BACKGROUND OF THE INVENTION

[0001] A capacitive microphone typically includes a diaphragm having an electrode attached to a flexible member and a backplate parallel to the flexible member attached to another electrode. The backplate is relatively rigid and typically includes a plurality of holes to allow air to move between the backplate and the flexible member. The backplate and flexible member form the parallel plates of a capacitor. Acoustic pressure on the diaphragm causes it to deflect, thereby changing the capacitance of the capacitor. The change in capacitance is processed by electronic circuitry to provide an electrical signal that corresponds to the change.

[0002] Similar kinds of sensors may be used for measuring pressure in other than acoustic applications.

[0003] Microelectrόmechanical system (MEMS) devices, including miniature microphones and other kinds of pressure sensors, are fabricated with techniques commonly used for making microelectronics such as integrated circuits. Potential uses for MEMS sensors include microphones for hearing aids and mobile telephones, and pressure sensors for vehicles.

BRIEF SUMMARY OF THE INVENTION

[0004] A microelectromechanical system (MEMS) transducer comprises a diaphragm and a back plate. The diaphragm and back plate are spaced apart and are made of semiconductor material using microelectronic fabrication techniques. The transducer also includes a pedestal comprising a cavity. The pedestal has a first end and a second end, and the back plate of the transducer is affixed to the first end, substantially covering the cavity. An extension member is affixed to the second end of the pedestal, contributing to an enlarged back volume for the transducer. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] A more detailed description of the invention will now be provided by way of example only with reference to the accompanying drawing. It should be appreciated, however, that the drawings should not be construed as limiting on the invention in any way. [0006] Figures IA- 1C illustrate top perspective, bottom perspective, and cross-section views of a MEMS transducer.

[0007] Figure ID shows the MEMS transducer of Figures IA- 1C attached to a substrate.

[0008] Figure 2 is a side view of a first wafer and a second wafer to be used to fabricate a MEMS transducer, in accordance with an example embodiment of the invention. [0009] Figure 3 illustrates a cross-sectional side view of the first wafer following patterning and etching of a cavity, in accordance with an example embodiment of the invention.

[0010] Figure 4 illustrates a cross-sectional side view of the first wafer and the second wafer bonded together, in accordance with an example embodiment of the invention.

[0011] Figure 5 illustrates a cross-sectional side view of the bonded wafers following patterning and etching of holes in the second wafer, in accordance with an example embodiment of the invention.

[0012] Figure 6 illustrates the attachment of a glass pedestal to the bonded wafers, in accordance with an example embodiment of the invention.

[0013] Figure 7 illustrates a cross sectional view of the bonded wafers following patterning and etching of a cavity and bond areas, in accordance with an example embodiment of the invention.

[0014] Figure 8 illustrates a cross sectional view of the bonded wafers after a grinding operation, in accordance with an example embodiment of the invention.

[0015] Figure 9 illustrates the deposition of metal onto the bonding areas, in accordance with an example embodiment of the invention.

[0016] Figure 10 illustrates the bonding of an extension member to the assembly of Figure 9, in accordance with an example embodiment of the invention.

[0017] Figure 11 illustrates a step in the fabrication of an extension member, in accordance with an example embodiment of the invention. [0018] Figure 12 illustrates another step in the fabrication of the example extension member.

[0019] Figure 13 illustrates another step in the fabrication of the example extension member. [0020] Figure 14 illustrates another step in the fabrication of the example extension member.

[0021] Figure 15 illustrates another step in the fabrication of the example extension member.

[0022] Figure 16 illustrates another step in the fabrication of the example extension member.

[0023] Figure 17 illustrates another step in the fabrication of the example extension member.

[0024] Figure 18 illustrates another step in the fabrication of the example extension member. [0025] Figure 19 illustrates another step in the fabrication of the example extension member.

[0026] Figure 20 illustrates the example extension member in an upright position.

[0027] Figure 21 illustrates the sensor of Figure 10 mounted in a package, in accordance with an example embodiment of the invention. [0028] Figure 22 illustrates the sensor of Figure 10 mounted in an alternative package, in accordance with an example embodiment of the invention.

[0029] Figure 23 shows an other alternative arrangement of components, in accordance with another example embodiment of the invention.

