MEMS sensor and method for manufacturing the same

Abstract

This invention provides a highly reliable MEMS sensor and a method for manufacturing the same, which have a capacitive transducer structure with a membrane containing metallic glass, exhibiting high impact resistance and fracture toughness, suppressing membrane stress fluctuations after substrate mounting and reflow, and being particularly suitable for the acoustic characteristics of microphones. [Solution] In a MEMS sensor 1 having a capacitive transducer structure in which a membrane MB and a back plate BP face each other with an air gap AG in between, the membrane MB is formed of a laminated film of a metal thin film X and a metallic glass MG having different stresses, and the stress of the metal thin film X cancels out the stress of the metallic glass MG, thereby reducing the overall stress of the membrane MB as a laminated film.

Description

Technical Field

[0001] The present invention relates to a MEMS sensor and a method for manufacturing the same, and particularly to a MEMS sensor using a metallic glass for a membrane.

Background Art

[0002] In recent years, small and high-performance capacitive MEMS microphones have been widely installed in general consumer mobile devices such as smartphones. Fig. 4 shows a cross-sectional view of a back-chamber type capacitive MEMS microphone 20. A MEMS sensor 21 is mounted on an organic substrate 23 together with an IC 22 for signal amplification and control, and is covered with a metal cap 24. External sound pressure passes through a sound hole 25 of the organic substrate 23 and reaches a cavity 26 of the MEMS sensor 21, vibrating a membrane 27. Along with this, a capacitance change due to a change in the distance between the membrane 27 and a back plate electrode 28 is taken out as an electrical signal. Conventionally, a polysilicon thin film has been used as the membrane 27 of the MEMS sensor 21, but there is a problem in that the resistance (fracture toughness) against crack propagation failure when a strong wind pressure is applied is weak.

[0003] One of the indexes indicating the characteristics of a microphone is low-frequency roll-off (LFRO). This is defined as the value of the frequency at which the sensitivity drops by 3 dB from the sensitivity at 1 KHz in the sensitivity-frequency characteristic. It is known that a capacitive MEMS microphone having a low LFRO characteristic has even weaker resistance to air burst (compressed air) and drop impact than a high LFRO product. Therefore, the LFRO of a capacitive MEMS microphone used in mobile devices or the like is set to a relatively high value of about ~100 Hz.

[0004] Meanwhile, in recent years, there has been a growing need for capacitive MEMS microphones with high signal-to-noise ratio (SNR) characteristics in the low-frequency range below 10 Hz, for applications such as in-vehicle active noise cancellation and human vital sign monitoring. To meet this demand, there is a growing need to improve the shock resistance against wind pressure and drops, which has been a challenge for conventional polysilicon thin-film membranes. As a result, various improvements to membrane structures have been proposed and put into practical use.

[0005] On the other hand, there have also been proposals to change the membrane material itself to a material with higher fracture toughness, and one of the candidates for this is metallic glass disclosed in Patent Document 1. Metallic glass has an amorphous solid structure and undergoes a transition from a supercooled liquid to glass (solid) during the cooling process. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 6132047 [Non-Patent Document 1] J. Sakurai, S. Hata and A. Shimokohbe: “Reduction of Electrical Resistivity in PdCuSi Thin Film Metallic Glass” Proc. Int. Conference on Advanced Technology in Experimental Mechanics 2003(ATEM'03), JSME-MMD, (2003). [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] Because metallic glass has an amorphous solid structure, it has the characteristic of being superior in impact resistance and fracture toughness compared to conventional polysilicon membranes that include a crystal structure. However, it has been reported that crystallization progresses when heated above the glass transition point (Tg) or when heated for a long time even below Tg (Non-Patent Document 1). As crystallization progresses, the resistance to crack propagation fracture deteriorates, so it becomes necessary to optimally design the temperature and time history of the heat process applied after forming the metallic glass film to be low temperature and short time.

