TWI860132B - Acoustic wave device and manufacturing method thereof - Google Patents

Abstract

An acoustic wave device and a method for manufacturing the same are provided. The acoustic wave device includes a piezoelectric substrate, an interdigital transducer, a pad, a passivation layer and a stack. The piezoelectric substrate has a surface, the interdigital transducer is disposed on the surface of the piezoelectric substrate, and the pad is disposed on the surface of the piezoelectric substrate and is electrically coupled to the interdigital transducer. The passivation layer is disposed on the interdigital transducer. The stack is disposed on the pad. The passivation layer is disposed in an open space.

Description

Sound wave device and method of manufacturing the same

The present invention relates to an acoustic wave device and a manufacturing method thereof, and in particular to a surface acoustic wave device and a manufacturing method thereof.

Surface acoustic wave (SAW) devices are used to convert and transmit electrical and acoustic signals, and are widely used in many fields. For example, SAW devices can be used in SAW filters, and SAW filters can filter noise, better retain wireless signals in the required frequency band, provide low transmission loss and anti-electromagnetic interference advantages, and are compact in size, so they are widely used in various communication products. In addition, SAW devices can also be used in resonators.

The SAW device can be assembled into a package body by flip chip bonding. The SAW device may include a piezoelectric substrate, an interdigital transducer (IDT) disposed on the piezoelectric substrate, and a solder layer (e.g., also referred to as a solder pad) electrically connected to the interdigital transducer. In addition, solder bumps containing tin, lead, or other metal materials may be formed on the solder layer, and through these solder bumps, the solder layer of the SAW device may be connected to, for example, an input solder layer, an output solder layer, and/or a ground solder layer disposed on the package body.

Electroplating or chemical plating processes may be used to form the connecting portion (e.g., copper pillars) of a SAW device. In such a process, various solutions may be used, which may cause corrosion or other adverse effects on the components (e.g., IDT) of the SAW device. Traditionally, a SAW device may be configured with a protective wall and a top cover to form a cavity covering the IDT. The IDT located in the cavity may be separated from the various solutions used in the electroplating process by the protective wall and the top cover. However, the protective wall and the top cover increase the size of the SAW device, which increases the cost and is not conducive to circuit miniaturization.

The present invention discloses a method for manufacturing an acoustic wave device, comprising: providing a piezoelectric substrate, wherein the piezoelectric substrate has a transducer and a welding layer, and the transducer is covered with a passivation layer; forming a photoresist layer to at least cover the transducer; patterning the photoresist layer to form a patterned photoresist layer, wherein the patterned photoresist layer covers the transducer and exposes the upper surface of the welding layer; forming a metal layer on the patterned photoresist layer; and peeling off the patterned photoresist layer.

The present invention also discloses an acoustic wave device, including a piezoelectric substrate, a transducer, a welding layer, a passivation layer, and a stack. The piezoelectric substrate has a surface. The transducer is disposed on the surface of the piezoelectric substrate. The welding layer is disposed on the surface of the piezoelectric substrate and is electrically connected to the transducer. The passivation layer is disposed on the transducer. The stack is disposed on the welding layer, wherein the passivation layer is disposed in an open space.

1:Sound wave device

10: Transducer

11: Stacking

20: Passivation layer

30: Welding layer

40: Seed layer

50:Metal layer

60: Solder ball

70: Piezoelectric substrate

70S: Surface

80: First photoresist layer

80p: First patterned photoresist layer

80S: Stairs

85: Second photoresist layer

85p: Second patterned photoresist layer

200: Manufacturing method

S201 to S211, S2011 to S2014, S203a and S205a, S203b and S205b: Steps

401 to 405, 501 to 505: Partial

6: Welding point

d1,d2,d3: thickness

G1,G2,g1: Gap

O1,O2: Opening

W1,W2: Width

Figure 1 is a cross-sectional schematic diagram of the acoustic wave device in an embodiment of the present invention.

Figure 2 is a schematic flow chart of the manufacturing method of the acoustic wave device in Figure 1.

Figures 3 to 8 show schematic diagrams of the steps of the method for manufacturing the acoustic wave device in Figure 1.

