CN101465628A - A kind of film bulk acoustic resonator and its preparation method - Google Patents

Abstract

The invention discloses a thin-film bulk acoustic resonator, which includes a substrate, a buffer layer, a piezoelectric layer and an electrode. The groove and the substrate form an air gap with a smooth upper convex edge and completely cover the air gap. The height of the lower top surface of the air gap is lower than the substrate, and has a flat surface and a gently changing boundary; ②The buffer layer The edge of the surface in contact with the air gap and adjacent to the substrate is in a smooth convex shape, the piezoelectric layer is arranged on the buffer layer, and the electrodes include a bottom electrode and a top electrode, and the bottom electrode is arranged in the piezoelectric layer on the buffer layer, The top electrode is disposed on top of the piezoelectric layer. The bulk acoustic wave resonator has a novel structure, and the method can produce a FBAR with a stable structure and low loss on a substrate without using a CMP process, which is beneficial for integration into a CMOS chip.

Description

A kind of film bulk acoustic resonator and its preparation method

technical field

The invention relates to the technical field of thin-film bulk acoustic wave resonators, in particular to a thin-film bulk acoustic wave resonator and a preparation method thereof.

Background technique

The multi-functional development of wireless communication terminals has put forward high technical requirements for radio frequency devices such as miniaturization, high frequency, high performance, low power consumption, and low cost. Traditional surface acoustic wave devices have large insertion loss in the high-frequency band above 2.4GHz, and microwave dielectric technology has good performance but is too bulky. Film Bulk Acoustic Resonator (FBAR) technology is a new radio frequency device technology that has emerged in recent years with the improvement of processing technology and the rapid development of modern wireless communication technology, especially personal wireless communication technology. It has the advantages of extremely high Q value (above 1000) and can be integrated on an IC chip, while effectively avoiding the shortcomings of SAW resonators and dielectric resonators.

FBAR is a thin-film device with a sandwich structure of electrode-piezoelectric film-electrode fabricated on the substrate material. PZT, ZnO and aluminum nitride are usually used as piezoelectric materials for FBAR. Among them, aluminum nitride has the highest sound velocity, so it is applied to higher frequencies, which meets the requirements of the development of high-frequency wireless communications. In addition, aluminum nitride has the advantages of low temperature loss, good chemical stability and relatively simple preparation process compared with the other two materials. In addition, materials such as zinc, lead, and zirconium are dangerous materials for CMOS processes because they will seriously reduce the lifetime of carriers in semiconductors, while aluminum nitride does not have this problem. Therefore, AlN is an ideal material for FBAR compatibility in CMOS devices.

The structure of FBAR includes air gap type, Bragg reflective type (SMR) and backside etching type. Among them, the air gap type FBAR has a higher Q value than the SMR type, and the loss is smaller; compared with the backside etching type FBAR, it does not need to remove a large area of the substrate, and its mechanical strength is higher. Therefore, the air-gap FBAR is the first choice for integration on CMOS devices.

Usually, the structure of the air-gap FBAR is shown in Fig. 1 . It includes a substrate, an air gap on the substrate, a bottom electrode, a piezoelectric layer and a top electrode sequentially fabricated across the air gap on the substrate. A common process method is: first anisotropically etches a pit on the silicon substrate, and then fills the pit with a sacrificial layer material, which can be Al, Mg or silicon dioxide. After the surface is polished by CMP, a layer of metal film is grown by sputtering, and the bottom electrode pattern is etched corresponding to the position above the sacrificial layer. Then a piezoelectric film is deposited on the bottom electrode, and after etching, the piezoelectric film covers the boundary of the pit on the substrate and exposes the lead-out end of the bottom electrode. Next, a metal film is deposited on the piezoelectric film, and the top electrode pattern is etched. Next, a window is etched on the piezoelectric layer by dry etching to partially expose the sacrificial layer. Finally, the sacrificial layer is released from the engraved release window, and the FBAR across the air gap on the substrate is fabricated.

