FIELD OF INVENTION The present invention relates generally to an electrodynamics microelectromechanical systems (MEMS) micro speaker, and more particularly to a method for fabrication of electrodynamics microelectromechanical systems (MEMS) micro speaker.

BACKGROUND OF INVENTION

During the last few years, there has been a growing market demand for embedded audio equipment in multimedia and mobile devices, such as cellular phone, smart phone and tablet personal computer. A small size, lightweight, and low cost micro speaker is particularly demanded for application, especially the cellular phones and hearing aids. Moreover, the devices are required to be slimmer, without compromising the existent of its performance. The main challenge is to make the better trade-off between power efficiency, higher sound level, better sound quality and integration.

The advantages of micromachining over conventional fabrication include precise dimensional control, integration of on-chip circuits and potential low cost owning to batch processing. Currently, the existing micro speakers are used in most of electronic mobile devices and they are usually fabricated using conventional or macroscopic machining technologies. In recent years, the fabrication of micro speaker for hearing instrument such as hearing aids, handphone, laptop, multimedia system, application using microelectromechanical systems (MEMS) technology is challenging because of certain critical requirements, including their small size, low driving voltage, high output sound pressure level, flat frequency response and low energy consumption. The microelectromechanical systems (MEMS) technologies refer to the fabrication devices with at least some of their dimension in the few millimeters to micrometer range. MEMS technologies offer attractive high precision fabricating process and interesting miniaturization potential. Besides, MEMS batch processing potential, there is a possibility to keep the final integration device with neighboring circuits and low cost. As such, it is potentially respectable for the microsystems industry, which could answer a demand of the problem aspect for the development micro speaker integrated on silicon or semiconductor substrate.

The limitation of conventional technologies in term integration are not far to be reached, and MEMS technologies present a promising potential for overcoming this limit. Many transduction principles have been implemented in MEMS micro speaker, such as piezoelectric, electrostatic, electrodynamics and thermo acoustic actuation. However, piezoelectric and thermo acoustic response has a major drawback in high fidelity transduction. Electrostatic principle, which broadly deployed in MEMS actuator has low power density but relatively requires high driving voltages. Electrodynamics actuation has a good alternative solution to answer the problem, although despite the technological challenge indeed of magnets integration.

In view of these and with the growing of the microsystems industry, it would be advantageous to provide an improved electrodynamics microelectromechanical systems (MEMS) micro speaker which offers smaller size, lightweight and low energy consumption. Accordingly, the present invention provides a method for fabrication of an improved electrodynamics MEMS micro speaker which facilitates simple assembly, low cost, slimmer in size without compromising the existent of its performance. Moreover, the improved electrodynamics MEMS micro speaker has a low driving voltage, high output sound pressure level, flat frequency response, and yet has a better tradeoff between power efficiency, higher sound level, better sound quality and integration.

SUMMARY OF THE INVENTION

The present invention provides a method for fabrication of an electrodynamics microelectromechanical systems (MEMS) micro speaker. Accordingly, the method includes: a) forming a first silicon wafer, i.e. wafer 1 , having a moving parts that serves a actuator system, said moving part includes flexible polyimide diaphragm and micro coil or voice coil; b) forming a second silicon wafer, i.e. wafer 2, with back cavity for a seating or supporting a permanent magnet; said second silicon wafer includes venting or acoustic hole on its back chamber; c) bonding the permanent magnet to the second silicon wafer using adhesive epoxy bonding followed by boding the both wafer 1 and wafer 2 together using adhesive epoxy bonding, to form the complete electrodynamics MEMS micro speaker; wherein the flexible polyimide diaphragm is formed by wet and dry etching of the first silicon wafer; such that a thin planar of the micro coil or voice coil is suspended on the flexible polyimide diaphragm.

In the preferred exemplary of the present invention, the micro coil or voice coil is a single or multi turn planar. By way of example but not limitation, the first silicon wafer, i.e. the wafer 1 , is preferably but not limiting, deposited with about 200 nm thick or more of low pressure chemical vapor deposition (LPCVD) Silicon-Nitride, such that nitride layer, i.e. the Silicon-Nitride layer is to be used as the masking and etch stop layer during potassium hydroxide (KOH) etching; and wherein patterning and etching of the silicon nitride for opening silicon-etching window on back side of the nitride layer is performed by using photolithography process. Accordingly, the wafer 1 is preferably immersed in KOH solution for silicon anisotropic wet etching to forming V-groove silicon cavity after the patterning the silicon-etching window. It will be appreciated that about 30-50 um thick silicon is remained as silicon diaphragm in concerning mechanical strength for subsequent wafer process handling. By way of example but not limitation, the silicon diaphragm i.e. the flexible polyimide diaphragm is deposited with a thin polyimide layer of about 2 um thick by spin coating process. Accordingly, the polyimide layer is to be cured at temperature 350-400 °C for about 1 hour.