Figure 1 is a cross-sectional schematic diagram of the acoustic wave device 1 in an embodiment of the present invention. The acoustic wave device 1 may be, for example, a surface acoustic wave (SAW) device. In some embodiments, the acoustic wave device 1 may receive a radio frequency signal from an antenna, convert the radio frequency signal into a sound wave, process the sound wave to generate a filtered signal, and output the filtered signal. The radio frequency signal and the filtered signal may be electrical signals. This is only an example to illustrate the use of the acoustic wave device 1, but the present invention is not limited thereto. In other embodiments, the acoustic wave device 1 may also be used for other purposes.

In some embodiments, the acoustic wave device 1 may include a piezoelectric substrate 70, a transducer 10, at least one welding layer 30, a passivation layer 20, and a stack 11, wherein the welding layer 30 may be electrically connected to the transducer 10. During the manufacturing process of the acoustic wave device 1, before, for example, electroplating, the transducer 10 may be covered with a passivation layer 20 and a photoresist layer to protect the transducer 10 from contamination or corrosion by a chemical solution. After the electroplating is completed, the photoresist layer may be removed, and the passivation layer 20 may remain on the transducer 10. The acoustic wave device 1 may have reduced weight, reduced circuit size, reduced manufacturing cost, and/or enhanced performance. In some embodiments, the area of the acoustic wave device 1 may be, for example, 1.1 millimeters (mm) * 0.9 mm.

In some embodiments, the piezoelectric substrate 70 has a surface 70S. The transducer 10 and the welding layer 30 are disposed on the surface 70S of the piezoelectric substrate 70. For example, the transducer 10 may include an interdigital transducer (IDT), which includes at least one input interdigital finger and one output interdigital finger. In some examples, the input interdigital finger and the output interdigital finger may be referred to as a pair of interdigital structures. The input interdigital finger and the output interdigital finger may be disposed adjacent to each other but not connected, for example, there may be a gap g1 therebetween. The passivation layer 20 may be disposed to cover the transducer 10. In some embodiments, the passivation layer 20 may be disposed in an open space (i.e., not a closed or sealed space). In other words, the passivation layer 20 is not disposed in a protective cavity.

As shown in FIG. 1 , the transducer 10 may have a thickness d2, which may be, for example, between 1000 angstroms (Å) and 1500 Å. Corresponding to the positions of the interdigitated fingers of the transducer 10, the passivation layer 20 may have a thickness d3, which may be, for example, between 200 Å and 300 Å. In addition, corresponding to the gaps (between the interdigitated fingers of the transducer 10), the passivation layer 20 may have a thickness d1, as shown in FIG. 1 , the thickness d1 may be greater than the thickness d3, but the present invention is not limited thereto. In other embodiments, the passivation layer 20 may be conformally formed on the transducer 10, in which case the thickness d1 may be substantially equal to the thickness d3. The passivation layer 20 may include materials such as silicon oxide (e.g., silicon dioxide) and silicon nitride. In the above examples, the input fingers and output fingers are only used for illustration and are not intended to limit the functions of the fingers, i.e., the input fingers may also have output functions, and/or the output fingers may also have input functions.

In some embodiments, the welding layer 30 is electrically connected to the transducer 10 to transmit a radio frequency signal to the transducer 10 or to receive a filtered signal from the transducer 10. In some embodiments, the transducer 10 and the welding layer 30 may be made of the same metal or different metals. For example, the transducer 10 and/or the welding layer 30 may include materials such as molybdenum (Mo), copper (Cu), aluminum (Al), gold (Au), platinum (Pt), tungsten (W), nickel (Ni), silver (Ag), tantalum (Ta) or a combination thereof. In addition, the transducer 10 and the welding layer 30 may be formed in the same process or in different processes. As shown in FIG. 1 , gaps G1 and G2 may be formed between the passivation layer 20 and the welding layer 30. However, in other embodiments not shown, there may be no gap G1 or G2 between the passivation layer 20 and the welding layer 30, that is, the passivation layer 20 may contact the welding layer 30.

In some embodiments, the piezoelectric substrate 70 is a single-layer structure, which may include the following piezoelectric materials: zinc oxide (ZnO), aluminum nitride (AlN), lithium niobate (LiTaO3, LT), lithium niobate (LN), quartz (QZ), lead titanate (PTO), lead zirconium titanate (PZT) and other materials or combinations thereof. In other embodiments, the piezoelectric substrate 70 may be a multi-layer structure, which may include a base layer and a piezoelectric layer disposed thereon, the base layer may include silicon, for example, and the piezoelectric layer may include at least one of the above-mentioned piezoelectric materials. In some other embodiments, the piezoelectric material may also include other types of piezoelectric single crystals, piezoelectric polycrystals (including piezoelectric ceramics), piezoelectric polymers and/or piezoelectric composites.