In order to reduce the manufacturing difficulty and improve the performance of FBAR, predecessors have proposed many improvement measures, such as "Improved Method for Making Thin Film Bulk Acoustic Resonator and the Structure of Thin Film Bulk Acoustic Resonator" by Agilent Technologies Co., Ltd. (published No.: CN1373556, publication date: 2002.10.09), it is proposed to add a channel on the substrate to facilitate the manufacture of the sacrificial layer release hole. "Method Of Producing Thin Film Bulk Acoustic Resonator" by Tetsuo Yamada et al. (public number: US7140084B2, publication date: 2006.11.28) proposed to reduce the surface roughness of the film and improve the quality of FBAR. However, these improved structures still have disadvantages: 1. In order to ensure good frequency stability of the FBAR, the area between the bottom electrode and the top electrode needs to be very flat. It is necessary to use high-precision CMP technology to smooth the silicon surface, which increases the requirements for processing equipment and thickness precision control of silicon wafers, and is not conducive to processing FBAR on CMOS chips. 2. Usually there is an amorphous transition region with a certain thickness on the piezoelectric layer. When the application frequency of FBAR rises from a few GHz to tens of GHz, the piezoelectric layer will be made thinner and thinner, and the piezoelectric film with preferred orientation The ratio of the layer thickness to the thickness of the amorphous transition region will decrease, which will lead to a larger insertion loss and lower Q value of the FBAR. This limits the development of FBAR towards high frequency.

Therefore, there is still such a need to improve the manufacturing technology of the air-gap type FBAR, that is, the surface of the flat FBAR resonance region can be obtained by using the conventional CMOS production process, so as to reduce the manufacturing difficulty of the FBAR and improve the compatibility with the CMOS circuit. And use appropriate technology to improve the quality of piezoelectric film, reduce the insertion loss of FBAR and increase the application frequency.

Contents of the invention

The problem to be solved by the present invention is: how to provide a thin-film bulk acoustic resonator and its preparation method. The CMP process is conducive to integration in CMOS chips.

The technical problem proposed by the present invention is solved like this: provide a kind of film bulk acoustic resonator, comprise substrate, buffer layer, piezoelectric layer and electrode, it is characterized in that:

①The upper end surface of the substrate is provided with a smooth groove and a buffer layer. The buffer layer spans the groove and the substrate to form an air gap with a smooth upper convex edge and completely covers the air gap. The lower top surface of the air gap Higher than the substrate, with a flat surface and gently varying boundaries;

② The edge of the buffer layer in contact with the air gap and adjacent to the substrate has a smooth convex shape, the piezoelectric layer is arranged on the buffer layer, and the electrodes include a bottom electrode and a top electrode, and the bottom electrode is arranged on the buffer layer Within the piezoelectric layer, the top electrode is disposed on top of the piezoelectric layer.

According to the film bulk acoustic resonator provided by the present invention, the buffer layer is an amorphous aluminum nitride layer, the piezoelectric layer is a c-axis oriented aluminum nitride layer, and the substrate is a silicon substrate.

The film bulk acoustic resonator provided by the present invention is characterized in that a sacrificial layer release window is provided, and the release window is located between the boundary of the air gap and the boundary of the electrode.

A method for preparing a thin-film bulk acoustic resonator, comprising the following steps:

① Prepare a layer of silicon nitride film on the substrate;

② Photoetching a window with the outline of the sacrificial layer on the silicon nitride film, that is, the outline of the air gap on the substrate, and etching away the silicon nitride at the window;

③ Form a silicon dioxide sacrificial layer at the window by local thermal oxidation, and control the thermal oxidation time to obtain the required thickness of the sacrificial layer. The silicon dioxide sacrificial layer has a flat surface and a gently changing boundary;

④Remove the remaining silicon nitride;

⑤Grow a layer of amorphous aluminum nitride at low temperature by sputtering, that is, the required buffer layer, to form an air gap;

⑥Grow a metal layer by sputtering on the upper surface of the buffer layer and photoetch the bottom electrode pattern;

⑦ After forming the bottom electrode, grow a piezoelectric layer on the bottom electrode and etch the piezoelectric layer pattern, the boundary of the piezoelectric layer pattern is larger than the boundary of the air gap;

⑧ Deposit the top electrode on the upper surface of the piezoelectric layer, and etch the pattern of the top electrode;

⑨ Release the sacrificial layer and dry.

A method for preparing a bulk acoustic wave resonator, comprising the following steps:

① Deposit a layer of 80-120nm silicon nitride on the polished silicon surface by CVD method, the polished surface is (100), (110) or (111) orientation, and then use photolithography to place The silicon nitride is removed;

② On the above-mentioned silicon surface grown with silicon nitride, a layer of silicon dioxide is grown in the area where the silicon nitride is removed by partial wet thermal oxidation treatment, and the thickness of the silicon dioxide is between 400nm and 800nm;

③Remove residual silicon nitride on the silicon surface;

④ After the above steps, grow a layer of 60-80nm amorphous aluminum nitride layer on the silicon surface, then sputter and grow a layer of metal on the amorphous aluminum nitride layer and photoetch the bottom electrode pattern. The electrode material is molybdenum, the thickness is 100-200nm, and the amorphous aluminum nitride is obtained by radio frequency sputtering under the conditions of less than 200°C and power density <5W·cm ^-2 ;

