FABRICATING AN INTEGRATED LOUDSPEAKER PISTON AND SUSPENSION

Abstract

A diaphragm and suspension for an electroacoustic transducer are formed by depositing a layer of compliant material on a first surface of a solid substrate and removing material from a second surface of the solid substrate. The removal leaves a block of substrate material suspended within an inner perimeter of an outer support ring of the substrate material by the compliant material, the block providing the diaphragm.

Claims

1. A method of forming an electroacoustic transducer having a diaphragm and suspension, the method comprising: depositing a layer of compliant material on a first surface of a solid substrate; and removing material from a second surface of the solid substrate, the removal leaving a block of substrate material suspended within an inner perimeter of an outer support ring of the substrate material by the compliant material, the block providing the diaphragm.

2. The method of claim 1, wherein the compliant material has an elastic strain limit of at least 50 percent.

3. The method of claim 1, wherein the compliant material has an elastic strain limit of at least 150 percent.

4. The method of claim 1, further comprising curing the compliant material.

5. The method of claim 1, wherein the compliant material comprises liquid silicone rubber (LSR).

6. The method of claim 1, wherein the step of removing material from the substrate comprises removing material from a portion of the substrate in some areas to form the block, and removing all material of the substrate in other areas to form a gap between the inner perimeter of the outer support ring and the suspended block.

7. The method of claim 6, wherein the step of removing material from the substrate comprises deep reactive ion etching (DRIE), material being removed from a portion of the substrate by a single DRIE etch, and material being removed from the entire substrate by multiple DRIE etches.

8. The method of claim 6, wherein the substrate comprises a silicon-on-insulator (SOI) wafer, and the step of depositing the layer of compliant material is performed after the step of removing material from a portion of the substrate to form the block, but before the step of removing all material from other areas to form the gap.

9. The method of claim 8, wherein the step of removing material from the substrate comprises deep reactive ion etching (DRIE), material being removed from a portion of the substrate by a single DRIE etch, and material being removed from the entire substrate by multiple DRIE etches through the main Si wafer, an etch of the insulator layer, and an etch of the top Si layer.

10. The method of claim 6, wherein the substrate comprises a silicon wafer, and the step of depositing the layer of compliant material is performed before the steps of removing material from the substrate.

11. The method of claim 1, wherein removing material from the substrate leaves the block having a side wall retaining most of the thickness of the substrate around an outer perimeter of the block facing the inner perimeter of the outer support ring, and a thinner portion of the substrate remaining bounded by the side wall leaving a void in the interior of the block.

12. The method of claim 11, further comprising attaching a bobbin to the block, the bobbin located adjacent to an inter perimeter of the side wall.

13. The method of claim 12, wherein the bobbin is attached to the block by adhesive, the adhesive being contained by the side wall such that it does not contact the suspension.

14. The method of claim 13, wherein the side wall of the block acts as an alignment guide for the attachment of the bobbin.

15. The method of claim 1, wherein removing material from the substrate leaves the outer support ring having a wall retaining most of the thickness of the substrate and forming the inner perimeter of the outer support ring, and a thinner portion of the substrate at the top of the wall forming a lip around an outer perimeter of the outer support ring.

16. The method of claim 15, further comprising attaching a ferromagnetic housing to the outer support ring, the housing located adjacent to an outer perimeter of the outer support ring wall and the lip.

17. The method of claim 16, wherein the housing is attached to the outer support ring by adhesive, the adhesive being prevented by the side wall from contacting the suspension between the block and the outer support ring.

18. The method of claim 16, wherein the outer support ring acts as an alignment guide for the attachment of the housing.

19. The method of claim 16, further comprising cutting through the compliant material at the location of the outer perimeter of the outer support ring, separating the block, the outer support ring, and the compliant layer suspending the block within the outer support ring from the substrate.

20. The method of claim 19, wherein an inner perimeter of the silicon substrate surrounding the outer support ring aligns a cutting tool for cutting through the compliant material.

21. The method of claim 19, wherein the step of cutting is performed after the step of attaching the ferromagnetic housing to the outer support ring.

22. The method of claim 21, wherein the ferromagnetic housing aligns a cutting tool for cutting through the compliant material.

