WO2005053357A1

WO2005053357A1 - Piezoelectric microspeaker with corrugated diaphragm

Info

Publication number: WO2005053357A1
Application number: PCT/KR2003/002582
Authority: WO
Inventors: Seung-Hwan Yi
Original assignee: Seung-Hwan Yi
Priority date: 2003-11-27
Filing date: 2003-11-27
Publication date: 2005-06-09
Also published as: AU2003304673A1

Abstract

The present invention relates to a piezoelectric micro-speaker with a corrugated diaphragm and a method for producing the same. The micro-speaker of the present invention can be easily manufactured with an improved output performance. The present invention provides a method for shaping an artificial corrugation to a compressive thin film of a diaphragm of the piezoelectric micro-speaker and providing a package structure onto the upper surface of the diaphragm. The piezoelectric micro-speaker of the present invention has an increased membrane deflection and an improved speaker output performance due to the amplification of sound-pressure by the package.

Description

Piezoelectric Microspeaker with Corrugated Diaphragm

Technical field The present invention relates to a piezoelectric micro-speaker and producing method thereof. More particularly, the present invention relates to a method for improving the function of a speaker by corrugating a diaphragm of the piezoelectric micro- speaker and by installing a package structure on an upper portion of the diaphragm, and a speaker produced by this method.

Background Art Currently, the use of a cellular phone is rapidly increasing due to convenience in personal communication and data transmission. The cellular phone is being developed in many ways to provide a smaller-sized, lighter-weighed, and higher quality one.

^■ Also, an integrated device which can include the functions of a microphone, a speaker and a buzzer has been developed in the area of the microphone and the speaker for a mobile phone handset. However, it is difficult to miniaturize the microphone and the micro-speaker without lowering the dynamic feature, so the applicable field thereof is limited.

That is, as for the currently commercialized micro-speaker, it is difficult to be applied in the field which requires a micro sound element such as hearing aid. Such problem also occurs in various fields which can use the micro-speaker, like home electronics such as a lap-top computer and an entertainment devices, toys including a sound generating toy, and card devices including a voice playback card.

Meanwhile, a technology of miniaturizing a mike and speaker on a silicon wafer by using a Micro Electro Mechanical System (MEMS) is disclosed recently.

Since the method for producing an acoustic transducer on a silicon wafer by using the MEMS can be performed by a semiconductor batch processing, the production cost can be reduced. Also, since a plurality of transducers and amplifiers can be integrated on a single chip, it is possible to miniaturiz the acoustic element including signal processing means. As such, this method has various advantages compared with the conventional art.

In general, the production of the speaker and the microphone using the MEMS technology mostly uses a piezoelectric film. That is because the transducer using the piezoelectric film does not need a permanent magnet and a driving coil, which are required for the conventional electro-dynamic type speaker. In addition, it is easier to produce the MEMS microphone than a condenser-type microphone, and the polarization-voltage is not required. Also, it has a wider dynamic range.

However, since the piezoelectric transducer using the conventional MEMS technology mostly uses a non-stoichiometric nitride membrane having a compressive residual stress, the microphone has a relatively low sensitivity, and the speaker has a low output. That is, the micro-speaker using the non-stoichiometric nitride having only the compressive- residual stress has the problems as follows:

1 ) the tone quality is not uniform due to the uneven wrinklings; 2) it is difficult to deposit the non-stoichiometric nitride film having a compressive residual stress; 3) since only the wrinklings are used, the deflection is not easy at the time of driving the piezoelectric film and thus the generation of the sound is limited; and 4) since there is no package structure for amplifying a sound-pressure, the sound-pressure generated under the same voltage is smaller than the conventional electro- dynamic type speaker.

Disclosure of the Invention In order to resolve the above-mentioned problems, the present invention is provided. It is an object of the present invention to provide a piezoelectric speaker, which is easier to produce and which has an excellent dynamic characteristic.

In order to achieve such object, the present invention discloses a method of corrugating a diaphragm of the piezoelectric micro-speaker and installing a package structure on an upper portion of the diaphragm.

Brief Description of the Drawings Figs, la-11 are cross-sectional views showing each producing step of a preferred embodiment of the method of producing the piezoelectric micro-speaker according to the present invention.

