Patent Grant
7301212
1. Field of the Invention
The present invention relates to a MEMS and, more particularly, to a MEMS microphone.
2. Description of the Related Art
A micro-electromechanical system (MEMS) is a microscopic machine that is fabricated using the same types of steps (e.g., the deposition of layers of material and the selective removal of the layers of material) that are used to fabricate conventional analog and digital CMOS circuits.
For example, one type of MEMS is a microphone. Microphones commonly use a micro-machined diaphragm (a thin layer of material suspended across an opening) that vibrates in response to pressure changes (e.g., sound waves). Microphones convert the pressure changes into electrical signals by measuring changes in the deformation of the diaphragm. The deformation of the diaphragm, in turn, can be detected by changes in the capacitance, piezoresistance, or piezoelectric effect of the diaphragm.
In operation, changes in air pressure (e.g., sound waves) cause diaphragm 112 to vibrate which, in turn, causes the end of piezocrystal 114 to be pushed and pulled. The pushing and pulling on the end of piezocrystal 114 oppositely charges the two sides of piezocrystal 114. The charges are proportional to the amount of pushing and pulling, and thus can be used to convert pressure waves into electrical signals which can then be amplified.
When a microphone is reduced in size to that of a MEMS, one concern that arises is sensitivity. This is because the size of the diaphragm of a MEMS microphone is so relatively small (e.g., less than a millimeter across), due to being formed across a cavity or a back side opening in a relatively-small semiconductor die.
As shown in
Interconnect structure 216, which electrically connects together the MOS transistors to form amplifiers and other devices, includes metal traces, contacts, intermetal vias, a top isolation layer 216A, and a number of surface vias 216B that are formed through top isolation layer 216A to be electrically connected to the structures that lie on the top surface of interconnect structure 216. In addition, the surface vias 216B are electrically connected to the MOS transistors and other devices via the metal traces, contacts, and inter-metal vias of interconnect structure 216.
As further shown in
Package top 222, in turn, has a top side 222T, a bottom side 222B, and side walls 222S that define an internal cavity 222C. The side walls 222S can optionally include micro-notches or micro-indentations 222G that prevent internal cavity 222C from being completely closed in response to a strong pressure wave.
When used, micro-indentations 222G control the speed with which the pressure within cavity 222C can be equalized with the surrounding pressure after cavity 222C has been closed. For example, micro-indentations 222G can be formed such that the pressure can not be equalized in less than 0.1 seconds (10 Hz). (Although the figures show micro-indentations 222G in only one side wall, any number of micro-indentations 222G in any number of side walls 222S can be used to achieve the desired pressure equalization speed.)
In accordance with the present invention, package top 222 functions either alone, or in combination with connector 220, as the diaphragm of microphone 200. Thus, since package top 222 is substantially larger than the top of semiconductor die 212, package top 222 provides a diaphragm that is substantially more sensitive than the diaphragm of a comparably-sized, prior-art MEMS microphone die.
As shown in the
The eight piezo-responsive leaf springs 230A, 230B, 230C, 230D, 230E, 230F, 230G, and 230H, in turn, are connected to the four conductive strips CM1, CM2, CM3, and CM4 that are connected to the bottom side of package top 222. When connected together, leaf springs 230A and 230B contact opposite ends of conductive strip CM1, while leaf springs 230C and 230D contact opposite ends of conductive strip CM2.
Similarly, leaf springs 230E and 230F contact opposite ends of conductive strip CM3, while leaf springs 230G and 230H contact opposite ends of conductive strip CM4. (The eight surface vias, eight leaf springs, and four conductive strips are exemplary, other numbers can alternately be used.)
In operation, when the pressure changes due to incoming pressure waves, the change in pressure causes package top 222 to vibrate. The vibration causes the piezo-responsive leaf springs 230A, 230B, 230C, 230D, 230E, 230F, 230G, and 230H to change position which, in turn, changes the strain placed on the piezo-responsive leaf springs 230A, 230B, 230C, 230D, 230E, 230F, 230G, and 230H.
The change in strain deforms the band gap structures of the piezo-responsive leaf springs 230A, 230B, 230C, 230D, 230E, 230F, 230G, and 230H. The deformed band gap structures change the mobility and density of the charge carriers which, in turn, changes the resistivity of the piezo-responsive leaf springs 230A, 230B, 230C, 230D, 230E, 230F, 230G, and 230H.
In this example, the changes in resistivity are detected by the Wheatstone Bridge circuit shown in
One of the advantages of the present invention is that microphone 200, which can be used in audio, ultrasonic, infrasonic, and hydrophonic applications, is substantially more sensitive than a comparably-sized MEMS microphone. This is because package top 222, which functions, in part, as the diaphragm, is substantially larger than the diaphragm of a comparably-sized MEMS microphone die. As a result, microphone 200 can detect much smaller variations in pressure (sound waves).
Alternately, rather than the leaf spring being formed from a piezo-responsive material, such as a piezoelectric or piezoresistive material, one or more leaf springs can be connected to a layer of piezo-responsive material to deform the piezo-responsive material, and alter the electrical response of the material.
As shown in the
In operation, as before, when the pressure changes due to incoming pressure waves, the change in pressure causes package top 222 to vibrate. The vibration causes the leaf springs 512A, 512B, 512C, and 512D to vary the location and amount of pressure that is exerted on piezo-responsive material 510 which, in turn, changes the electrical characteristics of piezo-responsive material 510. Thus, by detecting the change in the electrical characteristic (e.g., voltage or resistivity), the changes in pressure can be converted into an electrical signal.
In addition, the present invention applies equally well to capacitive microphones.
As shown in
As shown in
In operation, the first and second conductive layers 240 and 242 function as the plates of a capacitor, while top isolation layer 216A and the air that lies between plates 240 and 242 functions as the dielectric. To begin operation, a voltage is placed on conductive layer 240. This can be accomplished in a number of ways, such as using a switch and conducting leaf spring 230A to place the voltage on conductive layer 240.
When the pressure changes due to incoming sound waves, the change in pressure causes package top 222 to vibrate. The vibration causes the leaf springs 230A, 230B, 230C, 230D, 230E, 230F, 230G, and 230H to change position which changes the gap between the first and second plates 240 and 242 which, in turn, changes the capacitance across the first and second plates 240 and 242. The change in capacitance is detected and used to generate a signal that represents the incoming sound wave.
As shown in
Pressure equalization port 710, in turn, is formed to control the speed with which the pressure within cavity 222C can be equalized with the surrounding pressure. For example, port 710 can be formed such that the pressure can not be equalized in less than 0.1 seconds (10 Hz). This can be achieved by making port 710 small enough, or forming an object within port 710 to restrict air flow.
It should be understood that the above descriptions are examples of the present invention, and that various alternatives of the invention described herein may be employed in practicing the invention. For example, the MEMS microphone of the present invention need not be formed with MOS transistors and an interconnect structure.
Alternately, the MEMS microphone of the present invention can be formed such that connector 220 contacts only top isolation layer 216A, and electrical connections are made between connector 220 and an external device (e.g., electrical traces can be run from the point where the leaf springs contact top isolation layer 216A to a point where an external device can be electrically connected). Thus, it is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.
| Number | Name | Date | Kind |
|---|---|---|---|
| 20050218488 | Matsuo | Oct 2005 | A1 |