23. The method of claim 1, wherein the step of removing material forms a plurality of diaphragms and corresponding outer support rings over the area of the substrate.

24. The method of claim 23, further comprising attaching a plurality of bobbins to the diaphragms and a plurality of housings to the outer support rings, simultaneously, while the diaphragms and outer support rings remain attached to the substrate and each other by the layer of compliant material.

25. The method of claim 24, further comprising cutting through the compliant material at the locations of the plurality of outer support rings, the plurality of housings serving as alignment guides for a cutting tool.

26. A diaphragm and suspension assembly for an electroacoustic transducer, the assembly comprising: a piston comprising a disk of silicon having a flat surface and serving as the diaphragm; a support ring of silicon surrounding the piston and separated from the piston by a gap; a layer of compliant material adhered to a top surface of the support ring and to the flat surface of the piston, suspending the piston in the gap.

27. The piston and suspension assembly of claim 26, wherein the piston further comprises a void within the disk of silicon, bounded by a perimeter wall of the disk and the top surface of the disk.

28. The piston and suspension assembly of claim 26, wherein the support ring comprises an inner perimeter wall of silicon facing the gap, and an outer lip having less height than the inner perimeter wall.

29. The piston and suspension assembly of claim 26, wherein the compliant material has an elastic strain limit of at least 50 percent.

30. The piston and suspension assembly of claim 26, wherein the compliant material has an elastic strain limit of at least 150 percent.

31. The piston and suspension assembly of claim 26, wherein the compliant material has a Young's modulus and a thickness that together result in the compliant material surrounding the piston in the gap having a mechanical stiffness in the range of 5-100 N/m.

32. The piston and suspension assembly of claim 26, wherein the compliant material comprises liquid silicone rubber (LSR).

33. The piston and suspension assembly of claim 26, wherein the support ring has an outer diameter of around 4 mm.

34. The piston and suspension assembly of claim 26, wherein the piston has a thickness of between 10 and 100 m.

35. The piston and suspension assembly of claim 34, wherein the piston has a thickness of about 50 pm.

36. The piston and suspension assembly of claim 26, wherein the layer of compliant material is between 10 and 500 m thick.

37. The piston and suspension assembly of claim 26, wherein the layer of compliant material is around 50 m thick.

38. An electro-acoustic transducer comprising: a piston comprising a disk of silicon having a flat surface and serving as a diaphragm of the transducer; a support ring of silicon surrounding the piston and separated from the piston by a gap; a layer of compliant material adhered to a top surface of the support ring and to the flat surface of the piston, suspending the piston in the gap; a bobbin coupled to the piston; a ferromagnetic housing coupled to the support ring; and a magnet/voice-coil system coupled to the housing and bobbin for converting electrical current to motion of the piston.

39. The transducer of claim 38, wherein: the piston further comprises perimeter wall of the disk and the top surface of the disk, the perimeter wall and top surface bounding a void within the disk of silicon; and the bobbin is adjacent to an inner perimeter of the perimeter wall of the disk.

40. The transducer of claim 38, wherein: the support ring comprises an inner perimeter wall of silicon facing the gap, and an outer lip having less height than the inner perimeter wall; and the ferromagnetic housing is adjacent to an outer perimeter surface of the inner perimeter wall and a bottom surface of the outer lip.

41. A method of forming a diaphragm and suspension for an electroacoustic transducer from a silicon-on-insulator (SOI) wafer having a top layer of Si, an intermediate layer of SiO.sub.2, an inner layer of Si, and a bottom layer of SiO.sub.2, the method comprising: coating the bottom layer of SiO.sub.2 with first photoresist; masking the bottom of the wafer and exposing the wafer to a light source corresponding to the first photoresist; developing the photoresist; etching the bottom SiO.sub.2 layer, the etching masked by the photoresist; stripping the first photoresist and coating the bottom of the wafer with a second coat of photoresist; masking the bottom of the wafer and exposing the wafer to a light source corresponding to the second photoresist; developing the second photoresist; deep reactive ion etching (DRIE) through a first thickness of Si on the bottom of the wafer, less than the full thickness of the inner layer of Si, the etching masked by the second photoresist; stripping the second photoresist; DRIE etching from the bottom of the wafer through the complete thickness of the inner Si layer at the locations where the first DRIE etch was performed, the etching masked by the SiO.sub.2 left after the first etching of the SiO.sub.2, wherein portions of the inner Si layer having the first thickness remain in the area masked by the photoresist during the first DRIE etch, forming the plate of the diaphragm and the top surface of a support ring, and the areas masked by the SiO.sub.2 form walls of the diaphragm and support ring; etching the remaining portions of the bottom SiO.sub.2 layer and portions of the top SiO.sub.2 layer now exposed by the areas etched completely through the inner Si layer; applying a layer of liquid silicone rubber (LSR) on the top of the wafer; and etching through portions of the top Si layer exposed by the areas etched completely through the inner Si layer and upper SiO.sub.2 layer, leaving the diaphragm suspended from the support ring by the LSR where both layers of Si were removed.

