PACKAGE FOR MICROMACHINED SILICON CONDENSER MICROPHONE

Abstract: A microphone is disclosed having a case (512, 514), a motor die (518), and an integrated circuit die (526). The case defines a chamber. The motor die (518) is operably attached to the case within the chamber and partitions the chamber into a first compartment and a second compartment wherein the volume of the first compartment is larger than the volume of the second compartment. Furthermore, the integrated circuit die (526) is housed within the first compartment and operably connected to the motor die (518).

Description: PACKAGE FOR MICROMACHINED SILICON CONDENSER MICROPHONE DESCRIPTION Technical Field The present invention relates to miniature condenser microphones and, in particular, to a microphone packaging configuration that provides a high volume compliance and electrostatic shielding of the microphone's internal amplifier. Background of the Invention Miniature condenser microphones require careful design and material choices to achieve their performance specifications in a device which fits into the limited space available in their typical applications (e.g. hearing instruments). Typical performance specifications are: 1) high sensitivity; 2) low input-referred noise; 3) controlled frequency response; 4) immunity to external electromagnetic interference; and 5) stability of characteristics over the range of ambient humidity values encountered in use. Two major factors in the achievement of these performance goals are: 1) the volume compliance of the back chamber of the package (i.e., the higher the better); and 2) electrostatic shielding of the microphone's internal amplifier. Relationship Between Noise Performance and Volume of the Back Chamber Analysis of the internal noise of a condenser microphone indicates that the major components of the noise are: 1) Random fluctuations in displacement of the diaphragm / back chamber series combination, whose mean squared magnitude may be expressed as, < Pdisp where T is absolute temperature, k is Boltzmann's constant, p dιsp is an input acoustic pressure which would produce the same output as the displacement fluctuation and C dia and C^ are the volume compliances of the diaphragm and back chamber, respectively. 2) Random fluctuations in air flow through the barometric relief path (diaphragm pierce hole), whose mean squared magnitude may be expressed as, 2 kT < Pleak >= C "Ndia + c b, where p leak is an input acoustic pressure which would produce the same output as the air flow fluctuation, and 3 ) Random fluctuations in the electrical output of the microphone ' s internal amplifier, whose mean squared magnitude may be expressed as, < e .2 > < Pa ammpn >= 2 where p amp is an input acoustic pressure which would produce the same output as the amplifier fluctuation, <e 2 > is the mean squared fluctuation in the output voltage of the amplifier, s is the pressure sensitivity of the microphone (volts per unit pressure) at some frequency agreed upon for purposes of noise quotation. Random fluctuations due to these noise components occur over a range of frequencies. The formulas given are quantitatively exact for the case where no relative numerical weighting is given to fluctuations at different frequencies. For cases where greater numerical weighting is applied to fluctuations at some frequencies (which is quite common in noise quotations), numerical correction factors must be applied to these formulas. These correction factors depend solely on the weighting function being applied and upon the frequency response of the microphone. In the sense indicated directly by the formulas as given (no weighting according to frequency), any increase in the back chamber compliance C bc is beneficial in that it decreases the magnitude of input-referred noise and increases the sensitivity s. In an alternate sense where noise is quoted using relative weighting of a certain frequency range, the formulas still indicate that an increase in back chamber compliance is beneficial when comparing any two microphones which have both been constructed so as to meet a specified frequency response, since the same numerical correction factor would be applied to the formulas for both microphones. The compliance of the back chamber is proportional to its volume, being given by C - Σ*. Y "amb where γ \s a characteristic constant equal to 1.4 for air, V bc is the volume of the back chamber, and P amb is the ambient pressure. The only practical way of increasing the back chamber compliance is to increase its volume. Electrostatic Shielding The first amplifier of a condenser microphone is designed to respond to the flow of very small amounts of electrical charge to and from the motor. If the amplifier input is not sufficiently shielded, the motion of charged or biased objects in the surrounding environment can cause charge to flow into or out of the amplifier input, generating electrical interference with the acoustic signal. The packaging arrangement of a condenser microphone must comprise a conducting shield around the amplifier which intercepts the electrostatic fields from such nearby objects, but must also allow the acoustic pressure to reach the motor. Deficiencies in the Prior Art Prior miniature condenser microphones fall short of desired performance goals in at least two respects: 1) Humidity sensitivity of the compliance of polymer-diaphragm microphones; and 2) inadequate packaging configurations for miniature condenser microphones using non-polymeric diaphragms. Regarding humidity sensitivity of the compliance of polymer-diaphragm microphones, tensioned polymer films are widely used to build diaphragms in miniature condenser microphones. However, polymer films tend to absorb water from the vapor phase and swell slightly as they do so. Tensioned polymer diaphragms thus become looser or tighter with increase or decrease of ambient humidity, respectively, causing the sensitivity and input-referred noise to vary with humidity as well. Moreover, regarding the inadequate packaging configurations for miniature condenser microphones using non-polymeric