[0030] Figure 24 shows an extension member in accordance with another example embodiment of the invention.

[0031] Figure 25 shows the extension member of Figure 24 attached to a sensor, in accordance with an example embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0032] The sensor and method of fabricating the sensor will be described with reference to one particular embodiment of the sensor. It should be appreciated, as noted above, that this description is not intended to limit the invention. It should also be noted that the drawings illustrated are not drawn to scale and are given for illustrative purposes only. For example, the vertical scale of several of the figures is exaggerated.

[0033] Figures 1 A-IC illustrate one embodiment of a MEMS transducer 100. Figure IA is a top perspective view, Figure IB is a bottom perspective view, and Figure 1C shows transducer 100 in cross section. Diaphragm 101 and backplate 102 are fabricated using integrated circuit manufacturing techniques, and form the two plates of a parallel plate capacitor, with air gap 103 between them. The layers forming diaphragm 101 and backplate 102 are made of semiconductor material such as silicon. Bond pads 104, formed in the semiconductor material, provide connection points for electrically connecting the two capacitor plates to other components, for example by wire bonding, flip chip mounting, bump bonding, or other means. In transducer 100, the semiconductor layers are bonded to pedestal 105, which may be made of glass. Pedestal 105 includes a hole 106 approximately the diameter of diaphragm 101. Hole 106 provides a back volume for the transducer.

[0034] Transducer 100 may be mounted to a substrate in the fabrication of a packaged product, as is shown in Figure ID. In Figure ID ₅ adhesive 107 is placed between pedestal 105 and substrate 108. The size of the transducer back volume 110 is determined primarily by the depth and size of hole 106, but may also be affected by the thickness of adhesive layer 107. The size of the back volume 110 may affect the performance of a sensor such as transducer 100. For example, if transducer 100 is used as a microphone, a larger back volume may reduce the apparent stiffness of the air inside the back volume, and allow a higher sensitivity or signal-to-noise ratio for the microphone. This may be the result of the behavior of air residing between the diaphragm 101 and backplate 102. In general, as the diaphragm responds to sound, it squeezes and pushes air through the holes in backplate 102. Increased resistance to flow of air through the holes may provide increased damping to the diaphragm, with a corresponding reduction in sensitivity. In some applications, the back volume size may have other effects, for example affecting the frequency response of a pressure sensor. [0035] Figures 2-9 illustrate the fabrication of an example MEMS sensor similar to transducer 100.

[0036] Figure 2 is a side view of a first wafer 201 and a second wafer 202 to be used to fabricate a sensor. The first wafer 201 includes a first layer 203 of highly doped silicon, a second layer 204 of silicon substrate and an intermediate oxide layer 205. The first layer 203 may include p ⁺⁺ doped silicon and the second layer 204 may include an n-type substrate. Alternatively, the first layer 203 may include an n doped silicon and the second layer 204 may include a p-type substrate.

[0037] Typically, the first layer 203 is of the order of 4 microns thick and the oxide layer 205 is of the order of 2 microns thick. The thickness of these layers will generally depend on the characteristics required for the sensor. The second layer 204 may be larger than the first layer 203 and the oxide layer 205. For example, the second layer 204 may be in the order of 400 to 600 microns thick.

[0038] In this example embodiment, the second wafer 202 is formed from silicon. Other semiconductor materials may be used. The second wafer 202 is heavily doped and may be either p-type or n-type silicon. In other embodiments, different silicon surfaces or structures may be used.

[0039] It will be appreciated that the first wafer 201 includes a first major surface 206 formed from the heavily doped silicon of the first layer 203 and a second major surface 207. Likewise, the second wafer 202 includes a first major surface 208 and a second major surface 209. Oxide layers 210 have been formed on major surfaces 206, 207, 208, and 209.

