MEMS Pressure Sensor and Preparation Method thereof

Abstract

The invention discloses a MEMS pressure sensor, which includes a bulk silicon layer, a buried oxygen layer, a substrate, a varistor, a first passivation layer, an electrode layer, and a second passivation layer. The varistor is located on the upper surface of the buried oxygen layer, and the first passivation layer is a rectangular shell located on the upper surface of the buried oxygen layer; there is a through hole in the center of the top of the rectangular shell; the first passivation layer covers the varistor, and the gap between the first passivation layer and the varistor forms an isolation cavity. The electrode layer is located on the upper surface of the first passivation layer and is connected with the varistor via the through hole. The second passivation layer is located on the upper surface of the electrode layer.

Claims

1-9. (canceled)

10. A method for preparing the MEMS pressure sensor, comprising Step 4. preparing the first passivation layer (5) on the surface of the sacrificial layer by PECVD (plasma enhanced chemical vapor deposition) and photoetching, and forming the through hole (9) in the center of the first passivation layer (5) and the sacrificial layer by photoetching; Step 5. preparing the electrode layer (6) on the upper surface of the first passivation layer (5) by lift-off and electron beam evaporation, forming a sacrificial layer etch hole in the electrode layer (6) and the first passivation layer (5) by photoetching, and then forming the isolation cavity (4) between the first passivation layer (5) and the varistor (3) by etching the sacrificial layer; Step 6. depositing and forming the second passivation layer (7) on the upper surface of the electrode layer (6) by PECVD and photoetching, wherein the second passivation layer (7) seals the sacrificial layer etch hole, wherein the pressure sensor comprises a bulk silicon layer (1), a buried oxygen layer (10), a substrate (2), a varistor (3), a first passivation layer (5), an electrode layer (6), and a second passivation layer (7); the bulk silicon layer (1) is located on the upper surface of the substrate (2), the buried oxygen layer (10) is located on the upper surface of the bulk silicon layer (1), a cavity (11) is set inside the bulk silicon layer (1), and the bulk silicon layer (1) directly above the cavity (11) and the buried oxygen layer (10) jointly form a pressure sensitive film (8); the varistor (3) is located on the upper surface of the buried oxygen layer (10), and the first passivation layer (5) is a rectangular shell located on the upper surface of the buried oxygen layer (10); there is a through hole (9) in a center of the top of the rectangular shell; the first passivation layer (5) covers the varistor (3), and the gap between the first passivation layer (5) and the varistor (3) forms an isolation cavity (4); the electrode layer (6) is located on the upper surface of the first passivation layer (5) and is connected with the varistor (3) via the through hole (9); and the second passivation layer (7) is located on the upper surface of the electrode layer (6).

11. The method for preparing the MEMS pressure sensor of claim 10, wherein the pressure sensor comprises a total four varistors (3) mounted on the upper surface of the buried oxygen layer (10); the four varistors (3) are respectively mounted at the position directly above the midpoint of the four sides of the cavity (11), and each varistor (3) is separately provided with the first passivation layer (5), the electrode layer (6), and the second passivation layer (7); the four varistors (3) are connected by a Wheatstone bridge.

12. The method for preparing the MEMS pressure sensor of claim 10, wherein the electrode layer (6) completely covers the upper surface of the first passivation layer (5).

13. The method for preparing the MEMS pressure sensor of claim 10, wherein the material and thickness of the first passivation layer (5) are consistent with those of the second passivation layer (7).

14. The method for preparing the MEMS pressure sensor of claim 10, wherein the material of the substrate (2) is single crystal silicon or glass, and the thickness is 200-2000 μm.

15. The method for preparing the MEMS pressure sensor of claim 10, wherein the thickness of the pressure sensitive film (8) is 1-50 μm.

16. The method for preparing the MEMS pressure sensor of claim 10, wherein the material of the first passivation layer (5) is silicon dioxide or silicon nitride, and the thickness is 1-20 μm.

17. The method for preparing the MEMS pressure sensor of claim 16, wherein the material of the first passivation layer (5) is hydrogenated silicon dioxide or hydrogenated silicon nitride.

18. The method for preparing the MEMS pressure sensor of claim 17, further comprising: Step 1. preparing an SOI wafer, wherein the SOI wafer comprises the bulk silicon layer (1), the buried oxide layer (10) and a device layer; Step 2. forming the varistor (3) by photoetching and ion implantation on the device layer of the SOI wafer; Step 3. preparing a sacrificial layer on the upper surface and side wall of the varistor (3) by lift-off and electron beam evaporation; Step 7. forming an open cavity by performing photoetching on the lower surface of the bulk silicon layer (1); and Step 8. preparing a substrate (2), and bonding the substrate (2) with the bottom of the SOI wafer by anodic bonding, so as to form the cavity (11) and complete the preparation of the MEMS pressure sensor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 is a top view schematic of the varistor arrangement in the MEMS pressure sensor of the invention;

[0026] FIG. 2 is a schematic of Wheatstone bridge connection of the varistor in the MEMS pressure sensor of the invention;

[0027] FIG. 3 is a sectional view of the MEMS pressure sensor of the invention along the A-A′ direction in FIG. 1;

[0028] FIG. 4 is a structural representation corresponding to the preparation step 1 of the MEMS pressure sensor of the invention;

[0029] FIG. 5 is a structural representation corresponding to the preparation step 2 of the MEMS pressure sensor of the invention;

[0030] FIG. 6 is a structural representation corresponding to the preparation step 3 of the MEMS pressure sensor of the invention;

[0031] FIG. 7 is a structural representation corresponding to the preparation step 4 of the MEMS pressure sensor of the invention;

[0032] FIG. 8 is a structural representation corresponding to the preparation step 5 of the MEMS pressure sensor of the invention;

[0033] FIG. 9 is a structural representation corresponding to the preparation step 6 of the MEMS pressure sensor of the invention;

[0034] FIG. 10 is a structural representation corresponding to the preparation step 7 of the MEMS pressure sensor of the invention.