⑤ After the above steps, a layer of c-axis oriented aluminum nitride piezoelectric layer is grown on the silicon surface. The thickness of the aluminum nitride piezoelectric layer is determined according to the frequency range of the actual application. The c-axis oriented aluminum nitride piezoelectric layer Obtained by radio frequency sputtering under the conditions of temperature>250℃, power density>10W·cm ^-2 and nitrogen concentration>50%;

⑥ Etching out the pattern of the piezoelectric layer by wet method, the boundary contour of the piezoelectric layer pattern is larger than the boundary contour of the air gap, while etching the pattern of the piezoelectric layer, the release window of the sacrificial layer is also etched out, and the release window is located at between the air gap boundary and the electrode boundary;

⑦ Sputtering deposits the top electrode metal, and etches the top electrode pattern. The top electrode material is usually molybdenum, with a thickness of 100-200nm;

⑧ Release the sacrificial layer and dry.

Beneficial effects of the present invention: the present invention is innovative in structure, and it has a flat surface and an air gap with a gently changing boundary, which meets the flatness requirement of the resonance region between the FBAR bottom electrode and the top electrode, and is gentle at the same time The changing boundary avoids the problem of poor coverage of the RF sputtering coating on the steep steps, so the film at the boundary of the air gap has sufficient thickness to support the entire device; in addition, a buffer layer is provided, which has a flat surface in the resonant region and a flat surface in the resonant region. The boundary is connected to the substrate through a gentle arc surface, and the buffer layer is made of amorphous aluminum nitride, which is used to solve the problem of the bottom electrode material not adhering to the silicon dioxide or phosphosilicate glass (PSG) grown by thermal oxidation. To solve the problem of firmness, the amorphous aluminum nitride buffer layer has the same chemical properties as the piezoelectric layer material, and the redundant buffer layer can be removed at the same time when the piezoelectric layer is etched, without increasing the difficulty of the process.

Compared with the FBAR of the prior art, the preparation method of the present invention has the following advantages: first, the preparation of the sacrificial layer does not need to introduce a CMP process, and can be obtained by means of traditional oxidation equipment, so the requirements for the equipment of the production process are reduced, and it is more It is convenient to integrate into the CMOS process steps; secondly, the piezoelectric layer of FBAR produced by this technology can be made thinner without increasing the insertion loss of the device, so it is beneficial to make the application frequency of FBAR higher.

The method for making the sacrificial layer by local wet thermal oxidation on silicon is also a feature of the present invention, the sacrificial layer obtained on the polished silicon substrate by this method has a very smooth top surface, and has a gentle transition edge at the same time. The flat top surface meets the flatness requirements of the resonance region between the bottom electrode and the top electrode of FBAR, and its gentle transition edge ensures that the subsequent sputtering growth of each film layer will not appear due to the thin edge of the mechanical strength. high question. Therefore, the sacrificial layer obtained by this manufacturing method can replace the sacrificial layer formed by CMP polishing. The wet thermal oxidation process is a conventional CMOS process, which can be used in the FBAR manufacturing process without modification of the equipment.

Another feature of the preparation method of the present invention is that before depositing the bottom electrode metal, a layer of amorphous aluminum nitride with a thickness of about 100 nm is deposited on the partially thermally oxidized sacrificial layer by radio frequency sputtering. The role of crystalline aluminum nitride is to solve the problem of poor adhesion of the bottom electrode material on the silicon dioxide or phosphosilicate glass (PSG) grown by thermal oxidation. This problem is manifested in the fact that after the piezoelectric layer is grown, part of the bottom electrode will delamination from the sacrificial layer, resulting in cracking of the piezoelectric layer. This problem will be solved by adding an amorphous aluminum nitride layer between the bottom electrode and the sacrificial layer. At the same time, the use of amorphous aluminum nitride can provide a buffer layer with matching lattice constants for the bottom electrode and the piezoelectric layer, which is beneficial to the improvement of the preferred orientation performance of the piezoelectric layer using aluminum nitride as the material. The amorphous aluminum nitride material has the same chemical properties as the piezoelectric layer material, and the excess buffer layer can be removed at the same time when the piezoelectric layer is etched without increasing the difficulty of the process.

