality of holes; and
(g) a substrate with front and back sides, a back hole formed in the substrate with an opening in said back side, and a backplate region with acoustic holes formed between the back hole and front side of the substrate.
2. The silicon microphone sensing element of claim 1 wherein said substrate is comprised of conductive silicon.
3. The silicon microphone sensing element of claim 1 wherein the diaphragm and the horizontal sections of said rigid semiconductor layer are a single or composite layer comprised of doped polysilicon, silicon, nickel, gold, aluminum, nitride, or other semiconductor materials.
4. The silicon microphone sensing element of claim 1 wherein the first trench separates the diaphragm from the rigid semiconductor layer and the set of second trenches separates the electrical lead-out arm from the rigid semiconductor layer, said first trench and set of second trenches are connected to form a continuous opening around the diaphragm and electrical lead-out arm.
5. The silicon microphone sensing element of claim 1 wherein said soft constraint is a single or composite material layer comprised of parylene, Teflon, PMMA, PDMS, SU8 photoresist, or other materials having a higher elasticity than the diaphragm and a substantially lower Young's modulus than the diaphragm.
6. The silicon microphone sensing element of claim 1 wherein the first electrode and the second electrode are a composite layer comprised of Au/Cr or are a single layer or composite layer comprised of Al, Ti, Ta, Cu, Ni, or other conductive materials.
7. The silicon microphone sensing element of claim 1 wherein said dielectric stack is a single or composite sacrificial layer comprised of one or more of silicon oxide, TEOS, PSG, and silicon nitride.
8. The silicon microphone sensing element of claim 1 wherein said dielectric spacer stack is comprised of a silicon rich silicon nitride (SRN) layer.
9. The silicon microphone sensing element of claim 1 wherein said acoustic holes in the backplate region have a square or circular shape and are formed in multiple rows and columns.
10. The silicon microphone sensing element of claim 1 wherein the air gap is bounded by vertical sections of the rigid semiconductor layer.
11. The microphone sensing element of claim 1 further comprised of a hard mask formed on the back side of said substrate and surrounding the back hole wherein said hard mask is a single layer comprised of either a thermal oxide or a silicon nitride layer, or is a composite layer comprised of a thermal oxide layer and a silicon nitride layer.