DETAILED DESCRIPTION

[0035] According to the drawings, the invention is further described as follows:

[0036] As shown in FIG. 1 and FIG. 3, a MEMS pressure sensor includes a bulk silicon layer 1, a buried oxygen layer 10, a substrate 2, a varistor 3, a first passivation layer 5, an electrode layer 6, and a second passivation layer 7. The bulk silicon layer 1 is located on the upper surface of the substrate 2, the buried oxygen layer 10 is located on the upper surface of the bulk silicon layer 1, a rectangular cavity 11 is set inside the bulk silicon layer 1, and the bulk silicon layer 1 directly above the cavity 11 and the buried oxygen layer 10 jointly form a pressure sensitive film 8.

[0037] The four varistors 3 are located on the upper surface of the buried oxygen layer 10 and are respectively set at the position directly above the midpoint of the four sides of the cavity 11. Each varistor 3 is separately provided with the first passivation layer 5, the electrode layer 6, and the second passivation layer 7. Specifically, the first passivation layer 5 is a rectangular shell located on the upper surface of the buried oxygen layer 10, there is a through hole 9 in the center of the top of the rectangular shell, the first passivation layer 5 covers the varistor 3, and the gap between the first passivation layer 5 and the varistor 3 forms an isolation cavity 4; the electrode layer 6 is located on the upper surface of the first passivation layer 5, and is connected with the varistor 3 through the through hole 9 to lead out the electrode of the varistor 3; the second passivation layer 7 is located on the upper surface of the electrode layer 6. The four varistors (3) are connected by a Wheatstone bridge, as shown in FIG. 2. The electrode layer 6 completely covers the upper surface of the first passivation layer 5.

[0038] The material of the substrate 2 is single crystal silicon or glass, and the thickness is 200-2000 μm. The thickness of the pressure sensitive film 8 is 1-50 μm. The material of the first passivation layer 5 is silicon dioxide or silicon nitride, preferably hydrogenated silicon dioxide or hydrogenated silicon nitride; hydrogenation helps reduce and suppress the defect of the film, and the thickness of the layer is 1-20 μm. The material and thickness of the second passivation layer 5 are consistent with those of the first passivation layer 7, which can thus play a role in stress compensation and improve the mechanical strength and reliability of the sensor. The material of the electrode layer 6 is a metal, preferably one of Al, Ti, Au, Cu, and Pt.

[0039] The working principle of the MEMS pressure sensor of the present invention is as follows:

[0040] The pressure sensitive film 8 is deformed under the action of external pressure, and the deformation of the pressure sensitive film 8 causes the strain of the varistor 3; based on the piezoresistive effect, the resistance of the varistor 3 changes accordingly; the larger the environmental pressure, the larger the deformation of the pressure sensitive film 8, and the larger the corresponding change in the resistance of the varistor 3; the conversion of a pressure signal to an electrical signal is achieved by measuring the resistance change using the Wheatstone bridge.

[0041] A method for preparing the MEMS pressure sensor, including the following steps:

[0042] Step 1. preparing an N-type (100) SOI wafer, wherein the buried oxide layer 10 has a thickness of 2 μm, the bulk silicon layer 1 has a thickness of 300 μm, and the device layer has a thickness of 5 μm, as shown in FIG. 4.

[0043] Step 2. forming the varistor 3 by photoetching and ion implantation on the device layer of the SOI wafer, as shown in FIG. 5;

[0044] Step 3. preparing a 500 nm thick Ti as a sacrificial layer on the upper surface and side wall of the varistor 3 by lift-off and electron beam evaporation, as shown in FIG. 6;

[0045] Step 4. preparing a 2 μm thick hydrogenated silicon nitride layer as the first passivation layer 5 on the surface of the sacrificial layer by PECVD and photoetching, and forming the through hole 9 in the center of the first passivation layer 5 and the sacrificial layer by photoetching, as shown in FIG. 7;

[0046] Step 5. preparing a 3 μm thick Au as the electrode layer 6 on the upper surface of the first passivation layer 5 by lift-off and electron beam evaporation, forming a sacrificial layer etch hole in the electrode layer 6 and the first passivation layer 5 by photoetching, and then forming the isolation cavity 4 between the first passivation layer 5 and the varistor 3 by etching the sacrificial layer, as shown in FIG. 8;

[0047] Step 6. depositing a 2 μm thick hydrogenated silicon nitride layer on the upper surface of the electrode layer 6 by PECVD and photoetching to fill and seal the sacrificial layer etch hole, as shown in FIG. 9;

[0048] Step 7. forming an open cavity by performing photoetching on the lower surface of the bulk silicon layer 1, wherein the thickness of the bulk silicon layer on the top of the open cavity is 30 μm, as shown in FIG. 10;

[0049] Step 8. preparing a 400 μm thick BF33 glass sheet as the substrate 2, and tightly bonding the substrate 2 with the bottom of the SOI wafer by anodic bonding, so as to form the cavity 11 and complete the preparation of the MEMS pressure sensor, as shown in FIG. 1.

[0050] The above description is only a preferred embodiment of the invention. It should be pointed out that as far as a person of ordinary skill in the art is concerned, the person may implement some improvements and modifications on the premise of following the principle of the invention; however, such improvements and modifications shall be deemed to be within the coverage of protection of the invention.