[0008] In the process of conducting applied research on metallic glass, the inventors investigated the change in the stress (internal stress) of the metallic glass with respect to the peak temperature (Tp) and the progress of crystallization using various heating temperature profiles. As an example, the stress change from the initial stress value when a PdCuSi-based metallic glass thin film (Tg = 638K) sputter-deposited at room temperature was heated with different temperature profiles of Tp (<Tg) shown in FIG. 5 was plotted in FIG. 6. It can be seen from FIG. 6 that the stress change when heated at Tp = 260°C reaches about +300 MPa. In the manufacturing process of MEMS microphones, the final membrane stress is usually controlled to about 30 ± 10 MPa suitable for acoustic characteristics.

[0009] However, from the results of FIG. 6, it was found that when passing through a substrate mounting reflow process at 260°C in the final assembly process, the membrane stress changes to a large tensile stress exceeding +300 MPa, and as a result, acoustic characteristics such as a decrease in sensitivity and an increase in pull-in voltage Vp deteriorate. As a countermeasure, a method of assuming the amount of change in membrane stress in the reflow process in advance and setting the stress at the time of forming the metallic glass to a strong compressive stress of about -260 MPa can be considered. However, in the film-forming process of metallic glass with a strong compressive stress, problems such as deterioration of the in-plane stress distribution and film thickness distribution of the wafer, and changes in the composition ratio in the depth direction have occurred, and it has been found that there is a trade-off relationship in characteristics. Also, it has been found that there are problems with reliability, such as the final membrane stress being affected by variations in the temperature profile of the reflow process and the acoustic characteristics changing.

[0010] One of the objectives of the present invention is to provide a highly reliable MEMS sensor and a method for manufacturing the same, which have a capacitive transducer structure having a membrane containing metallic glass, high impact resistance and fracture toughness, suppression of membrane stress fluctuations after substrate mounting reflow, and are particularly suitable for the acoustic characteristics of microphones. [Means for solving the problem]

[0011] To solve the above problems, a MEMS sensor according to one embodiment is a capacitive transducer structure in which a membrane and a back plate face each other with an air gap in between, wherein the membrane is formed by laminating a metal thin film X and a metallic glass MG, the direction and absolute value of stress being different from each other, the absolute value of the stress in the metallic glass MG is balanced by the absolute value of the stress in the metal thin film X, and the stress of the entire membrane is controlled in the tensile direction. The metal thin film X may be any of the metals Mo, Al, Ni, Fe, Cr, Mg, Co, W, or Zr. The metallic glass MG may be any of the Pd-based metallic glass, Zr-based metallic glass, Fe-based metallic glass, Ni-based metallic glass, Mg-based metallic glass, Co-based metallic glass, or Cu-based metallic glass.

[0012] Furthermore, in a method for manufacturing a MEMS sensor according to one embodiment, a first insulating film, a second insulating film, and an electrode film are sequentially formed on a silicon substrate, a spacer layer, a membrane, and electrodes are sequentially formed, and after forming through holes in the silicon substrate, sacrificial layer etching is performed. In this method for manufacturing a MEMS sensor, the membrane is formed by sputtering a thin metal film X, followed by continuous sputtering of a metallic glass MG, with the film stress during deposition of both the thin metal film X and the metallic glass MG being compressive stress, and then preheating. The preheating in the formation of the membrane may be performed in a range of 260°C to 300°C. The thin metal film X may consist of any of the metals Mo, Al, Ni, Fe, Cr, Mg, Co, W, or Zr. The metallic glass MG may be any of Pd-based metallic glass, Zr-based metallic glass, Fe-based metallic glass, Ni-based metallic glass, Mg-based metallic glass, Co-based metallic glass, or Cu-based metallic glass. [Effects of the Invention]