In some embodiments, the stack 11 is disposed on the solder layer 30, and the stack 11 includes a seed layer 40, a metal layer 50, and a solder ball 60 stacked in sequence. In some examples, the seed layer 40 can be omitted, so that the metal layer 50 is directly formed on the solder layer 30. For example, the seed layer 40 can include materials such as titanium (Ti), nickel (Ni) or their alloys. The metal layer 50 can include materials such as copper (Cu), aluminum (Al), nickel (Ni), tin (Sn), silver (Ag) or their alloys. The solder ball 60 can include materials such as tinned (Sn) or lead (Pb). In some examples, the thickness of the metal layer 50 can be greater than the thickness of the seed layer 40. In some examples, metal layer 50 may be referred to as under-bump metallization (UBM).

FIG. 2 is a flow chart of a manufacturing method 200 of the acoustic wave device 1. The manufacturing method 200 includes steps S201 to S211. Step S201 is used to form a patterned photoresist layer on the transducer to at least cover the transducer. Steps S203 and S205 are used to form a seed layer and a metal layer, respectively, and step S207 is used to peel off the patterned photoresist layer. Steps S209 and S211 are used to form solder balls in sequence. Any reasonable technical changes or step adjustments are within the scope of the present invention. The details of steps S201 to S211 are as follows: Step S201: forming a patterned photoresist layer to at least cover the transducer; Step S203: forming a seed layer on the patterned photoresist layer; Step S205: forming a metal layer on the seed layer; Step S207: peeling off the patterned photoresist layer; Step S209: performing screen printing on the metal layer to form welding points; Step S211: performing back soldering on the welding points to form solder balls.

The manufacturing method 200 is further described below with reference to FIGS. 3 to 8.

FIG. 3 is a schematic diagram of step S201, which may include steps S2011 to S2013. In step S2011, a piezoelectric substrate 70 is provided, and a transducer 10 and a welding layer 30 are formed on a surface 70S of the piezoelectric substrate 70, and the welding layer 30 can electrically connect the transducer 10. A passivation layer 20 is covered on the transducer 10. In some examples, the passivation layer 20 can protect the transducer 10 from particles and solutions. In step S2012, a first photoresist layer 80 is formed to at least cover the transducer 10. In the present embodiment, the first photoresist layer 80 covers the transducer 10, the passivation layer 20 and the welding layer 30 on the piezoelectric substrate 70. In some examples, the first photoresist layer 80 can be formed by processes such as coating and baking. Then, in step S2013, the first photoresist layer 80 is patterned to form a first patterned photoresist layer 80p, wherein the first patterned photoresist layer 80p exposes the upper surface of the welding layer 30. In detail, the first patterned photoresist layer 80p may include at least one first opening O1, and the upper surface of the welding layer 30 may be exposed from the first opening O1. In some embodiments, the first photoresist layer 80 may be patterned by a photolithography process. For example, the first photoresist layer 80 may be a positive photoresist or a negative photoresist. Taking the positive photoresist as an example, the photoresist material on the welding layer 30 may be removed after irradiation (e.g., by ultraviolet light, deep ultraviolet light, electron beam, ion beam or X-ray irradiation) to form the first opening O1. Thus, through step S201, a first patterned photoresist layer 80p is formed, which at least covers the transducer 10, and further covers the passivation layer 20 on the transducer 10.

FIG. 4 is a schematic diagram of steps S203 and S205. After step S201, a seed layer 40 is formed on the upper surface of the welding layer 30 and the first patterned photoresist layer 80p (step S203). As shown in FIG. 4, the seed layer 40 may include a first portion 401 and a second portion 402 extending horizontally, the first portion 401 being formed on the upper surface of the welding layer 30, and the second portion 402 being formed on the upper surface of the first patterned photoresist layer 80p. The seed layer 40 may be used to provide a better surface for the subsequently formed metal layer 50, and may also be used to increase the thickness. In some examples, the seed layer 40 may be formed, for example, by sputter deposition, and may provide sufficient conductivity for subsequent electroplating processes. In some examples, the seed layer 40 may further include a third portion 403 extending vertically, which may be formed on the sidewall of the first patterned photoresist layer 80p. For example, the sidewall of the first patterned photoresist layer 80p may be partially adjacent to the sidewall of the welding layer 30. In some examples, the third portion 403 of the seed layer 40 is thinner than the first portion 401 and thinner than the second portion 402. Further, the seed layer 40 may not include the third portion 403, that is, the seed layer 40 may not be formed on the sidewall of the first patterned photoresist layer 80p. In this way, by step S203, the seed layer 40 is formed on the first patterned photoresist layer 80p.