diaphragms, the prior art in which miniature condenser microphones have been constructed from non-polymeric diaphragm films (e.g. those available using silicon wafer processing technology) has, either intentionally or unintentionally, embodied improvement over the polymer diaphragm microphones with respect to humidity sensitivity. However, the packaging configurations used in prior art have been such that they do not assign a large fraction of the package interior volume to the back chamber, which detracts from reaching the performance goals at a given device size for reasons presented below. Summary of the Invention The present invention provides a packaging configuration for a diaphragm/backplate component (referred to henceforth as a motor die) created by silicon wafer processing. Such diaphragms provide excellent stability with respect to humidity changes. The packaging configuration creates a back chamber of greater volume (and hence volume compliance) for a given microphone size than has been provided previously in the packaging of such motor die, and provides a more complete electrostatic shield for the internal amplifier than realized previously. Brief Description of the Drawings In the accompanying drawings forming part of the specification, and in which like numerals are employed to designate like parts throughout the same, FIGURE 1 is a cross-sectional side view of a microphone case having a bottom cup with a motor die cemented directly thereon; FIGURE 2 is a cross-sectional side view of a microphone case having a bottom cup with a motor die attached by a support ring cemented to both the cup and the die; FIGURE 3 is a cross-sectional side view of an embodiment of a microphone case having a motor die connected to an integrated circuit die by a multi-conductor flexible lead structure; FIGURE 4 is a cross-sectional side view of another embodiment of a microphone case having a motor die connected to an integrated circuit die by a multi-conductor flexible lead structure; FIGURE 5 is a cross-sectional side view of yet another embodiment of a microphone case having a motor die connected to an integrated circuit die by a multi- conductor flexible lead structure; FIGURE 6 is a cross-sectional side view of a microphone case having a motor die connected to an integrated circuit by using a wire-bonding method; and FIGURE 7 is a cross-sectional side view of a microphone case having a motor die attached to an integrated circuit by using a direct die-to-die connection. Detailed Description While this invention is susceptible of embodiments in many different forms, there is shown in the drawings and will herein be described in detail a preferred embodiment of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiments illustrated. Formation of Back Chamber and Location of Motor Die Referring to FIGURE 1 , the outer case of a microphone 10 is formed from two cup-shaped pieces (known as the bottom cup 12 and the top cup 14). When these two pieces are joined, they largely define the exterior size and available internal volume of the microphone. The path for entry of acoustic input pressure passes through an inlet slot 16 cut in the bottom cup at or near the bottom. As shown in FIGURE 1, a motor die 18 spans the lateral cross section of the bottom cup 12. In this arrangement the diaphragm 19 of the motor die 18 partitions the available internal volume of the case into regions which serve as the back chamber (which in FIGURE 1 are regions above the diaphragm of the motor die) and regions which comprise the aforementioned acoustic path to the exterior (regions below the diaphragm of the motor die). In the embodiment shown in FIGURE 1 the motor die is placed in the bottom of the cup. This maximizes the volume of the back chamber which is created within the available space. As previously described above and as will be appreciated by those skilled in the art, this is beneficial to the performance of the microphone with respect to sensitivity and input-referred noise. Preferably, the motor die forms an airtight seal between the back chamber and the front acoustic path. Assembly methods for realizing this are: 1 ) cement 20 the motor die to the inside walls of the bottom cup 12; and 2) as shown in FIGURE 2, cement the motor die 118 to a support ring 122 which then is cemented to the bottom cup 112. Location of Integrated Circuit Die and Means of Electrical Connection Integrated circuits providing such functions as biasing of the microphone backplate and amplification of the motor output are located on one or more die separate from the silicon motor die, in the back volume region. Electrically conducting material is used to construct the top and bottom cups, so that this arrangement establishes electrical shielding of the amplifier against electrostatic interference. Within the back volume region, the required electrical connections are established by any of several alternative means between the motor die and the integrated circuits. External electrical access to some device terminals such as supply voltage, output signal and circuit ground is established by means of electrically conducting elements which pass through the wall of the case and in turn are connected to the required terminals on the integrated circuits. Various embodiments of this electrical connection arrangement can be categorized according to the method used for electrical connections and according to the orientation and mounting methods. In three embodiments, shown in FIGURES 3, 4 and 5, the connection between the silicon motor die and one or more integrated circuit die defining an amplifier, for example, is preferably accomplished using a multi-conductor flexible lead structure. In all cases an additional section or extension of the flexible lead structure to be used for external access is arranged to pass through the j oint between top and bottom cups, resting on an intermediate plate as it does so. In the embodiment depicted in FIGURE 3, the flexible lead 224 is bent through a large angle in such a way that one side of the flexible lead faces downward toward the silicon motor die 218 