42. A method of forming a piston and suspension for an electroacoustic transducer, the method comprising: growing first and second layers of SiO.sub.2 on top and bottom surfaces of a Si wafer; depositing a layer of Cr on the first layer of SiO.sub.2; coating a layer of liquid silicone rubber (LSR) on the Cr layer; coating the top and bottom of the wafer with photoresist; masking the bottom of the wafer and exposing the wafer to a light source corresponding to the photoresist; developing the photoresist; reactive ion etching (RIE) or HF etching the bottom SiO.sub.2 layer; stripping the exposed photoresist and coating the wafer with a new coat of photoresist; again masking the bottom of the wafer and exposing the wafer to a light source corresponding to the photoresist; again developing the photoresist; deep reactive ion etching (DRIE) through a first thickness of Si on the bottom of the wafer; stripping the bottom layer of photoresist; DRIE etching from the bottom of the wafer through the complete thickness of Si at the locations where the first DRIE etch was performed, the etching masked by the SiO.sub.2, wherein portions of the Si having the first thickness remain in the area masked by the photoresist during the first DRIE etch, forming the plate of the diaphragm and the top surface of a support ring, the areas masked by the SiO.sub.2 form rings of the diaphragm and support ring, and the diaphragm is suspended from the support ring by the LSR where the Si was completely removed; and removing the remaining exposed SiO.sub.2 and photoresist.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG. 1 shows a cross-sectional view of a complete electro-acoustical transducer.

[0045] FIGS. 2A, 2B, and 2C show a top perspective, bottom perspective, and cross-sectional view of the diaphragm and suspension of the transducer.

[0046] FIGS. 3A and 3B show an assembly process for the transducer.

[0047] FIG. 4 shows a partial sectional view with dimensions of an example of the transducer.

[0048] FIG. 5A through 5K and 6A through 6M show MEMS fabrication processes for the piston and suspension of the transducer.

DESCRIPTION

[0049] As shown in FIG. 1, an electro-acoustic transducer 100 built using the technique disclosed below includes a diaphragm 102 suspended from a support ring 104 by a suspension 106. Unlike conventional loudspeaker suspensions, the suspension 106 consists of a layer of compliant material extending over the entire surface of the diaphragm, as shown more clearly in FIG. 2A. The diaphragm itself also differs from typical loudspeaker diaphragms, in that its radiating surface is a flat plane, hence we refer to it as a piston. The remaining parts of the transducer match those of a conventional electro-dynamic loudspeaker: a voice coil 108 wound around a bobbin 110, surrounding a coin 112 and magnet 114. The coin 112 and magnet 114 are connected to the support ring by a back plate 116 and housing 118, which, like the coin, are formed of ferromagnetic material, such as steel. Electrical current flowing through the voice coil within the field produced by the magnet 114 and shaped by the ferromagnetic parts produces a force on the voice coil in the axial direction. This is transferred to the piston 102 by the bobbin 110, resulting in motion of the piston, and the production of sound. The same effects can be used in reverse to produce current from sound, i.e., using the transducer as a microphone or other type of pressure sensor. In other examples, the voice coil is stationary and the magnet moves. Such a small transducer is described, aside from the fabrication of the piston and suspension as disclosed below, in U.S. patent application Ser. No. 15/182,069, Miniature Device Having an Acoustic Diaphragm, filed Jun. 14, 2016, the entire contents of which are incorporated here by reference.