[0013] According to one embodiment of the MEMS sensor, the membrane has a laminated structure of a metal thin film X and a metallic glass MG, where the direction and absolute value of stress are different from each other. This balances the stress of the metallic glass MG with the stress of the metal thin film X, controlling the stress of the entire membrane to an appropriate value in the tensile direction. As a result, a membrane with high impact resistance and fracture toughness can be made while maintaining good sensor characteristics. Furthermore, according to the manufacturing method of the MEMS sensor according to one embodiment, the metal thin film X and the metallic glass MG are continuously deposited, and the stress of both during deposition is set to compressive stress. Subsequent preheating causes the stress of the metallic glass MG to change predominantly in the tensile direction, opposite to the stress during deposition, compared to the metal thin film X, thereby promoting stress balance with the metal thin film X. Moreover, the introduction of preheating makes it possible to minimize the stress change of the metallic glass MG due to the heating process during the substrate mounting process, which is necessary to maintain stable sensor characteristics. [Brief explanation of the drawing]

[0014] [Figure 1]This is a cross-sectional view showing the structure of a MEMS sensor according to an embodiment of the present invention. [Figure 2] This diagram shows the manufacturing process flow of the MEMS sensor shown in Figure 1. [Figure 3] This graph shows typical MEMS microphones. [Figure 4] This is a graph showing the temperature profile. [Figure 5] This graph shows the stress change in a PdCuSi-based metallic glass thin film due to heating. [Figure 6] This graph shows the stress change when heated according to the temperature profile in Figure 5. [Modes for carrying out the invention]

[0015] Hereinafter, embodiments of the present invention will be described with reference to Figures 1 to 3. In the following embodiments, parts that are the same or equivalent to each other will be denoted by the same reference numerals.

[0016] <About the structure> Figure 1 shows the structure of the MEMS sensor according to the present invention. The MEMS sensor 1 employs a capacitive transducer structure. The membrane MB is composed of a laminated film of a metal thin film (X) and a metallic glass (MG). These two films have different absolute compressive stresses when they are formed, but the stress of the laminated film is controlled to be low in the tensile direction by the heat treatment described later. That is, the compressive stress of the metal thin film X balances and cancels out the change in tensile stress of the metallic glass MG after heat treatment. On the other hand, the back plate BP is composed of a silicon nitride film 4, an electrode film 5, and a thin silicon nitride film 6. The thin silicon nitride film 6 is provided to avoid a short circuit due to contact with the electrode film 5 when the membrane vibrates greatly.

[0017] The membrane MB and the backplate BP face each other with an air gap AG therebetween, and the air gap AG is formed at an appropriate interval according to the thickness of the spacer layer 7. An acoustic hole AH is formed in the backplate BP, and the external sound pressure entering the cavity CA passes through this hole and reaches the membrane MB. Note that the cavity CA is formed by a through hole 11 in the silicon substrate 2 and a cavity in the silicon oxide film 3 overlapping therewith. Further, a vent hole BH (not shown) is provided in the membrane MB, and an appropriate acoustic resistance is formed according to its shape and size.

[0018] A bias electrode 9 is connected to the electrode film 5 of the backplate BP, and a signal extraction electrode 10 is connected to the membrane MB. Note that reference numeral 8 denotes a surface protection film, which provides electrical insulation around the bias electrode 9 and the membrane MB and functions as a protection film during the sacrificial layer etching described later. Due to the above structure, the sound pressure entering the cavity CA passes through the acoustic hole AH, reaches the air gap AG, and vibrates the membrane MB. The change in the distance between the membrane MB and the electrode film 5 due to this vibration results in a change in the capacitance value, which is taken out as an electrical signal from the signal extraction electrode 10.

[0019] As shown in the figure, since the membrane MB is provided on a laminated structure of films made of semiconductor materials in a high-temperature film formation process, it is not affected by the heat treatment of these semiconductor materials in the semiconductor process. Also, due to such a structure, after the previous process, heat ray irradiation such as laser or ion milling is possible, and adjustment of accidental properties such as correction of the center of gravity position and dispersion of stress can be performed by SLS (Selective Laser Sintering) or partial removal of materials.

[0020] <Regarding the manufacturing method> Fig. 2 shows the manufacturing process flow of the MEMS sensor according to the present invention. In the present embodiment, the case where Mo is used as X and PdCuSi is used as MG in the metal thin film X / metal glass MG laminated membrane will be described.