Fig. 2 is a process flow showing a preferred embodiment of a method of producing the piezoelectric micro-speaker according to the present invention.

* Explanations of the main reference numbers in the drawings *

100: silicon substrate 102, 102': oxide film 104: corrugation 106: compressive film 108, 108': bottom electrode 110: piezoelectric film 112: bottom insulator film 114, 114': top electrode 116: top insulator film 118: anchor region 120: thick photoresistor 122: seed film 124: top photoresistor 126: non-electrolytic nickel layer 128: cavity

Detailed Description of the Invention The method for producing the piezoelectric micro-speaker according to the present invention comprises the steps of:

1) etching an outer portion of a side of a silicon substrate to form a corrugation on the substrate; 2) forming a compressive thin film on both sides of the substrate; 3) forming a bottom electrode at the center portion of the substrate surface surrounded by the corrugation and at an outer portion of the corrugation; _4) forming a piezoelectric thin film which covers the bottom electrode formed at the center portion of the substrate; 5) forming an insulator film over the substrate surface; 6) forming a top electrode on the position on the insulator film of the center portion of the substrate which does not overlap with the bottom electrode at the outer portion of the corrugation; 7) forming a top insulator film on the substrate surface; and 8) removing the silicon substrate at the center portion and the bottom portion of the corrugation.

Also, the method for producing the piezoelectric micro-speaker according to the present invention comprises the steps of: ^' 1) forming an anchor region by removing an insulator film on an outer portion of the substrate surface forming the piezoelectric micro-speaker; 2) forming a thick photoresistor layer on the substrate surface excluding the anchor region; 3) forming a seed film over the substrate surface; 4) forming a top photoresistor layer on the outside of the anchor region and on a partial area of the center portion of the substrate; 5) growing a non-electrolytic nickel layer over the substrate surface excluding the top photoresistor layer; and 6) removing the top photoresistor layer, the seed layer contacting the top photoresistor layer and the thick photoresistor layer.

The method for producing the piezoelectric micro-speaker according to another preferred embodiment of the present invention comprises the steps of:

1) cleaning a wafer and growing an oxide film (approximately 1 μ m) for initial cleaning, protecting the backside of the water and making an artificial corrugation structure; 2) patterning and Si etching a surface for forming a surface corrugation; 3) etching the oxide film and depositing a compressive membrane(approximately 1 μ m), which grows or deposits the compressive membrane (i.e., a non-stoichiometric nitride film, an oxide/nitride film or a nitride/oxide/nitride film); 4) depositing and patterning a bottom electrode (Al; approximately 0.5 μ m), which locates the bottom aluminum electrode inside of the corrugation structure at the time of patterning; 5) depositing and etching a piezoelectric film (ZnO, A1N, etc.; within approximately 0.5 μ m), which dry-etches the piezoelectric film or which uses chemicals described in the thesis of the present applicant in order to prevent damage of the bottom electrode; 6) depositing and patterning an insulator film (parylene-C or parylene-D; within approximately 0.2 μ m), which deposits the insulator film in order to prevent an electrical short between the bottom electrode and a porous piezoelectric material, and which patterns the insulator film in order to secure more stable contact of the contacting part of the top electrode material; 7) depositing and patterning the top electrode (Al or a bilayer of Au/Cr; within approximately 0.5 μ m), which locates the top electrode to be inside of the corrugation structure, and which makes the top electrode become a bit smaller than the bottom electrode in order to prevent an electrical short between the top and bottom electrodes; 8) parylene-C or parylene-D depositing and patterning a thick film (approximately 1 μ m), which deposits the thick film in order to enhance the mechanical strength at the time of cutting into single chip or to control the stress of the end of the membrane, and which patterns the thick film in order to secure an anchor region for securing an adhering force of a Ni/Cr seed layer; 9) patterning and etching a backside compressive film, which determines the size of the membrane, patterns in order to etch the silicon in an anisotropic etching solution (KOH solution) thereby to form the membrane, and proceeds with the etching of the compressive film in a reactive ion etching system after the patterning; 10) applying and patterning a thick photoresistor, and depositing a Ni/Cr seed film, which applies the thick photoresistor in order to secure a cavity and patterns the thick photoresistor in order that the seed layer is attached to the silicon at only the desired position, which hardens the thick photoresistor for approximately 30 minutes at a high temperature of 150 degree in order to prevent the deterioration of the Ni/Cr seed layer at the time of depositing, and which deposits the Ni Cr seed layer within a thickness of approximately 0.1 μ m in overall by a thermal depositing or sputtering method; 11) applying and patterning a thin photoresistor, and plating a non- electrolytic nickel, which applies a thin photoresistor in order to define the lift-off of unnecessary parts and a sound discharge gate after the non-electrolytic nickel plating, and- which plates non-electrolytic nickel under the processing condition disclosed in the thesis of the present applicant for forming a package structure at a low temperature thereby depositing an approximately 5 μ m-thick metal thin film; 12) removing a photoresistor and a backside silicon, which proceeds with a ultrasonic cleaning of the thin film and the thick photoresistor on the surface and the bottom portion of the nickel film for forming a cavity in an acetone solution for approximately 10 seconds and then leaves the films in the acetone solution for a predetermined time in order to completely remove the photoresistor in and outsides, and which protects the machined element over the surface using a silicon etching device and controls the silicon in an anisotropic etching solution for a predetermined time in order to form a membrane; and I 13) cutting a chip and bonding a wire, which separates the element to a single element after the silicon anisotropic etching and electrically contacting each electrode to a predetermined contacting part to complete the element.