[0040] Oxide layers 210 are may be formed on the major surfaces 206-209 of the wafers 201 and 202 through thermal growth or a deposition process. Forming oxide layers 210 on both major surfaces 206-207 and 208-209 wafers 201 and 202 respectively reduces the risk of distorting the wafers that may occur if oxide were only formed on one major surface on each wafer. That being said, in an alternative embodiment to that illustrated in Figure 2, an oxide layer 210 may be only formed on the first major surface 206 of the first wafer 201 and the first major surface 208 of the second wafer 202. The thickness of the oxide layers 210 may be less than the thickness of the first and second wafers 201 and 202. [0041] In fabricating the sensor, the first wafer 201 and the second wafer 202 are initially processed separately before being bonded together and further processed. It will also be appreciated that while the figures and the following discussion describe the fabrication of a single sensor, in actual practice many sensors may be produced by the processing of wafers containing multiple sensor substrates, and the sensors separated after fabrication in a process known as singulation or dicing. [0042] Figure 3 illustrates the first wafer 201 in which a cavity 301 has been patterned and etched. In particular, the cavity 301 has been patterned and etched through the oxide layer 210 on the first major surface 206 of the first layer 203 of the first wafer 201, and into the first layer 203 of the first wafer 201. In this step, a portion of the heavily doped silicon forming the first layer 203 is etched away to produce a thin section 302 of the heavily doped silicon of the first layer 203.

[0043] The thickness of the thin section 302 will determine the properties of the sensor eventually fabricated as this thin section 302 of highly doped silicon will form the flexible member of the diaphragm of the sensor, as illustrated in the following drawings.

[0044] A wet or dry silicon etch may be employed in this step. In one embodiment a reactive ion etch (RIE) is used to form the cavity 301. Generally, the etch is a time etch. Therefore, the final thickness of the thin section 302, and consequently the flexible member of the diaphragm, is dependent on the etching time. Further, the desired shape of the cavity 301 will generally be dictated by the desired properties of the sensor.

[0045] Figure 4 shows the first and second wafers 201 and 202 bonded together. The major surfaces bonded together, via respective oxide layers 210, are the first major surface 206 of the first wafer 201 and the first major surface 208 of the second wafer 202. In one embodiment the wafers 201 and 202 are bonded together through their respective oxide layers 210 using fusion bonding.

[0046] In bonding the wafers 201 and 202 together, an air gap 401 is formed between the wafers 201 and 202 corresponding with the cavity 301 formed in a previous etching step.

[0047] As is shown in Figure 5, a plurality of holes 501 are then patterned and etched into the highly doped silicon of the second wafer 202 in a region associated with the air gap 401 and, therefore, the thin section 302. When the holes 501 are formed, a global etch is conducted such that the holes 501 extend through to the air gap 401. In effect, holes 501 are formed that extend through the second wafer 202 to the air gap 401. Holes 501 may be formed by deep reactive ion etching (DRIE). At this or another convenient time, a vent hole 502 may be formed through wafer 202 and first layer 203 of wafer 201.

[0048] Figure 6 illustrates the attachment of a pedestal 601 to the bonded wafers. Pedestal 601 is preferably made of glass, for example Pyrex, and is preferably attached to wafer 202 by anodic bonding or another form of wafer bonding. Other materials and attachment methods are also possible. For example, pedestal 601 may be made of silicon or another semiconductor material. Pedestal 601 may be attached to the bonded wafers by fusion bonding, with an adhesive, or by other means. Pedestal 601 has a first end and a second end. Back plate 604 of the transducer is affixed to the first end of pedestal 601. In Figure 6, this is the bottom of pedestal 601, although one of skill in the art may recognize that a finished transducer may be used in any orientation, and the first end of pedestal 601 may not always be the bottom end. Pedestal 601 may be previously prepared to include a cavity 602 that generally corresponds to the lateral dimensions of cavity 401. Alternatively, pedestal 601 may be formed by bonding a solid wafer to silicon wafer 202, and then etching cavity 602 from the glass wafer. For example, a masking layer of chrome and gold may be deposited onto the glass wafer and the cavity 602 may be formed by wet or dry etching, for example using HF. Preferably, the height of pedestal 601 is many times the combined remaining thickness wafers 201 and 202. For example, pedestal 601 may be up to several millimeters in height. [0049] As is shown in Figure 7, another DRIE etch process is performed to remove parts of first wafer 201, forming cavity 701 and bonding pad locations 702 and 703. As is evident in Figure 7, bonding pad location 702 is an exposed surface of layer 203 of first wafer 201, from which diaphragm 705 is formed. Bonding pad location 703 is an exposed surface of second wafer 202, from which forms the backplate 604 of the MEMS microphone. After cavity 701 is formed, vent hole 502 passes completely through wafer 202 and layer 203 of wafer 201 to cavity 701. Vent hole 502 may be formed adjacent back plate 604 and diaphragm 705 as shown, or may include a passage through diaphragm 705.