After step S203, as shown in FIG. 4, a metal layer 50 is formed on the seed layer 40 (step S205). The metal layer 50 may include a first portion 501 and a second portion 502 extending horizontally, the first portion 501 being formed on the first portion 401 of the seed layer 40, and the second portion 502 being formed on the second portion 402 of the seed layer 40. In some examples, the metal layer 50 may be formed by, for example, evaporation, sputtering, electroplating, or electroless plating. In some examples, the metal layer 50 may further include a third portion 503 extending vertically, which may be formed on the third portion 403 of the seed layer 40. In some examples, the third portion 503 of the metal layer 50 is thinner than the first portion 501 and the second portion 502. Furthermore, the metal layer 50 may not include the third portion 503, that is, the metal layer 50 may not be formed on the third portion 403 of the seed layer 40. Thus, by step S205, the metal layer 50 is formed on the seed layer 40. It should be noted that the terms "upper" or "lower" in this article are not limited to the horizontal position relationship, and may also include the vertical position relationship.

As described above, in some embodiments, step S203 may be omitted, that is, step S205 may be performed directly after step S201, so that a metal layer 50 is formed on the upper surface of the welding layer 30 and the first patterned photoresist layer 80p. In the process of forming the seed layer 40 and/or the metal layer 50, at least one electrolyte may be used. Since the transducer 10 is covered by the first patterned photoresist layer 80p and the passivation layer 20, it can be protected from the influence of the electrolyte, that is, the first patterned photoresist layer 80p can be used to protect the transducer 10 from contamination or corrosion by the electrolyte.

FIG. 5 is a schematic diagram showing steps S207 to S211. After step S205, as shown in FIG. 5, at least a portion of the first patterned photoresist layer 80p is removed by a stripping or lift-off process in step S207. In this example, the second portion 402 of the seed layer 40 and the second portion 502 of the metal layer 50 located on the first patterned photoresist layer 80p can be substantially removed together, that is, the main portion is removed and a small amount of residual portion can remain. In addition, the first portion 401 of the seed layer 40 and the first portion 501 of the metal layer 50 can remain on the soldering layer 30. Specifically, the first portion 401 of the seed layer 40 has a stronger adhesion strength with the soldering layer 30 than the adhesion strength between the first patterned photoresist layer 80p and the passivation layer 20 and/or the adhesion strength between the first patterned photoresist layer 80p and the piezoelectric substrate 70. In other words, the adhesion between the first portion 401 of the seed layer 40 and the soldering layer 30 may be greater than the adhesion between the first patterned photoresist layer 80p and the passivation layer 20, and may also be greater than the adhesion between the first patterned photoresist layer 80p and the piezoelectric substrate 70. Furthermore, in step S207, the third portion 403 of the seed layer 40 and/or the third portion 503 of the metal layer 50 may also be substantially removed.

In some examples, step S207 can be performed by a high-voltage stripping machine. For example, the high-voltage stripping machine may include a processing tank, an ultrasonic device, and a high-voltage spraying device. For example, the piezoelectric substrate 70 (and the layers thereon) can be placed in the processing tank, vibrated by an ultrasonic device, and sprayed with a solvent by a high-voltage spraying device to substantially remove the first patterned photoresist layer 80p and the seed layer 40 and the metal layer 50 thereon. This example is only used for illustration and is not intended to limit the present invention. In other cases, step S207 can also be performed by other types of stripping machines.

In some embodiments, the third portion 403 (if present) of the seed layer 40 and the third portion 503 (if present) of the metal layer 50 are thinner and thus can be easily damaged, so that the high-pressure spraying solvent used in step S207 can remove the first patterned photoresist layer 80p on the transducer 10 through the damaged third portion 403 of the seed layer 40 and the damaged third portion 503 of the metal layer 50, thereby removing the second portion 402 of the seed layer 40 and the second portion 502 of the metal layer 50 on the first patterned photoresist layer 80p. After step S207, the passivation layer 20 can still remain on the transducer 10. In some additional embodiments, a cleaning step may be optionally performed by plasma treatment (not shown) during and/or after step 207. Thus, by step S207, the first patterned photoresist layer 80p is stripped.