In addition, the boundary of the sacrificial layer formed by local thermal oxidation is larger than the boundary of the bottom electrode and the apex electrode, so that the release window of the sacrificial layer penetrating the piezoelectric layer and the buffer layer can be successfully etched once by wet etching. Finally, dilute HF or HF buffer to release the sacrificial layer from the etched window. Therefore, the FBAR manufactured by the present invention will not increase the complexity of the process.

Description of drawings

Fig. 1 shows the structural diagram of the air-gap type FBAR in the prior art

Fig. 2 shows the sectional view of the air-gap type FBAR made by the present invention;

Fig. 3 shows a cross-sectional view of growing a layer of silicon nitride film by CVD method on the substrate silicon, and photolithographically removing the silicon nitride corresponding to the sacrificial layer region;

Figure 4 shows a cross-sectional view after growing silicon dioxide on a substrate by a localized thermal oxidation method;

Figure 5 shows a cross-sectional view after removing the remaining silicon nitride;

Figure 6 shows a cross-sectional view after the bottom electrode has been grown and etched on the sputtered amorphous aluminum nitride layer;

Figure 7 shows a cross-sectional view after depositing an aluminum nitride piezoelectric layer;

8 shows a cross-sectional view after etching the aluminum nitride boundary and the release window on the aluminum nitride;

FIG. 9 shows a cross-sectional view of the FBAR after growing the top electrode.

Detailed ways

The present invention will be further described below in conjunction with accompanying drawing:

The new FBAR structure provided by the present invention is shown in Figure 2, including substrate 1, air gap 2, buffer layer (amorphous aluminum nitride layer), bottom electrode 4, aluminum nitride piezoelectric layer 5, top electrode 13 and release windows 6,7.

The substrate material of the FBAR provided by the present invention is silicon, the height of the top surface of the air gap 2 is higher than the upper surface of the substrate 1 , and the height of the lower top surface of the air gap 2 is lower than the upper surface of the substrate 1 . There is a layer of amorphous aluminum nitride buffer layer 3 on the upper surface of the air gap 2, the amorphous aluminum nitride buffer layer 3 straddles the boundary of the air gap, and the thickness is not greater than 0.10 microns. On the amorphous aluminum nitride buffer layer 3 are a bottom electrode 4 , an aluminum nitride piezoelectric layer 5 , and a top electrode 13 in sequence. The overlapping area of the bottom electrode 4 and the top electrode 13 perpendicular to the direction of the upper surface of the substrate is required to be within the boundary of the air gap 2 .

The following are specific embodiments of the present invention:

1) Deposit a layer of silicon nitride with a thickness of about 100 nm on the surface of the polished silicon 1 by CVD. The polished surface may be (100), (110) or (111) oriented. Then, the silicon nitride corresponding to the area of the sacrificial layer is removed by photolithography, leaving the silicon nitride at the part where 8 and 9 are located, as shown in FIG. 3 .

2) On the silicon surface on which the silicon nitride 8 and 9 have been grown, a layer of silicon dioxide 10 is grown in the area where the silicon nitride is removed by wet thermal oxidation treatment. The silicon dioxide has a thickness between 400nm and 800nm. As shown in Figure 4.

3) After the above steps, remove the residual silicon nitride 8, 9 on the silicon surface, leaving the silicon dioxide 10 part. As shown in Figure 5.

4) After the above steps, an amorphous aluminum nitride layer 11 of about 100 nm is grown on the silicon surface, and then a layer of metal is grown on the amorphous aluminum nitride layer 11 by sputtering and the bottom electrode pattern 4 is photoetched out. The material of the bottom electrode is usually molybdenum, and the thickness is 100-200nm. As shown in Figure 6. The amorphous aluminum nitride layer 11 can be obtained by radio frequency sputtering under the conditions of low temperature (<200°C) and low power density (<5W·cm ^-2 ).

5) After the above steps, grow a c-axis oriented aluminum nitride layer 12 on the silicon surface. The thickness of the aluminum nitride layer 12 is determined according to the practical frequency range. As shown in Figure 7. The c-axis oriented aluminum nitride layer 12 can be obtained by radio frequency sputtering under the conditions of temperature>250°C, power density>10W·cm ⁻² and nitrogen concentration>50%.

6) Etching the aluminum nitride layer 12 by wet etching to obtain the piezoelectric layer pattern 5 . The boundary contour of the piezoelectric layer pattern 5 is larger than the boundary contour of the air gap. When the piezoelectric layer pattern 5 is etched, the release windows 6 and 7 of the sacrificial layer are also etched. The release window 6, 7 is located between the air gap boundary and the electrode boundary. Because the material in this region is aluminum nitride, the release windows 6 and 7 can be easily etched through until the sacrificial layer 10 is exposed. As shown in Figure 8.