[0050] The remaining portion of layer 204 of first wafer 201 is then thinned in a grinding operation. The result of this operation is shown in Figure 8. Note that in Figure 8 and the following figures, the assembly is inverted as compared with the previous figures.

[0051] Figure 9 illustrates the deposition of metal onto bonding areas 702 and 703. For example, aluminum pads 901 and 902 may be formed on bonding areas 702 and 703 respectively by vapor deposition. Prior to vapor deposition, a shadow mask 903 may be placed over the wafers to ensure that only bonding areas 702 and 703 receive deposited metal. The shadow mask is removed after the metal deposition is complete. Pads 901 and 902 allow electrical connections to be made to the two plates of the capacitor formed by diaphragm 705 and backplate 604. For example, pads 901 and 902 may be independently connected by wire bonds to a substrate on which the sensor is eventually mounted.

[0052] In some cases, it is desirable to make the lateral dimensions of the sensor as small as possible, so as to produce as many sensors as possible from a semiconductor wafer. However, as the size of the sensor is reduced, the size of the back volume is also reduced, and the performance of the sensor may be compromised. In example embodiments, a sensor as shown in Figure 9 may have a square footprint of 1.2mm by 1.2mm, or a square footprint of 1.7mm by 1.7 mm, or another shape or size. For example, the sensor need not be square, but could be rectangular, circular, or have another foot print shape. Similarly, the sensor may have one or more lateral dimensions less than 1.2mm, larger than 1.7mm, up to a dimension of several millimeters, or any size between.

[0053] Figure 10 illustrates an apparatus and method for extending or enlarging the back volume of a sensor, in accordance with an example embodiment of the invention. In this example, an extension member 1001 is affixed to the bottom of pedestal 601. Extension member 1001 may be made, for example of a semiconductor material such as silicon, germanium, or another semiconductor material. Alternatively, extension member 1001 may be made of glass, or any other suitable material. Preferably, extension member 1001 is made of a material that can withstand the temperatures used in packaging a semiconductor product, which may reach 260 ⁰ C or more. In the example of Figure 10, extension member 1001 may be affixed to pedestal 601 or another part of the sensor by an adhesive, for example room temperature vulcanizing (RTV) silicone adhesive, or another compliant or substantially rigid adhesive. Alternatively, extension member 1001 may be attached to the sensor or pedestal by fusion bonding, by anodic bonding, by eutectic bonding, or by another means. The result is completed transducer 1000.

[0054] Figures 11-20 illustrate the fabrication of an extension member, in accordance with an example embodiment of the invention. Preferably, the extension member is made from a material that is not electrically conductive, or from a semiconductor material using semiconductor circuit fabrication processes. Each of Figures 11-14 shows a cross-section view of a wafer segment at various stages in the fabrication of the extension member, and also shows a perspective view of the wafer segment. Each of Figures 15-20 shows a cross- section view, and also both upper and lower perspective views of the wafer at the corresponding fabrication stage. [0055] In Figure 11, a wafer 1100 includes oxide layers 1101 on its upper and lower surfaces. The oxide layer may be, for example, on the order of 0.5 to 1.0 micrometer in thickness, although other thicknesses are possible. In one example embodiment, wafer 1100 may be, for example about 250 to 600 micrometers in thickness. Other thicknesses less than 250 microns or greater than 600 microns are also possible. In Figure 12, a resist coat 1201 has been applied to the upper surface of wafer 1100.

[0056] In Figure 13, the resist coat 1201 has been patterned and baked, outlining a rectangular area 1301 on the upper surface of wafer 1100. In Figure 14, a resist coat 1401 has been applied to the lower surface of wafer 1100. In Figure 15, resist coat 1401 has been patterned and baked, outlining a generally circular area 1501 on the lower surface of wafer 1100. While areas 1301 and 1501 are shown as rectangular and circular respectively, either or both may be rectangular, circular, oval, polygonal, or have any other suitable shape.