Next, as shown in FIG. 5 , in step S209 , screen printing (e.g., printing solder paste) is performed on the metal layer 50 (e.g., on the first portion 501 of the metal layer 50 ) to form a soldering point 6. Then, as shown in FIG. 5 , in step S211 , the soldering point 6 is reflowed to form a solder ball 60 , which may be a metal solder ball, for example. In some embodiments, the first portion 401 of the seed layer 40 on the soldering layer 30 , the first portion 501 of the metal layer 50 , and the solder ball 60 may be referred to as a stack 11 . Thus, the solder ball 60 is formed by steps S209 and S211 .

In a further embodiment, step S201 may additionally include step S2014. As shown in FIG. 6, a second patterned photoresist layer 85p is formed on the first patterned photoresist layer 80p (step S2014), and the second patterned photoresist layer 85p exposes the upper surface of the welding layer 30. The step of forming the second patterned photoresist layer 85p may be similar to the step of forming the first patterned photoresist layer 80p (e.g., step S2012 and step S2013). In some embodiments, the step of forming the second patterned photoresist layer 85p may include first forming a second photoresist layer 85 (not shown) and then patterning the second photoresist layer 85 to form a patterned second patterned photoresist layer 85p.

In some embodiments, a second photoresist layer 85 (not shown) may be formed after step S2012 (forming the first photoresist layer 80, see FIG. 3), and then the first photoresist layer 80 and the second photoresist layer 85 may be patterned to form a first patterned photoresist layer 80p and a second patterned photoresist layer 85p, respectively. In this example, the first photoresist layer 80 may be patterned by a first photolithography process, and then the second photoresist layer 85 may be patterned by a second photolithography process. In other embodiments, patterning the second photoresist layer 85 and patterning the first photoresist layer 80 may be performed simultaneously, for example, the first photoresist layer 80 and the second photoresist layer 85 may be patterned by the same photolithography process at the same time.

In other embodiments, a second photoresist layer 85 (not shown) may be formed after step S2013 (forming a first patterned photoresist layer 80p, see FIG. 3), and then the second photoresist layer 85 may be patterned to form a second patterned photoresist layer 85p.

As shown in FIG. 6 , the second patterned photoresist layer 85p may include at least one second opening O2, and the upper surface of the welding layer 30 may be exposed from the second opening O2. In some embodiments, the second opening O2 may be substantially aligned with the first opening O1. For example, in a direction perpendicular to the surface of the piezoelectric substrate 70, the second opening O2 may partially overlap with the first opening O1. The second opening O2 may be located on the first opening O1, and the width W2 of the second opening O2 is wider than the width W1 of the first opening O1, so as to form a step 80S at the junction of the first patterned photoresist layer 80p and the second patterned photoresist layer 85p, but the present invention is not limited thereto. In other embodiments, the width W2 of the second opening O2 may be equal to the width W1 of the first opening O1, so that no step 80S is formed at the junction of the first patterned photoresist layer 80p and the second patterned photoresist layer 85p. The total thickness of the first patterned photoresist layer 80p and the second patterned photoresist layer 85p may be between 20-30 micrometers. In this embodiment, the combination of the first patterned photoresist layer 80p and the second patterned photoresist layer 85p may provide a greater thickness, so that the seed layer formed in the subsequent step S203 has a thinner vertical portion (e.g., a thinner third portion 403), which is conducive to performing step S207.

Similar to step S203 and step S205, Figures 7 and 8 show step S203a and step S205a, respectively.

In step S203a, a seed layer 40 is formed on the upper surface of the solder layer 30 and the second patterned photoresist layer 85p. As shown in FIG. 7 , the seed layer 40 is further formed on the step 80S (if any). In detail, the seed layer 40 may include a first portion 401, a second portion 402, a third portion 403, a fourth portion 404 and a fifth portion 405, wherein the first portion 401 extends horizontally and is formed on the upper surface of the welding layer 30, the second portion 402 extends horizontally and is formed on the step 80S (i.e., on the upper surface of the first patterned photoresist layer 80p), the third portion 403 extends vertically and is formed on the side wall of the step 80S (i.e., on the side wall of the first patterned photoresist layer 80p), the fourth portion 404 extends vertically and is formed on the side wall of the second patterned photoresist layer 85p, and the fifth portion 405 extends horizontally and is formed on the upper surface of the second patterned photoresist layer 85p. In some embodiments, among the portions of the seed layer 40, the vertically extending portion (e.g., the third portion 403 and/or the fourth portion 404) may be thinner than the horizontally extending portion (e.g., the first portion 401, the second portion 402, and/or the fifth portion 405). For example, the third portion 403 is thinner than the first portion 401 and the second portion 402, and the fourth portion 404 is thinner than the second portion 402 and the fifth portion 405. In some embodiments, the seed layer 40 may not include the third portion 403 and/or the fourth portion 404.