[0050] One potential material for the compliant suspension is liquid silicone rubber (LSR), a product based on polydimethylsiloxane (PDMS). To properly suspend the piston, while allowing it to move as needed at acoustic frequencies, the material of the suspension should have an elastic strain limit of at least 50 percent and a Young's modulus and thickness resulting in mechanical stiffness of the suspension in the range of 5-100 N/m. Various elastomers will meet this requirement. LSR is one example. In addition, even larger elastic strain limits, as high as 100 or 150 percent may be desired to accommodate large forces applied to the transducer when an ear-sealing earbud of which it is a component is inserted into or removed from an ear canal. Conversely, for applications where less displacement is needed, an elastic strain limit as low as 10 percent may be sufficient.

[0051] The piston and suspension are shown in more detail in FIGS. 2A-2C. FIGS. 2A and 2B show top and bottom views of the piston and suspension surrounded by the silicon substrate 200 from which they are formed. In FIG. 2A, the layer of material 202 (wavy lines) from which the suspension 106 is formed can be seen to extend over the entire top surface 204 of the piston 102, and over the support ring 206 that forms the top edge of the housing 104 in FIG. 1. The material 202 is cut out above the gap between the support ring 206 and the surrounding substrate in FIGS. 2A and 2C but intact in FIG. 2B, to assist in visualizing the construction. The bottom view 2B and side sectional view 2C show that the underside of the piston may consist of a pattern of rings 208 and ribs 210, with voids 212 between them etched in the silicon. This provides stiffness to the silicon piston while decreasing its weight relative to a solid disk. In other examples, a flat plate of silicon is sufficiently stiff, and the ribs and rings are not needed for stiffness, though similar structures, or just the outermost ring 208, may be needed due to the fabrication process, as discussed below. The sectional view also shows a layer 216 of SiO2, which will be explained below.

[0052] FIGS. 3A and 3B show one example of how the piston and suspension can be connected to the rest of the transducer. In FIG. 3A, the housing and bobbin, with the magnet, coin, back plate, and voice coil already assembled to them, are dipped into a shallow pool of adhesive 300 in order to apply a uniform bead of adhesive to one end of the housing. Preferably, the bead is sized to fill the gap between the outer support ring and the inner surface of the housing without excessive squeeze-out of adhesive. In other examples, the magnet, coin, and back plate are not attached until later. Then, in FIG. 3B, the bobbin is set on the piston 102, and the housing 118 is set on the outer ring 206. The adhesive is cured, and the transducer is ready for further processing, such as attaching or dressing lead-outs from the voice coil. In some example, the lead-outs extending from the voice coil are dressed before the bobbin is attached to the piston. In some examples, the bobbin and housing are attached to the piston and ring, respectively, before the ring is cut away from the rest of the substrate. This can make it easier to fix the location of the piston and ring when making the attachment. Further, a large number of bobbins and housings can be attached to a full wafer of pistons and rings all at once, using an appropriate fixture.

[0053] FIG. 4 shows a detail of the cross-section of the transducer, with dimensions of one example implementation. Other implementations may have quite different dimensions. In this example, the suspension is formed from a layer 202 of liquid silicone rubber (LSR) 10-500 m thick depending on desired suspension stiffness, formed by spin-coating the LSR on the silicon substrate. In some examples, the LSR layer is 30-80 m thick, and in one particular example, it is about 50 m thick. The piston top is between 10 and 100 m thick, and in some cases around 50 p.m thick, and is separated from the LSR by a 0.25-2 m thick layer of SiO2 thermal oxide and/or 5-50 nm of Cr or other suitable material, as discussed below with regard to the fabrication process. The outer ring 208 of the piston 102 is 50 m thick, and it is separated from the support ring 206 by a small gap 214 of around 300 m. The support ring provides an adhesion area for the LSR at the top surface of the substrate, and includes a thinner wall, around 75 m thick, extending down the inner face of the gap, providing a lip where the wall of the main housing may be attached. These dimensions allow the completed transducer to have an outer diameter only 4 mm acrosssubstantially smaller than typical electrodynamic (voice coil moving a diaphragm) transducers (only one outer edge is shown in FIG. 4). Smaller sizes may be achieved, though with less space available inside the bobbin for the magnet and coin. With a magnet as small as 1.5 mm, a total transducer diameter of 3 mm may be achieved. Larger sizes may also be built using this method, though the piston may need to be thicker or have more reinforcing ribs as the aspect ratio (diameter to height) increases.