[0057] In Figure 16, both sides of wafer 1100 have been etched to remove the portions of oxide layers 1101 not protected by resist coat layers 1201 and 1401. In Figure 17, a cavity 1701 has been created in the upper surface of wafer 1100, outlined by resist coat 1201. Cavity 1701 may be formed, for example, by deep reactive ion etching (DRIE).

[0058] In Figure 18, the lower surface of wafer 1100 has been similarly etched to form hole 1801, outlined by resist coat 1401. In Figure 19, the resist and oxide layers have been stripped off, leaving completed extension member 1001. In Figure 20, extension member 1001 has been inverted into its usual orientation. Of course, extension member 1001 and any sensor it is comprised in may be used in any orientation.

[0059] While extension member 1001 is shown as being generally square with a rectangular chamber and a generally circular opening for interfacing with the transducer back hole, one of skill in the art will recognize that many other shapes are possible. For example, the chamber may have a cross-sectional shape that is rectangular, polygonal, round, oval, elliptical, bounded by some other curve, or of another shape. Similarly, the opening for interfacing with the sensor chip may also be square, rectangular, polygonal, oval, elliptical, or have some other shape. Neither the opening nor the chamber need have a constant cross section. For example, the chamber may have angled walls that include draft.

[0060] In Figure 20, dimension "a" indicates the size of the opening that interfaces with the transducer back hole. The size of the opening may be chosen to achieve particular sensor performance parameters. Rather than one opening, a plurality of smaller openings may be provided, and the plurality of smaller openings may be fabricated by steps essentially the same as those described for fabricating extension member 1001.

[0061] Figure 21 shows the transducer 1000 of the previous figures mounted in an example package 2100. Package 2100 comprises a substrate 2101, which may be for example a printed circuit board, a ceramic substrate, a semiconductor substrate, a flexible circuit, or another kind of substrate. Transducer 1000 is may be attached to substrate 2101 using an adhesive layer 2102. The adhesive used may be, for example, a room temperature vulcanization (RTV) silicone adhesive available from the General Electric Company of Fairfield, Connecticut, USA. Many other attachment methods are possible, including any suitable method used for attaching semiconductor chips to substrates. In the example arrangement of Figure 21, back volume 2108 is enlarged, as compared with a system in which pedestal 601 is affixed to substrate 2101 directly without the benefit of extension member 1001. This extended back volume 2108 may provide enhanced performance of transducer 1000, and may allow the optimization of various performance parameters by selection of the dimensions of extension member 1001, in conjunction with the design of other parts of the transducer. In the example package of Figure 2I ₃ transducer 1000 is electrically connected to substrate 2101 by wire bonds 2103 and 2104. One or more additional electronic components 2105 may also be attached and electrically connected to substrate 2101. A cover 2106 may be placed over transducer 1000 and attached to substrate 2101. In this example, cover 2106 comprises an aperture 2107 through which sound may enter and reach transducer 1000. Cover 2106 may be made, for example, of a metal such as stamped sheet steel or copper, and may be plated. Cover 2106 may also be made of a polymer, and may be covered with a conductive layer. Cover 2106 provides protection for transducer 1000, and may be electrically connected to traces on substrate 2101, for example to provide protection from electromagnetic interferences or other interference signals. Many other package configurations are possible. [0062] In Figure 21 , extension member 1001 has the same footprint as pedestal 601 and the rest of transducer 1000. In this embodiment, wafers comprising an array of sensor diaphragms and backplates may be fabricated and then laminated with an array of pedestals and another wafer having an array of extension members formed in it. The individual sensors may then be separated by dicing or singulation. Other assembly methods may be used.

[0063] Figure 22 shows transducer 1000 mounted in an alternative package 2200. Package 2200 is a "reverse mount" package having substrate 2201 that includes an aperture 2202 through which sound may enter and reach transducer 1000. In this example, cover 2203 does not include an aperture. Cover 2203 may be made of materials like those of which cover 1106 may be made, and may be electrically connected to circuitry on substrate 2201. In some embodiments, a package may be provided with apertures in both a cover and a substrate.