In step S205a, a metal layer 50 is formed on the seed layer 40. As shown in FIG. 8 , the metal layer 50 may include a first portion 501, a second portion 502, a third portion 503, a fourth portion 504, and a fifth portion 505, which are respectively located on the first portion 401, the second portion 402, the third portion 403, the fourth portion 404, and the fifth portion 405 of the seed layer 40. Similarly, the third portion 503 of the metal layer 50 is thinner than the first portion 501 and the second portion 502, and the fourth portion 504 of the metal layer 50 is thinner than the second portion 502 and the fifth portion 505. In some embodiments, the metal layer 50 may not include the third portion 503 and/or the fourth portion 504.

In some embodiments, step S203a may be omitted, that is, step S205a may be performed directly after step S201, thereby forming a metal layer 50 on the upper surface of the welding layer 30 and the second patterned photoresist layer 85p.

As described above, the combination of the first patterned photoresist layer 80p and the second patterned photoresist layer 85p can provide a greater thickness, so that the seed layer 40 and the metal layer 50 formed subsequently have thinner vertical portions, so that they can be more easily destroyed, so that the high-pressure spraying solvent used in step S207 can more easily remove the first patterned photoresist layer 80p and/or the second patterned photoresist layer 85p through the destroyed vertical portions.

According to the embodiment of the present invention, the acoustic wave device 1 can be manufactured relatively cheaply and simply. For example, in the step of forming the metal layer 50 (e.g., step S205), the first patterned photoresist layer 80p can cover the passivation layer 20 and the transducer 10 to isolate the transducer 10 from the electrolyte and prevent it from being contaminated or corroded. In addition, in the stripping (or stripping) process, the first patterned photoresist layer 80p is removed, and the passivation layer 20 can be retained on the transducer 10 to protect the transducer 10 from damage during the stripping (or stripping) process, thereby maintaining the transducer 10 in an ideal shape and size, thereby maintaining or improving the performance (such as quality factor and/or target frequency) of the acoustic wave device 1. In other words, without the passivation layer 20, the stripping process may change the shape and/or size of the transducer 10, causing a frequency shift. Furthermore, a second patterned photoresist layer 85p may be additionally formed on the first patterned photoresist layer 80p, and the combination thereof may provide a greater thickness, which is beneficial for performing subsequent stripping steps, thereby providing a better yield.

According to the embodiment of the present invention, the acoustic wave device 1 can omit the protective wall and the top cover, which is conducive to the miniaturization of the acoustic wave device 1 and the simplification of the manufacturing process. In addition, the acoustic wave device 1 can have a more accurate operating frequency. Therefore, the present invention provides a cost-effective, compact and superior acoustic wave device. The above is only a preferred embodiment of the present invention, and all equal changes and modifications made according to the scope of the patent application of the present invention should be within the scope of the present invention.

1:Sound wave device

10: Transducer

11: Stacking

20: Passivation layer

30: Welding layer

40: Seed layer

50:Metal layer

60: Solder ball

70: Piezoelectric substrate

70S: Surface

d1,d2,d3: thickness

G1,G2,g1: Gap

Claims (19)