7) Deposit the top electrode metal by sputtering, and etch the top electrode pattern 13 . The material of the top electrode is usually molybdenum, and the thickness is 100-200nm. As shown in Figure 9.

8) The sacrificial layer is released, exposing the air gap 2 . dry. as shown in picture 2.

Claims (5)

1, a kind of film bulk acoustic resonator, comprise substrate, piezoelectric layer and electrode, it is characterized in that: ①The upper end surface of the substrate is provided with a smooth groove and a buffer layer. The buffer layer spans the groove and the substrate to form an air gap with a smooth upper convex edge and completely covers the air gap. The lower top surface of the air gap Higher than the substrate, with a flat surface and gently varying boundaries; ② The edge of the buffer layer in contact with the air gap and adjacent to the substrate has a smooth convex shape, the piezoelectric layer is arranged on the buffer layer, and the electrodes include a bottom electrode and a top electrode, and the bottom electrode is arranged on the buffer layer Within the piezoelectric layer, the top electrode is disposed on top of the piezoelectric layer. 2. The thin film bulk acoustic resonator according to claim 1, wherein the buffer layer is an amorphous aluminum nitride layer, the piezoelectric layer is an aluminum nitride piezoelectric layer, and the substrate is a silicon substrate . 3. The thin film bulk acoustic resonator according to claim 1, characterized in that a sacrificial layer release window is provided, and the release window is located between the boundary of the air gap and the boundary of the electrode. 4. A method for preparing a thin-film bulk acoustic resonator, comprising the following steps: ① Prepare a layer of silicon nitride film on the substrate; ② Photoetching a window with the outline of the sacrificial layer on the silicon nitride film, that is, the outline of the air gap on the substrate, and etching away the silicon nitride at the window; ③ Form a silicon dioxide sacrificial layer at the window by local thermal oxidation, and control the thermal oxidation time to obtain the required thickness of the sacrificial layer. The silicon dioxide sacrificial layer has a flat surface and a gently changing boundary; ④Remove the remaining silicon nitride; ⑤Grow a layer of amorphous aluminum nitride at low temperature by sputtering, that is, the required buffer layer, to form an air gap; ⑥Grow a metal layer by sputtering on the upper surface of the buffer layer and photoetch the bottom electrode pattern; ⑦ After forming the bottom electrode, grow a piezoelectric layer on the bottom electrode and etch the piezoelectric layer pattern, the boundary of the piezoelectric layer pattern is larger than the boundary of the air gap; ⑧ Deposit the top electrode on the upper surface of the piezoelectric layer, and etch the pattern of the top electrode; ⑨ Release the sacrificial layer and dry. 5. A method for preparing a bulk acoustic wave resonator, comprising the following steps: ① Deposit a layer of 80-120nm silicon nitride on the polished silicon surface by CVD method, the polished surface is (100), (110) or (111) orientation, and then use photolithography to place The silicon nitride is removed; ② On the above-mentioned silicon surface grown with silicon nitride, a layer of silicon dioxide is grown in the area where the silicon nitride is removed by partial wet thermal oxidation treatment, and the thickness of the silicon dioxide is between 400nm and 800nm; ③Remove residual silicon nitride on the silicon surface; ④ After the above steps, grow a layer of 60-80nm amorphous aluminum nitride layer on the silicon surface, then sputter and grow a layer of metal on the amorphous aluminum nitride layer and photoetch the bottom electrode pattern. The electrode material is molybdenum, the thickness is 100-200nm, and the amorphous aluminum nitride is obtained by radio frequency sputtering under the conditions of less than 200°C and power density <5W·cm ^-2 ; ⑤ After the above steps, a layer of c-axis oriented aluminum nitride piezoelectric layer is grown on the silicon surface. The thickness of the aluminum nitride piezoelectric layer is determined according to the frequency range of the actual application. The c-axis oriented aluminum nitride piezoelectric layer It can be obtained by radio frequency sputtering under the conditions of temperature>250℃, power density>10W·cm ^-2 and nitrogen concentration>50%; ⑥ Etching out the pattern of the piezoelectric layer by wet method, the boundary contour of the piezoelectric layer pattern is larger than the boundary contour of the air gap, while etching the pattern of the piezoelectric layer, the release window of the sacrificial layer is also etched out, and the release window is located at between the air gap boundary and the electrode boundary; ⑦ Deposit the top electrode metal by sputtering, and etch the top electrode pattern. The top electrode material is molybdenum, and the thickness is 100-200nm; ⑧ Release the sacrificial layer and dry.

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