[0064] While Figures 21 and 22 show transducer 1000, including an extension member 2204, mounted to a substrate, other applications are also possible. For example, a transducer such as example transducer 1000 may be mounted in a pre-molded cavity package, attached to a silicon submount, or mounted on top of an integrated circuit. The attachment may be die-to-die. And while Figures 21 and 22 show electrical connections made from transducer 1000 by wire bonds such as bonds 2103 and 2104, other attachment methods may be used. For example, a transducer according to an embodiment of the invention may be flip-chip mounted or bump bonded to another component, and such a mounting may make electrical connections, may affix the transducer mechanically to the other component, or both. In some embodiments, metallic bumps made of gold or another metal may be used for bump-bonding the transducer to another component.

[0065] In Figure 22, extension member 2204 does not have the same footprint as pedestal 601 and the rest of transducer 1000. In this case, extension member 2204 may be assembled to transducer 1000 during packaging of the transducer, and not before transducer 1000 is singulated from an array of similar devices.

[0066] Figure 23 shows another alternative arrangement of components, in accordance with another example embodiment of the invention. In Figure 23, an encapsulation layer 2301 has been affixed to transducer 1000 before transducer 1000 is mounted onto substrate 2302. Encapsulation layer 2301 may be made, for example, of silicon or another semiconductor material, or another suitable material. [0067] Figure 24 shows another extension member 2401, in accordance with another example embodiment of the invention. Extension member 2401 is fabricated in a way similar to extension member 1001, except that no opening is provided through the member 2401. Extension member 2401 may be used as shown in the example embodiment of Figure 25. In Figure 25, extension member 2401 is bonded to one side of pedestal 601, using a bonding material 2501. For example, bonding material 2501 may be an RTV adhesive or another kind of pliable adhesive, a substantially rigid adhesive 2506, or any other suitable bonding material. Pedestal 601 and extension member 2401 are bonded to a substrate 2502 using any suitable kind of die attach material or other adhesive, but leaving a center channel 2503 unbonded so that back chamber 2504 includes both the back hole of the transducer and the chamber provided by extension member 2401.

[0068] A transducer according to an embodiment of the invention may be used in low- pressure applications, for example as a microphone that responds to pressure fluctuations of an acoustic signal, or may be used in higher pressure applications. For example, a transducer according to a embodiment of the invention may be used to measure the pressure of various gasses or liquids in a motor vehicle engine. Various fabrication parameters may be adjusted depending on the intended application of a sensor. For example, a microphone transducer may have a thin, flexible diaphragm, while a sensor designed for higher pressure applications may be fabricated with a comparatively thicker, stiffer diaphragm. For the purposes of this disclosure, an acoustic signal may be but is not limited to a signal audible to humans. An acoustic signal may be a signal having an amplitude below the threshold of human hearing, or an amplitude above a level that may cause damage to a human ear. An acoustic signal may include or be composed entirely of one or more frequency components outside the range of human delectability. For example, an acoustic signal may have a frequency or frequencies below 20 Hertz, or may have a frequency or frequencies above 20 kHz. A signal in this upper frequency range is sometimes referred to as a ultrasonic signal.

[0069] One of skill in the art will also recognize the order of operations may be varied from the order described above. For example, hole 1801 may be deep etched before cavity 1701.

[0070] While the above examples are shown in the context of a capacitive sensor, an extension member in accordance with an embodiment of the invention may be used with other kinds of sensors as well. For example, a piezoresistive or piezoelectric sensor may comprise a diaphragm and use a back volume, but lack the back plate shown in the Figures. Even so, the shape and size of the back volume may affect the performance of a piezoresistive or piezoelectric sensor, and an extension member similar to member 1001 may be used to enhance the performance of such a sensor.

[0071] The sensor and arrangements embodying the invention may provide a number of advantages. The sensor allows for an arrangement having a large and well controlled back volume. The shape and size of the back volume may be adjusted by the selection of the thicknesses of pedestal 601 and wafer 1100 and the shape and size of cavity 1701 and hole 1801. Especially with regard to acoustic applications, back volume is important to the acoustic performance of a device as it affects sensitivity and frequency response. In some embodiments, a microphone with an extension member may have a sensitivity approximately 2 dB higher than the sensitivity of a similar microphone not having an extension member.

[0072] The foregoing describes the invention including preferred forms thereof. Alterations and modifications as will be obvious to those of skill in the art are intended to be incorporated in the scope hereof as defined by the accompanying claims.