[0054] As shown in this example, the bobbin has an outer diameter matched to the inner diameter of the outer ring of the piston, so that the bobbin is contained inside the outer ring. This design contains any extra adhesive to the inside of the piston and outside of the housing ring, i.e., away from the gap between the piston and the housing, unlike in the example of FIG. 3B. Similarly, attaching the housing 118 to the outer periphery of the support ring keeps the adhesive for that joint out of the gap.

[0055] FIGS. 5A-5K show a cross-section of a silicon wafer as it goes through an example MEMS fabrication process to form the piston and suspension. Other MEMS processes, with different technologies used for patterning, masking, and etching may be used, with accordingly different process steps. The etch depths mentioned below are based on a 300 m thick Si wafer and may be adjusted to achieve the desired characteristics of the Si piston, e.g., mechanical stiffness, moving mass, etc. The process steps are as follows: [0056] 1. Layers (504, 506) of thermal oxide (SiO.sub.2) are grown on the top and bottom surfaces of a 300 m thick Silicon wafer 502. (FIG. 5A) [0057] 2. A 5-50 nm thick layer 508 of Chromium is deposited on the top by physical vapor deposition (PVD). The Cr will serve as an etch-stop for later steps; other appropriate materials may be used. (FIG. 5B) [0058] 3. A 50 m thick layer 510 of LSR is spin-coated on top of the Cr and cured. Thinner or thicker layers of LSR may be used, based on the properties of the LSR and the desired amount of excursion and stiffness in the speaker. (FIG. 5C) [0059] 4. Photoresist 512, 514 is spin-coated onto both sides. (FIG. 5D) [0060] 5. The bottom side is masked (516) and exposed to an appropriate light source to activate the photoresist 512. (FIG. 5E) [0061] 6. The photoresist layer is developed and used to mask reactive ion etching (RIE) or HF etching of the bottom SiO2 layer 506. (FIG. 5F) [0062] 7. The developed photoresist 512 on at least the lower surface is stripped and a new coating 518 is spin-coated. (FIG. 5G) [0063] 8. Another mask 522 is used to expose the photoresist 518 on the bottom side. (FIG. 5H) [0064] 9. The photoresist 518 is developed and used to mask deep reactive ion etching (DRIE) through 50 m of the bottom of the Si wafer to create channels 524, 525 (note that these are circular channels in the wafer, viewed twice each in the cross-section). (FIG. 5I) [0065] 10. The bottom layer of photoresist 518 is stripped, and DRIE is used again to etch through the remaining 250 p.m of the silicon wafer (FIG. 5J). Where the first DRIE etch was performed, the second etch goes completely through the wafer, extending the channels 524, 525 to the SiO.sub.2 layer 504; the area that was protected by the second mask during the 50 m etch remains 50 m thick, as only 250 m is removed, forming the plate 526 of the piston and the top surface of the support ring. The areas protected by the first mask remain protected by the SiO.sub.2 506 left behind after the RIE etch in step 6, and form the rings of the piston and housing and any other full thickness features, such as the stiffening ribs and rings mentioned above (not shown). In some examples, full-thickness features are also used to manage the DRIE process. [0066] 11. The remaining SiO.sub.2 506 at the bottom layer and at the top of the now-open channels 524, 525 between the piston and the housing is removed using RIE or HF, with the Cr layer 508 serving as an etch-stop to prevent the RIE or HF from etching the underside of the LSR layer 510 after etching the top SiO.sub.2 layer 504 via the channels 524, 525. (FIG. 5K). The remaining photoresist layer 514 covering the LSR 510 is stripped.