The polyimide layer is then deposited with a thin metal, i.e. titanium and gold Ti/Au using sputtering deposition. By the way of example but not limitation, the titanium and gold Ti/Au is preferably deposited on the polyimide layer with thickness deposition Ti/Au of 50nm and 300-500 nm respectively. It should be noted that the sputtered gold layer is used as planar micro coil or voice coil that is patterned using lift-off process. It will be appreciated that the remaining silicon diaphragm and silicon nitride are to be removed to perform as polyimide layer and the planar micro coil or voice coil as a vibration part of the electrodynamics MEMS micro speaker.

In accordance with preferred exemplary of the present invention, the second silicon wafer, i.e. the wafer 2, is preferably deposited with 200 nm thick or more of low pressure chemical vapor deposition (LPCVD) Silicon-Nitride, such that nitride layer, i.e. the Silicon-Nitride layer is to be is used as the masking and etch stop layer during potassium hydroxide (KOH) etching; and wherein patterning and etching of the silicon nitride for opening silicon etching window on the front and back side of the nitride layer is performed by using photolithography process. Accordingly, the wafer 2 is preferably immersed in KOH solution for silicon anisotropic wet etching to form a silicon cavity on front side, and venting or acoustic hole on back side of the silicon wafer after the patterning the window of the wafer 2. It will be appreciated that the patterned front side of the silicon wafer, i.e. the wafer 2 is immersed in KOH solution to etching silicon wafer to certain depth or predetermined depth, which is dependent to thickness of the permanent magnet to be used. By way of example but not limitation, the permanent magnet is a disc permanent magnet (NdFeB) at preferred dimension of 1 .6 mm diameter and 8 mm thickness. Preferably but not limiting, the depth silicon wafer etching is etched around 100-200 urn depth, so that to achievable distance between the flexible polyimide diaphragm and top surface magnet about 50 urn to 100 urn.

In accordance with the preferred exemplary, the patterned venting or acoustic hole window on the back-side silicon wafer is preferably immersed in the same KOH solution to etch through remained silicon wafer to formed hole in silicon cavity.

The present invention consists of several novel features and a combination of parts hereinafter fully described and illustrated in the accompanying description and drawings, it being understood that various changes in the details may be made without departing from the scope of the invention or sacrificing any of the advantages of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, wherein:

FIG. 1 a shows a schematic structure of an electrodynamics microelectromechanical systems (MEMS) micro speaker in perspective or in three-dimensional view in accordance with preferred exemplary of present invention;

FIG. 1 b shows a schematic diagram in cross-sectional view of the electrodynamics MEMS micro speaker structure of FIG. 1 a, in accordance with preferred exemplary of present invention; FIG. 2 shows a process flow for wafer 1 suspended on flexible polyimide diaphragm with embedded single or multi-turn planar micro coil on a silicon wafer according to preferred exemplary of the present invention;

FIG. 3 shows a process flow for wafer 2, for permanent magnet seating and acoustic hole for evacuate air in a back chamber according to preferred exemplary of the present invention;

FIG. 4 shows a bonding process of the wafer 1 to the wafer 2, and the permanent magnet according to preferred exemplary of the present invention; FIG. 5 shows a photograph of the electrodynamics MEMS micro speaker in earphone packaging according to preferred exemplary of the present invention;

FIG. 6 shows a photograph of frequency response versus dB SPL of the electrodynamics MEMS micro speaker according to preferred exemplary of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for fabrication of an electrodynamics microelectromechanical systems (MEMS) micro speaker. Hereinafter, this specification will describe the present invention according to the preferred exemplary, methods and/or embodiments of the present invention. However, it is to be understood that limiting the description to the preferred exemplary of the invention is merely to facilitate discussion of the present invention and it is envisioned that those skilled in the art may devise various modifications and equivalents without departing from the scope of the appended claims.

The microelectromechanical systems (MEMS) technology is based to microelectronics fabrication and conveys the advantages of miniaturization, multiple components and on chip signal processing to the design and construction of integration microstructures. The MEMS technologies present a promising potential for overcoming the limit.

The method for fabrication of electrodynamics microelectromechanical systems (MEMS) micro speaker according to the preferred exemplary of the present invention will now be described in detail with reference to accompanying FIGS. 1 to 6, either individually or in any combination thereof.