The piezoelectric micro-speaker according to the present invention is characterized in being produced by the above method.

Hereinafter, the present invention will be explained with reference to the accompanying drawings. Figs, la- 11 are cross-sectional views showing each producing step of a preferred embodiment of the method of producing the piezoelectric micro-speaker according to the present invention. That is, Fig. 11 is a cross-sectional view of final step showing a speaker which is produced by the method for producing a piezoelectric micro- speaker according to the present invention.

Fig. 2 is a view showing a preferred embodiment of the method of producing the piezoelectric micro-speaker according to the present invention.

After cleaning a silicon substrate 100, a 1 μ m-thick oxide film 102, 102' is grown on a substrate (200). The outside of a- surface of the silicon substrate 100 is etched to form corrugation 104 on the substrate (202). The corrugation 104 has a shape of surrounding the membrane in which a compressive thin film is deposited. The oxide film 102 is patterned in order to etch the silicon for forming the corrugation 104.

The oxide film 102 on both sides of the substrate 100 is removed by a wet-etching and 1 μ m-thick compressive thin films 106 are deposited (204). The compressive film 106 is selected from one of a non-stoichiometric nitride film, a bilayer of oxide film and nitride film, and a triple layer of nitride film, oxide film and nitride film. Since the compressive thin film 106 uses a film having a compressive residual stress, if the silicon is removed, wrinkles are formed around the membrane. Thus, it is used to make the upward/downward movement of the membrane become easier than the film having a tensile residual stress.

0.5 μ m-thick Al is deposited and patterned on the center portion of the substrate surface and a portion of the outside of the corrugation to form a bottom electrode 108, 108' (206). The bottom electrode 108 on the center portion is located inside of the corrugation. A piezoelectric thin film 110 covering the bottom electrode 108 formed on the center portion of the substrate is formed (208). The piezoelectric thin film 110 is selected from one of ZnO or A1N, and it is deposited and etched after computing its thickness considering the residual stress of the whole film to be produced. At this time, the dry etching can be used in order to minimize damages of the bottom electrode 108. Also, the piezoelectric thin film is patterned and etched such that the sides of the bottom electrode 108 are completely covered for the insulation between the top and bottom electrodes.

A bottom insulator film 112 is formed on the overall substrate surface (210). The bottom insulator film 112 is selected from one of parylene-C and parylene-D, and deposited with a thickness of 0.2 μ m. The bottom insulator 112 is deposited in order to prevent the electrical short between the bottom electrode and the porous piezoelectric material.

A top electrode 114, 114' is formed on the position on the bottom insulator film

112 of the center portion of the substrate so that it does not overlap with the bottom electrode 108' outside of the corrugation (212). The top electrode 114, 114' is deposited and patterned with a thickness of 0.5 μ m by Al having a thickness of 0.5 μ m or less, or by a predetermined thickness of Au/Cr bilayer considering the residual stress. The top electrode 114 on the center portion is also located inside of the corrugation structure and made to become a bit smaller than the bottom electrode 108. Thus, the electrical short and coupling between the top and bottom electrodes are prevented at the maximum.