[0021] First, a silicon substrate 2 is prepared, and silicon oxide film 3, silicon nitride film 4, electrode film 5, and thin silicon nitride film 6 are deposited on its surface in that order (Figure 2(a)). Here, silicon oxide film 3 may be formed by thermal oxidation, and silicon nitride films 4 and 6 may be formed by plasma CVD. Electrode film 5 consists of a conductive polysilicon film, and may be deposited by depositing a polysilicon film while adding impurities, or a non-doped polysilicon film may be deposited first, and then impurities may be added by ion implantation or the like.

[0022] Next, using known lithography and etching techniques, the silicon nitride film 4, electrode film 5, and thin silicon nitride film 6 are processed to a predetermined size to form a back plate BP. On this back plate, NSG (Non-doped Silicate Glass) is deposited and patterned to form a spacer layer 7 (Figure 2(b)). The above processing creates acoustic holes AH in the back plate BP, which serve as pathways for the etching solution in the sacrificial layer etching described later. The patterning also exposes the electrode film 5, enabling electrical connection with the bias electrode 9 to be formed later.

[0023] Next, a silicon nitride film is deposited and patterned across the entire surface using known lithography and etching techniques to form a surface protective film 8. Then, the membrane MB is formed, followed by the bias electrode 9 and signal extraction electrode 10 (Figure 2(c)). The membrane MB has a Mo / PdCuSi multilayer structure and is formed by sputtering using a target that has been amorphized by supercooling. The initial deposition conditions involved sputtering a Mo film (thickness 55 nm) with a compressive stress of -1.1 GPa, followed by continuous sputtering of a PdCuSi film (thickness 440 nm) with a compressive stress of -125 MPa. The stress of the multilayer film at this time was approximately -230 MPa. Subsequently, preheating was performed at Tp=260°C in the temperature profile shown in Figure 5. The stress change in the lower Mo film during heating was almost 0, but the upper PdCuSi experienced a tensile stress change of approximately +300 MPa (see Figure 6, Tp=260°C point). As a result, the stress on the entire laminated membrane after heating was controlled to approximately +33 MPa, which is suitable for acoustic properties. Patterning of the membrane MB can be performed either before or after this preheating. This patterning creates vent holes BH (not shown).

[0024] Next, the silicon substrate 2 is back-ground by CMP, and through-holes 11 extending from the back surface of the silicon substrate 2 to the silicon oxide film 3 are formed using known lithography and etching techniques, followed by the formation of an air gap AG and a cavity CA (Figure 2(d)). Here, the formation of the air gap AG and cavity CA is performed by sacrificial layer etching. Sacrificial layer etching is performed by immersing the wafer in a hydrofluoric acid-based mixed acid aqueous solution, flowing the etching solution through the through-holes 11 and vent holes BH, and etching a portion of the spacer layer 7 and a portion of the silicon oxide film 3. Through the above process, the MEMS sensor 1 shown in Figure 1 is formed.

[0025] <Regarding the effects> Figure 3 shows the stress changes when a Mo / PdCuSi multilayer film is heated (preheated) at Tp=260°C, and then subjected to up to 5 additional heating cycles at the same temperature profile. As mentioned above, the stress immediately after Mo / PdCuSi film deposition is approximately -230 MPa (see the 0th cycle on the X axis in Figure 3), and the stress changes to approximately +33 MPa after the subsequent 260°C preheating (see the 1st cycle). It can be seen that even after repeating the reflow process at the same temperature profile of Tp=260°C up to 5 times, the membrane stress remains controlled within the range of 30±10 MPa, which is suitable for acoustic properties. Furthermore, transmission electron microscopy observation confirmed that crystallization of PdCuSi does not occur with respect to this repeated heating.