Referring to FIGS. 1 a and 1 b, the electrodynamics microelectromechanical systems (MEMS) micro speaker (100) is generally constructed in two silicon wafers (i.e. wafer 1 and wafer 2) (200 and 300). Accordingly, each part of the silicon wafer (200, 300), before packaging, is provided as substrate or frame for a suspended planar micro coil or voice coil (202) on flexible polyimide diaphragm (204), and for seating permanent magnet (302). The electrodynamics MEMS micro speaker (100) is further provided with a venting or acoustic hole (304) for air in its back chamber (206), such that to allow it to move in and out. Particularly, the electrodynamics MEMS micro speaker includes two-wafer bonded structure, i.e. the wafer 1 (200) and the wafer 2 (300), having a suspended flexible polyimide diaphragm (204) and a single or multi turn of planar micro coil or voice coil (202) that is fabricated on the wafer- 1 (200); and the permanent magnet (NdFeB) (302) is preferably bonded to the wafer-2 (300). By way of example but not limitation, said wafer-1 (200) and wafer-2 (300) are preferably bonded together with epoxy adhesive bonding. The step process fabrication of each silicon wafer (i.e. the wafer 1 and wafer 2) will be described hereinafter.

In the preferred exemplary of the present invention, the method for fabrication of the electrodynamics MEMS micro speaker (100) generally includes the fabrication of moving part, i.e. the flexible polyimide diaphragm (204), the planar micro coil or voice coil (202) which serves as actuator system, its cavity (306) and the venting or acoustic hole (304) as formed the back chamber (206), and also for the seating permanent magnet (302). The method also includes the bonding method for permanent magnet (302) to the silicon substrate and the two silicon wafer, i.e. wafer 1 (200) and wafer 2 (300).

It should be noted that the main components to makeup the electrodynamics MEMS micro speaker (100) are the planar micro coil or voice coil (202) attached to the flexible polyimide diaphragm (204), and the permanent magnet (302). It will be appreciated that any signal or current flowing through the planar micro coil or voice coil (202) within the micro speaker's magnetic field shall generate a force that moves the flexible polyimide diaphragm (204).

According to Lorentz Force Law {F _Lorenlz ), this force is equal to the product of a magnetic flux density (B) time to the length of coil (I) time the current flowing in coil (/): BH

In the micro speaker's planar micro or voice coils (202), electrons travel (or oscillate) in a common cylindrical path at a constant speed, generating an alternating magnetic field, known as a Biot-Savart field. As such, the micro speakers are served as transducers such that they convert electrical energy or signal (e.g. current flow) to mechanical (acoustical) energy. Without the interaction of the stationary magnetic field and the field created by the constantly varying electric charges, no acoustical output would result, and all of the power amplifier's current would be dissipated simply as heat in the voice coil (202).

In accordance with preferred exemplary of the present invention, the important of fabrication method of the electrodynamics MEMS micro speaker (100) includes:

a) forming a part of wafer i.e. the wafer 1 (200) and devices wafer by MEMS-based processing (batch processing);

b) forming a flexible polyimide diaphragm (204) by wet and dry etching on back of a first silicon wafer, i.e. the wafer 1 (200);

c) forming a very thin planar micro coil or voice coil (202) suspended on the flexible polyimide diaphragm (204)

d) forming back cavity (306) for the seating or supporting permanent magnet (302) and the venting or acoustic hole (304) on the back chamber (206), and front a second silicon wafer, i.e. the wafer 2 (300) by wet etching silicon wafer; e) bonding the permanent magnet (302) on the center of a second part of silicon wafer; and

f) bonding the silicon wafers i.e. the wafer 1 (200) to the wafer 2 (300) such that the completed electrodynamics MEMS micro speaker (100) is formed.

The fabrication method for forming the flexible polyimide diaphragm (204) and the single or multi turn planar micro coil or voice coil (202) on the first silicon wafer, i.e. the wafer 1 (200) is shown in FIG. 2. By way of example but not limitation, a 200 nm thick low pressure chemical vapor deposition (LPCVD) Silicon-Nitride (212) was deposited on the silicon wafer 1 (200). This nitride film, i.e. the Silicon-Nitride (212), is used as the masking and etch stop layer during potassium hydroxide (KOH) etching.

It will be appreciated that patterning and etching silicon nitride (212) for opening silicon- etching window (214) on back side of the nitride layer is preferably performed by photolithography process, as shown in FIG. 2(a). After patterning the window, the wafer 1 (200) is preferably immersed in KOH solution for silicon anisotropic wet etching to forming V-groove silicon cavity. Concerning mechanical strength for subsequent wafer process handling, it will be appreciated that, for example but not limiting, 30-50 um thick silicon was remained as silicon diaphragm (FIG. 2(b)).