[0067] The process shown above etches a channel 525 through the wafer around the outer support ring, allowing the piston/support ring/suspension unit to be cut out of the substrate. Many such units can be formed simultaneously in a single substrate, held in place by the LSR layer, and cut out as needed by either mechanical means, RIE, or laser-cutting. The inner wall of the bulk Si remaining outside the outermost channel 525 may serve as an alignment guide to the cutting process. As noted above, housings and bobbins may be attached to the support rings and pistons in bulk before they are cut out of the substrate, and the housings may also serve as alignment guides for the cutting operation. Curing the LSR layer helps control the pretension in the surround, to make the stiffness of the surround more linear. Without pretension, bending stiffness dominates near the neutral axial position of the piston (with no magnetic forces applied to the voice coil). At some piston excursion, the tensile stresses in the surround begin to dominate and cause the stiffness to increase. The pretension due to curing makes the overall stiffness greater but much more linear. In some examples, curing the LSR at 150 C. roughly doubles the near-neutral position stiffness.

[0068] Another process flow is shown in FIG. 6A through 6M. This process begins with a Silicon-on-insulator (SOI) wafer 600 and delays the application of the LSR layer to late in the process, which may be more compatible with some MEMS fabrication workflows. The process steps are as follows: [0069] 1. The process begins with a SOI wafer having a first layer 602 of Si, oxide layers 604 and 608 on either side of the first Si layer, and a very thin (2-10 m) second Si layer 606 bonded on top. (FIG. 6A) [0070] 2. A single layer 610 of photoresist is applied to the bottom of the wafer. (FIG. 6B) [0071] 3. The bottom side is masked (612) and exposed to an appropriate light source to activate the photoresist 610. (FIG. 6C) [0072] 4. The photoresist layer is developed and used to mask reactive ion etching (RIE) or HF etching of the bottom SiO.sub.2 layer 608. (FIG. 6D-E) [0073] 5. The developed photoresist 610 is stripped and a new coating 614 is spin-coated. (FIG. 6F) [0074] 6. Another mask 616 is used to expose the photoresist 614 on the bottom side. (FIG. 6G) [0075] 7. The photoresist 614 is developed to create a new mask that covers the remaining SiO.sub.2 608 and part of the main silicon layer 602. (FIG. 6H) [0076] 8. Deep reactive ion etching (DRIE) through 50 m of the bottom of the Si layer 602, masked by the photoresist 614, creates channels 618, 620 (note again that these are circular channels in the wafer, viewed twice each in the cross-section). (FIG. 6I) [0077] 9. The bottom layer of photoresist 614 is stripped, and DRIE is used again to etch through the remaining 250 m of the silicon wafer (FIG. 6J). As before, where the first DRIE etch was performed, the second etch goes completely through the wafer, extending the channels 618, 620 to the top Si02 layer 604; the area that was protected by the second mask during the 50 m etch remains 50 m thick, as only 250 m is removed, forming the plate 622 of the piston and the top surface of the support ring. The areas protected by the first mask remain protected by the SiO.sub.2 608 left behind after the RIE etch in step 4, and form the rings of the piston and support ring and any other full thickness features, such as the stiffening ribs and rings mentioned above (not shown). In some examples, full-thickness features are also used to manage the DRIE process. [0078] 10. The remaining Si02 608 at the bottom layer and at the top of the now-open channels 618, 620 between the piston and the housing is removed using RIE or HF. (FIG. 6K) [0079] 11. A 50 m thick layer 622 of LSR is now spin-coated on top of the top Si layer 606 and cured. Thinner or thicker layers of LSR may be used, based on the properties of the LSR and the desired amount of excursion and stiffness in the speaker. (FIG. 6L) [0080] 12. To release the piston 622, the Si of the thin top layer 606 is etched using an isotropic XeF.sub.2 etch. This etch is effectively masked by the much thicker (even where nearly etched through) bottom Si layer 602while 5 m of the piston layer may be lost, 45 m remain, combined with the 5 m of the top layer that are protected between the bottom layer and the LSR. Vertical Si areas will not be etched as they are still protected by a passivation layer deposited during the DRIE step. Other isotropic or anisotropic etching techniques (e.g., RIE using chlorine or fluorine chemistries, KOH, TMAH) may be used instead of XeF2 for this release step.

[0081] As compared to the first example, because the LSR is added late in the process, the top layer of photoresist is not needed.

[0082] A number of implementations have been described. Nevertheless, it will be understood that additional modifications may be made without departing from the scope of the inventive concepts described herein, and, accordingly, other embodiments are within the scope of the following claims.