[0026] Thus, the inventors have newly discovered that the introduction of preheating can suppress stress fluctuations during the subsequent substrate mounting reflow process. The preheating temperature needs to be set to Tp = 260°C or higher, which is normally used in the reflow process. However, for all metallic glass materials tested in the experiment, it was found that the maximum heating temperature must be 300°C or lower in order to maintain the amorphous solid structure after heating.

[0027] Although embodiments of the present invention have been described above, various modifications are possible based on the spirit of the invention. For example, in the above embodiments, the insulating film supporting the backplate and membrane was formed of silicon nitride or silicon oxide, but other insulating films may also be used. However, the insulating film as a spacer layer must have a higher etching rate than the insulating film between the silicon substrate and the backplate.

[0028] Furthermore, although Mo was used as the thin metal film X in the laminated film constituting the membrane, other metals can be used that allow for easy stress control during film formation and exhibit little stress change with temperature up to approximately 300°C. In other words, Mo can be replaced with any of the metals Al, Ni, Fe, Cr, Mg, Co, W, or Zr.

[0029] Furthermore, while Pd-based metallic glass was used as the metallic glass MG in the laminated film constituting the membrane, any of the following metallic glasses may be used: Zr-based metallic glass, Fe-based metallic glass, Ni-based metallic glass, Mg-based metallic glass, Co-based metallic glass, or Cu-based metallic glass. [Explanation of Symbols]

[0030] 1 MEMS sensor 2. Silicon substrate 3. Silicon oxide film 4. Silicon nitride film 5 Electrode membrane 6. Thin silicon nitride film 7 Spacer layer 8 Surface protective layer 9. Bias electrode 10 Signal extraction electrodes 11 Through holes 20 MEMS Microphone 21 MEMS sensors 22 IC 23 Organic substrates 24 metal caps 25 sound holes 26 Cavity 27 Membrane 28 Backplate electrodes AG Air Gap AH Acoustic Hall BP Backplate CA Cavity MB Membrane

Claims

1. A capacitive transducer structure MEMS sensor in which a membrane and a backplate face each other with an air gap in between, The aforementioned membrane is formed by laminating a thin metal film X and a metallic glass MG, each having a different stress direction and absolute value. A MEMS sensor characterized in that the absolute value of the stress of the metallic glass MG is balanced by the absolute value of the stress of the metallic thin film X, and the stress of the entire membrane is controlled in the tensile direction.

2. The MEMS sensor according to claim 1, characterized in that the metal thin film X is made of any of the metals Mo, Al, Ni, Fe, Cr, Mg, Co, W, or Zr.

3. The MEMS sensor according to claim 1, characterized in that the metallic glass MG is any of Pd-based metallic glass, Zr-based metallic glass, Fe-based metallic glass, Ni-based metallic glass, Mg-based metallic glass, Co-based metallic glass, or Cu-based metallic glass.

4. In a method for manufacturing a MEMS sensor, in which a first insulating film, a second insulating film, and an electrode film are sequentially formed on a silicon substrate, a spacer layer, a membrane, and electrodes are sequentially formed, through holes are formed in the silicon substrate, and then sacrificial layer etching is performed, A method for manufacturing a MEMS sensor, characterized in that the membrane is formed by sputtering a thin metal film X, followed by continuous sputtering a metallic glass MG, wherein the film stress during deposition of both the thin metal film X and the metallic glass MG is set to compressive stress, and then preheating is performed.

5. The method for manufacturing a MEMS sensor according to claim 4, characterized in that the preheating in the formation of the membrane is performed in a range of 260°C to 300°C.

6. The method for manufacturing a MEMS sensor according to claim 4 or 5, characterized in that the metal thin film X is made of any of the metals Mo, Al, Ni, Fe, Cr, Mg, Co, W, or Zr.

7. A method for manufacturing a MEMS sensor according to claim 4 or 5, characterized in that the metallic glass MG is one of Pd-based metallic glass, Zr-based metallic glass, Fe-based metallic glass, Ni-based metallic glass, Mg-based metallic glass, Co-based metallic glass, or Cu-based metallic glass.

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