at the connection to the motor die and upward toward the integrated circuit die 226 (active side facing downward) at the connection to the integrated circuit die. The embodiment of FIGURE 4 also uses only one side of the flexible lead 324 for connection but with more gentle bending, in this case by orienting the integrated circuit die 326 with its active surface facing upwards. In the embodiment of FIGURE 5 both sides of the flexible lead 424 are used for electrical connection. No large bends in the flexible lead are required. The three embodiments of FIGURES 3, 4 and 5 also indicate three different methods for mechanical attachment and support of the integrated circuit die. In the embodiment of FIGURE 3 the integrated circuit die 226 is attached to the nearby inside surface of the case. In FIGURE 4 the integrated circuit 326 derives its support from the flexible lead 324. By locating the integrated circuit connection close to the point where the flexible lead 324 is attached to the intermediate plate 330, a mechanically stiff mounting of the integrated circuit die 326 can be realized. Moreover, the integrated circuit die 326 can be nested in a notch in the intermediate plate 330. In FIGURE 5 the integrated circuit die 426 is attached to the intermediate plate 430. FIGURE 6 shows an embodiment in which the connection between the silicon motor die 518 and one or more integrated circuit die 526 is accomplished using wire-bonding methods. The conducting elements for external access are embodied in electrically conductive paths 534 which are disposed on the surface of a ceramic substrate 536 which passes through the joint between top cup 514 and bottom cup 512, resting on an intermediate plate 530 as it does so. In an embodiment using wire-bonding methods for electrical connection, all points to be connected preferably are rigidly supported and in their final position before the connection is made. In FIGURE 6 the integrated circuit die 526 is mechanically attached to the aforementioned ceramic substrate 536 and the bonded wires 540 are arranged such that one end of each wire is bonded to locations on the ceramic substrate 536, specifically either the aforementioned electrically conductive paths 534 for external access or additional electrically conductive areas on the ceramic substrate. Bonded wire connections are formed between the integrated circuit 526 and the appropriate conducting paths 534 or areas on the ceramic substrate 536, and additional bonded wire connections are formed between the silicon motor die 518 and the appropriate conducting paths 534 or areas on the ceramic substrate 536. Means for facilitating external electrical connections to the conducting paths 534 on the ceramic substrate 536 can be accomplished by solder bumps 542 or the like. Connection of the integrated circuit die to the silicon motor die can be also be realized by direct die-to-die connection using thermocompression bonding or solder reflow methods. The resulting embodiment is shown in FIGURE 7. The integrated circuit 626 is oriented with its active surface facing downward and placed at the appropriate location on the silicon motor die 618. The locations of connection terminals on the integrated circuit die 626 are lined up with locations of connection terminals on the silicon motor die 618 so that the thermocompression bonding or solder reflow process causes the appropriate electrical connection to be made. The conducting elements for external access are once again embodied in electrically conductive paths 634 which are disposed on the surface of a ceramic substrate 636 which passes through the joint between top cup 614 and bottom cup 612, resting on an intermediate plate 630 as before. Bonded wires 640 from the conductive paths 634 on the ceramic substrate 636 to conductive areas on the silicon motor die 618 are used to establish the connections for external access. While the specific embodiments have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention and the scope of protection is only limited by the scope of the accompanying Claims.

Claims: CLAIMS I CLAIM: 1. A microphone comprising: a case defining a chamber; a motor die operably attached to the case within the chamber and partitioning the chamber into a first compartment having a volume and a second compartment having a volume wherein the volume of the first compartment is larger than the volume of the second compartment; and an integrated circuit die within the first compartment and operably connected to the motor die. 2. The microphone of claim 1 wherein the motor die is attached to the case by cement. 3. The microphone of claim 1 wherein the motor die is cemented to a support ring attached to the case. 4. The microphone of claim 1 wherein the case is constructed of a material for providing electrostatic shielding of the integrated circuit die. 5. The microphone of claim 4 wherein the material includes metal. 6. The microphone of claim 1 wherein the motor die is operably connected to the integrated circuit die by a multi-conductor flexible lead structure. 7. The microphone of claim 6 wherein the integrated circuit die is attached to the case. 8. The microphone of claim 7 wherein the case includes an intermediate plate within the first compartment and the integrated circuit die operably attached to the intermediate plate. 9. The microphone of claim 1 wherein the motor die is attached to the integrated circuit die by a direct die-to-die connection. 10. The microphone of claim 1 wherein the case includes an intermediate plate within the first compartment and a substrate attached to the plate and extending outside of the first compartment. 11. The microphone of claim 10 wherein the substrate includes a plurality of conductive paths operably connected to integrated circuit die and the motor die.

Inventors: LOEPPERT PETER V, SCHAFER DAVID E, LOEPPERT, PETER, V., SCHAFER, DAVID, E.
Applicants: KNOWLES ELECTRONICS LLC, LOEPPERT PETER V, SCHAFER DAVID E, KNOWLES ELECTRONICS, LLC, LOEPPERT, PETER, V., SCHAFER, DAVID, E.
Application year: 2000
Main classification: H04R25
IPC codes: H04R19, H04R25