Referring now to FIG. 2(c), a thin polyimide (216) layer was then preferably deposited on silicon diaphragm, i.e. the flexible polyimide diaphragm (204) by spin coating process, preferably but not limiting, with 2 um thickness. This layer was preferably, but not limiting, cured at temperature 350-400 °C for about 1 hour. Next, a thin metal (titanium and gold) (218) was then deposited on the polyimide (216) layer using sputtering deposition, with preferably, but not limiting, thickness deposition Ti/Au of 50nm and 300-500 nm respectively. A very thin titanium layer functioned as adhesion layer between gold and polyimide layer. In FIG. 2(d), the sputtered gold layer is preferably used as planar micro coil or voice coil (202) which was patterned using lift-off process. Finally, as shown in FIG. 2(e) the remaining silicon diaphragm (204) and silicon nitride (212) were removed to perform as polyimide (216) layer and the planar micro coil or voice coil (202) as a vibration part of the electrodynamics MEMS micro speaker (100).

The fabrication method for forming of a back cavity (306) for a seating or supporting permanent magnet (302), and venting or acoustic hole (304) on a second silicon wafer, i.e. the wafer 2 (300), the forming of a part is shown in FIG. 3. It will be appreciated that the wafer 2 (300) functioned for a permanent magnet seating, the supporting silicon cavity (306) and the venting or acoustic hole (304) silicon wafer was formed by potassium hydroxide (KOH) solution as similar as in process etching silicon wafer in the wafer- 1 (200).

By way of example but not limitation, a 200 nm thick low pressure chemical vapor deposition (LPCVD) Silicon-Nitride (312) was deposited on the wafer. This nitride film, i.e. the Silicon-Nitride (312), is used as the masking and etch stop layer during potassium hydroxide (KOH) etching.

It will be appreciated that patterning and etching silicon nitride (312) for opening silicon etching window (314) on the front and back side nitride layer is used photolithography process, as shown in FIG. 3(a). After patterning the window the wafer 2 (300) is preferably immersed in KOH solution for silicon anisotropic wet etching to form a silicon cavity (306) on front side, and venting or acoustic hole (304) on back side of the silicon wafer. Due to different of the etching depth on the both side of the silicon wafer, process fabrication this part is divided in two step process.

Firstly, the patterned front side silicon wafer, i.e. the wafer 2 (300) is preferably immersed in KOH solution to etching silicon wafer to certain depth or predetermined depth, which is dependent to thickness of the permanent magnet (302) to be used. By way of example but not by way of limitation, a disc permanent magnet (NdFeB) (302) of dimension 1 .6 mm diameter and 8 mm thickness is preferably mounted at wafer 2 (300). To achievable distance between the flexible polyimide diaphragm (204) and top surface magnet about 50 urn to 100 urn, the depth silicon wafer etching was etched around 100- 200 urn depth, as shown in FIG. 3(b).

Secondly, the patterned venting or acoustic hole (304) window on the back-side silicon wafer was then immersed in the same KOH solution to etch through remained silicon wafer to formed hole in silicon cavity (306), as shown in FIG. 3(c), while the front side silicon wafer was protected to avoided etch silicon in both side, as same as in first processed silicon cavity.

After the parts, e.g. the wafer 1 (200) and wafer 2 (300), have been fabricated, next process is to bond the both wafer 1 (200) and wafer 2 (300) to form a complete electrodynamics MEMS micro speaker (100) device. The fabrication method for bonding silicon wafer 1 (200) and wafer 2 (300) for complete device, and also bonding permanent magnet (302) to silicon wafer 2 (300), is shown in FIG. 4. Accordingly, the permanent magnet (302) is first mounted and bonded on the center of wafer 2 (300) using adhesive epoxy bonding, and then the wafer 1 (200) and wafer 2 (300) are to be bonded together using adhesive epoxy bonding, to form the complete electrodynamics MEMS micro speaker (100).

For requirement test performance of the electrodynamics MEMS micro speaker (100), said electrodynamics MEMS micro speaker (100) is preferably packaged as earphone packaging for in-ear canal microphone application. As shown in FIG. 5, the electrodynamics MEMS micro speaker (100) is preferably mounted and packaged in earphone packaging. It will be appreciated that the performance of electrodynamics MEMS micro speaker (100) is preferably measured using FineSPL software to test its frequency response versus dB sound pressure level, as shown in FIG. 6.

It is to be understood that both the set forth examples and detailed description are exemplary and explanatory only, and are not restrictive of the invention. As such, the set forth examples and detailed description, including methodology, sizes, temperatures, times, materials and its contents or any other variable parameters and methods as described above should not be construed as limiting, but as the best mode contemplated by the inventor for carrying out the invention.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the principle and scope of the invention, and all such modifications as would obvious to one skilled in the art intended to be included within the scope of following claims.