At this time, the top electrode forms an integral electrode or an electrode separated by two parts, in order to generate the deformation of the piezoelectric film at the maximum. That is, if the two top electrodes are used as terminals for applying the voltage, the bottom electrode is used as a common electrode.

A top insulator film 116 is formed over the overall substrate surface (214). At this time, the parylene-C or parylene-D is deposited with a thickness of 1 μ m. This film 116 is deposited in order to enhance the mechanical strength at the time of cutting by the single chip in the last step and to control the stress of the end of the membrane.

An anchor region is formed by removing an insulator film 112, 116 on a portion of the outside of the substrate surface. This is to secure the adhering force of the seed layer 122. A compressive thin film 102' of the backside substrate is patterned to be removed (218). By this, the final area of the membrane is defined. After this patterning, the etching of the compressive film 102' proceeds by the reactive ion etching system.

A thick photoresistor 120 is formed on the substrate surface excluding the anchor region. The thick photoresistor 120 is applied in order to secure a cavity 128 between the membrane and the package using a non-electrolytic nickel film. At this time, the thick photoresistor 120 is hardened for 30 minutes at a temperature of 150 degree, in order to minimize the deterioration generated at the time of depositing the seed layer and to prevent micro cracks.

A seed thin film 122 is formed over the overall substrate surface (222). The seed film 122 is deposited with a thickness of 0.1 μ m by thermal depositing or sputtering with nickel-chrome alloy.

Atop photoresistor layer 124 is formed outside of the anchor region 118 and on a partial area of the center portion of the substrate (224). The thin photoresistor 124 is applied in order to form the non-electrolytic nickel layer in the region excluding the sound discharge gate, and to define and remove unnecessary parts on the element region after plating the non-electrolytic nickel.

A non-electrolytic nickel layer 126 is formed on the substrate surface excluding the top photoresistor layer 124 (226). The non-electrolytic nickel plating 126 is to form a package structure at a low temperature of 100 degree centigrade or less, and a metal thin film having a thickness of 5 μ m-10 μ m is deposited by a non-electrolytic plating method. At this time, the structure shown in Fig. 11 is produced by controlling the composition, the temperature and the pH of the plating solution for the residual stress of the plated nickel.

A thin film 124 and the thick photoresistor 120 under the surface and the bottom portion of the nickel layer are removed by an ultrasonic cleaning process in an acetone solution for a few seconds in order to form the cavity 128 (228). At the time of removing the top photoresistor layer 124, since the seed layer 122 under the bottom portion of the photoresistor, the acetone also removes the thick photoresistor layer 120 on the bottom portion of the seed layer through the sound discharging gate, which is formed at this time.

^' A membrane is formed by etching the silicon substrate 100 on the center portion and the bottom portion of the corrugation in a potassium hydroxide solution (KOH solution) (230). At this time, the machined element on the substrate is protected by using a silicon etching device and the silicon 100 is etched in an anisotropy etching solution for a predetermined time, in order to form the membrane.

A chip is cut and wire-bonded (232). The element is separated to an each single element after the silicon anisotropic etching and each electrode is electrically contacted to a predetermined contacting part to complete the element.

The operating theory of the piezoelectric micro-speaker according to the present invention is as follows: when arranging an electrode (an Al electrode, an electrode having Al in the bottom portion and Au/Cr in the upper portion, or a primary and secondary electrodes of the upper portion; the bottom electrode is used as a common electrode) located in the top and the bottom portions of the piezoelectric film to form a vibration plate having a sandwich structure and applying alternating current voltage outside thereto, the piezoelectric film is contracted or expanded according to the change of the polarity of the applied voltage. The contraction and the expansion according to the applied voltage cause the upward/downward movement of the membrane deposited on the silicon substrate. Accordingly, the air space of the top and bottom portion of the membrane moves.

At this time, due to the unique property of the membrane, which is produced to have the compressive residual stress, the artificial corrugation structure and the controlled residual stress of the membrane in overall, the membrane can easily cause a vibration having a big amplitude according to the alternating current voltage. That is, the combination of the unique property of the membrane itself with the artificial corrugation structure generates the vibration having the bigger amplitude more easily than the speaker having no artificial corrugation structure.