A method for manufacturing an acoustic wave device comprises: providing a piezoelectric substrate, wherein a transducer and a welding layer are disposed on the piezoelectric substrate, and a passivation layer is covered on the transducer; forming a first photoresist layer on the piezoelectric substrate; patterning the first photoresist layer to form a first patterned photoresist layer, wherein the first patterned photoresist layer covers an upper surface of the passivation layer and exposes an upper surface of the welding layer; forming a metal layer on the upper surface of the welding layer and the first patterned photoresist layer; and peeling off the first patterned photoresist layer. The method as described in claim 1 further comprises: before forming the metal layer, forming a seed layer on the upper surface of the welding layer and the first patterned photoresist layer, wherein the seed layer includes a first portion located on the upper surface of the welding layer and a second portion located on an upper surface of the first patterned photoresist layer; and forming the metal layer on the seed layer, wherein the metal layer includes: a first portion located on the first portion of the seed layer; and a second portion located on the second portion of the seed layer. The method as described in claim 2, wherein: the seed layer further includes a third portion located on a side wall of the first patterned photoresist layer, the third portion of the seed layer is thinner than the first portion of the seed layer and the second portion of the seed layer; and the metal layer further includes a third portion located on the third portion of the seed layer, the third portion of the metal layer is thinner than the first portion of the metal layer and the second portion of the metal layer. The method as described in claim 2, wherein stripping the first patterned photoresist layer comprises: performing a stripping process by a high-pressure stripping machine to remove at least a portion of the first patterned photoresist layer, wherein the second portion of the seed layer and the second portion of the metal layer are removed, and the first portion of the seed layer and the first portion of the metal layer are retained. The method as described in claim 1, wherein the first patterned photoresist layer includes at least one first opening, and the at least one first opening exposes the upper surface of the welding layer. The method as described in claim 1, wherein the step of forming the metal layer comprises: forming the metal layer by an electroplating process, wherein an electrolyte is used in the electroplating process, and wherein in the electroplating process, the first patterned photoresist layer and the passivation layer separate the transducer from the electrolyte. The method as described in claim 1 further includes forming a second photoresist layer on the first photoresist layer after forming the first photoresist layer. The method as described in claim 7 further comprises: patterning the second photoresist layer to form a second patterned photoresist layer, wherein the second patterned photoresist layer exposes the upper surface of the welding layer; wherein patterning the second photoresist layer is performed after patterning the first photoresist layer; or patterning the second photoresist layer and patterning the first photoresist layer are performed simultaneously. The method as described in claim 8, wherein: The second patterned photoresist layer includes at least one second opening, and the at least one second opening exposes the upper surface of the welding layer; and in the vertical direction, the at least one second opening at least partially overlaps with the at least one first opening. The method as described in claim 9, wherein the at least one second opening is located on the at least one first opening and is wider than the at least one first opening, so that a step is formed at a junction of the first patterned photoresist layer and the second patterned photoresist layer. The method as described in claim 10 further includes forming a seed layer before forming the metal layer; wherein the seed layer includes: a first portion located on the upper surface of the welding layer; a second portion located on an upper surface of the first patterned photoresist layer; a third portion located on a side wall of the first patterned photoresist layer; a fourth portion located on a side wall of the second patterned photoresist layer; and a fifth portion located on an upper surface of the second patterned photoresist layer. The method as described in claim 11, wherein stripping the first patterned photoresist layer comprises: performing a stripping process by a high-pressure stripper to remove at least a portion of the first patterned photoresist layer and at least a portion of the second patterned photoresist layer; wherein the second portion of the seed layer and the fifth portion of the seed layer are removed, and the first portion of the seed layer is retained. The method as described in claim 7, wherein the total thickness of the first patterned photoresist layer and the second patterned photoresist layer is between 20-30 microns. The method as described in claim 1, wherein providing the piezoelectric substrate comprises: forming the transducer on a first surface of the piezoelectric substrate; forming the welding layer on the first surface of the piezoelectric substrate, the welding layer electrically connected to the transducer; forming the passivation layer on the transducer and the welding layer; and patterning the passivation layer to expose the upper surface of the welding layer. A method as described in claim 14, wherein a gap is formed between the passivation layer and the welding layer. The method as described in claim 1 further comprises: performing a screen printing on the metal layer to form a welding point; and performing back soldering on the welding point to form a solder ball. An acoustic wave device comprises: a piezoelectric substrate having a first surface; a transducer disposed on the first surface of the piezoelectric substrate; a welding layer disposed on the first surface of the piezoelectric substrate and electrically connected to the transducer; a passivation layer disposed on the transducer; and a stack disposed on the welding layer; wherein the passivation layer is disposed in an open space. An acoustic wave device as described in claim 17, wherein the stack includes a seed layer, a metal layer, and a solder ball stacked in sequence. An acoustic wave device as described in claim 17, wherein the piezoelectric substrate comprises: a base layer; and a piezoelectric layer disposed on the base layer.

Images