At this moment, the vibration of the membrane causes the pressure variation in the air space in the upper portion of the membrane, and the sound-pressure is generated accordingly. The generated sound-pressure is amplified by a package structure having a cavity located in the upper portion of the membrane, and the amplified sound-pressure is delivered to the air through the sound-pressure discharge gate formed on the package, so that the micro-speaker can have the bigger sound-pressure.

Industrial Applicability The piezoelectric micro-speaker of the present invention has effects of the easy production, the improvement of the dynamic function of the speaker membrane by the corrugation, and the improvement of the speaker output by the package. In addition, the present invention provides the micro-speaker in which the generation of the sound-pressure is increased by the combination of the membrane structure having a bigger amplitude with the artificial corrugation structure, which amplifies the generated sound-pressure is structurally amplified by the packages structure having a cavity, and the membrane is protected from a mechanical shock.

It is obvious for a person skilled in the art that such speaker of the present invention can be used for a voice generating device in a communication field, a micro- hearing aid for medical purpose, a voice generating device for toys, a thin film speaker used for a voice-playback type post card, and so on. Although the invention has been shown and described with respect to a preferred embodiment, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims. As shown in the accompanying claims, the scope of the present invention includes at least 1) a speaker structure having a membrane comprising a nitride film, a nitride/oxide film or a nitride/oxide/nitride film in order to form a membrane having a compressive residual stress; 2) a micro-speaker formed by a corrugation structure; 3) a micro-speaker using the combination of items 1) with 2); 4) a micro-speaker comprising a package having a cavity produced by micromachining in order to amplify the sound volume of the structure having items 1), 2) and 3); 5) a micro-speaker, produced by using an electrolytic or non-electrolytic plating technology for the metal in terms of forming a package having a cavity; and 6) a micro-speaker with the structures in all of the above items using piezoelectric films, such as ZnO and A1N for the speaker driving.

Claims

What is claimed is:

1. A method for producing a piezoelectric micro-speaker comprising the steps of:

1) etching an outer portion of a side of a silicon substrate to form corrugation on the substrate; 2) forming a compressive film on both sides of the substrate; 3) forming a bottom electrode at the center portion of the substrate surface surrounded by the corrugation and at an outer portion of the corrugation; 4) forming a piezoelectric film which covers the bottom electrode formed on the center portion of the substrate; 5) forming a bottom insulator film on the substrate surface; 6) forming a top electrode on the insulator film of the center portion of the substrate so that it cannot not overlap with the bottom electrode at the outer portion of the corrugation; 7) forming a top insulator film on the substrate surface; and 8) removing the silicon substrate at the center portion and the bottom portion of the corrugation.

2. The method for producing the piezoelectric micro-speaker as claimed in claim 1, wherein the compressive film in the step 2) is selected from the group of a non- stoichiometric nitride film, a bilayer of oxide film and mtride film, and a triple layer of nitride film, oxide film and mtride film.

3. The method for producing the piezoelectric micro-speaker as claimed in claim 1, wherein the piezoelectric film in the step 4) is ZnO or A1N.

4. The method for producing the piezoelectric micro-speaker as claimed in claim 1 , wherein the top and the bottom insulator films are one of parylene-C and parylene-D.

5. The method for producing the piezoelectric micro-speaker as claimed in claim 1, wherein the bottom electrode is a common electrode, and the top electrode is separated to two electrodes.

• 6. A method for producing a piezoelectric micro-speaker comprising the steps of:

a. forming an anchor region by removing an insulator film on an outer portion of the substrate surface on which the piezoelectric micro-speaker is formed; b. forming a thick photoresistor layer on the substrate surface excluding the anchor region; c. forming a seed film on the substrate surface; d. forming a top photoresistor layer at the outside of the anchor and a partial area of the center portion of the substrate; e. forming a non-electrolytic nickel layer on the substrate surface excluding the top photoresistor layer; and f. removing the top photoresistor layer, the seed layer contacting the top photoresistor layer and the thick photoresistor layer.

7. A piezoelectric micro-speaker, which is produced by a method in one of claims-6.