Flip-chip force sensor array

Abstract

The application provides a flip type force sensor array, which comprises an array type MEMS force sensor and a flexible circuit board. The array type MEMS force sensor is composed of m*n MEMS force sensors, wherein m>=1 and n>=1; the MEMS force sensor is selected from a cross beam structure and is composed of a force sensing film, a Wheatstone sensitive bridge, a frame and electrodes; the force sensing film is located at the center position of the cross beam; the Wheatstone sensitive bridge is composed of four resistance strips which are consistent in mode and are respectively located in the maximum stress area of the cross beam; the four electrodes are etched from a silicon wafer and are distributed at the four right angle positions of the frame; the flexible circuit board is provided with m*n MEMS force sensor packaging pads, each packaging pad comprises four pads and a bump, and the bump is located at the center position of the packaging pad. The measured force is directly transmitted to the sensitive area of the MEMS force sensor through the metal bump on the flexible plate, so that the accurate measurement of the pressure is realized.

Description

Technical fields:

[0001] This invention belongs to the field of microelectronic sensor technology and relates to an inverted force sensor. Background technology:

[0002] Sensors are the core of wearable technology, enabling the acquisition of various health data related to the human body. Sensors used for wearable health monitoring include pressure sensors, accelerometers, and photoelectric sensors. Among these, pressure sensors are widely used in wearable monitoring devices due to their simple principle and wide range of implementation methods. Flexible pressure sensors, with their bendable and stretchable properties, meet the performance requirements of wearable health monitoring and are therefore gradually becoming a research hotspot in both scientific research and commercial application fields.

[0003] Flexible electronics can combine sensors, batteries, antennas, light-emitting diodes, and other components with flexible substrates in various ways. Force sensors, which convert external load force signals into electrical signals, are an indispensable part of the flexible electronic device family. Flexible pressure sensors have achieved many remarkable successes in the biomedical field. Among them, continuous vital sign monitoring is an attractive application of flexible pressure sensors. Unlike traditional healthcare devices that only provide snapshots of the human body's physiological state, flexible sensors can measure physiological abnormalities in real time, enabling early detection and diagnosis, and providing a safer and more comfortable way to protect human health. Flexible pressure sensors can be attached to the human body to capture various physiological parameters, such as heart rate, blood pressure, and pulse rate. Compared with traditional pressure sensors, flexible pressure sensors require both flexibility and pressure sensing functionality. The sensor's structural design and force transmission are key issues affecting sensor performance. Existing flexible pressure sensors use a thin-film structure with soft adhesive to measure pressure. The pressure sensor is connected to the circuit board via gold wire leads. This method is prone to damage during force transmission, and the pressure signal is significantly affected by the soft adhesive, easily distorted and attenuated, directly impacting the test accuracy of the pressure sensor. In addition, the performance parameters of the adhesive are affected by the application environment. If the time is too long, curing may occur, which will eventually affect the effectiveness of pressure signal transmission.

[0004] Therefore, this invention proposes a flip-chip force sensor array, which starts from silicon-based sensors and organically combines them with flexible circuit boards and flexible films. Through the optimized design of the internal structure and force transmission method of the pressure sensor, real-time monitoring of flexible pressure sensor signals is achieved. The array design provides compensation for the integrity of the pressure signal, thereby improving the testing accuracy of wearable health monitoring. Summary of the Invention:

[0005] The purpose of this invention is to provide an inverted force sensor array in which the force to be measured is directly transmitted to the sensitive area of ​​the MEMS force sensor through metal bumps on a flexible plate, thereby achieving accurate pressure measurement. It features low signal attenuation, high accuracy, small size, and easy mass production.

[0006] Another object of the present invention is to provide a method for fabricating a MEMS force sensor.

[0007] The technical solution of this invention is a flip-chip force sensor array, comprising an array of MEMS force sensors and a flexible circuit board. The array of MEMS force sensors consists of m×n MEMS force sensors, where m≥1 and n≥1. The MEMS force sensors employ a crossbeam structure, comprising a force-sensing film, a Wheatstone bridge, a frame, and electrodes. The force-sensing film is located at the center of the crossbeam. The Wheatstone bridge consists of four resistor strips, arranged in a consistent manner and located within the area of ​​maximum stress on the crossbeam. The four electrodes are distributed at the four right-angle positions of the frame. The flexible circuit board has m×n MEMS force sensor packaging pads, each packaging pad comprising four pads and one bump, with the bump located at the center of the packaging pad.

[0008] The four electrodes of the aforementioned MEMS force sensor are etched from a silicon wafer to form silicon pillars, which differ in height from the plane of the force-sensitive thin film by approximately 40–50 μm.

[0009] The aforementioned MEMS force sensor is soldered to the flexible circuit board in an inverted manner.

[0010] The bumps on the flexible board packaging pads are spherical, with a diameter less than or equal to the length of the force-sensitive film area and a height of about 45-50 μm. The bumps are achieved by opening windows in the pads and adding immersion gold, without any electrical connection. They are used to contact the force-sensitive film and act as a force conductor to transmit the measured force to the force-sensitive film.

[0011] The above-mentioned method for fabricating a MEMS force sensor includes the following steps:

[0012] (1) Wafer preparation: silicon wafer surface is oxidized to generate SiO2, and piezoresistive pattern is formed by photolithography.

[0013] (2) Ion implantation, followed by adhesive removal;

[0014] (3) Sputtering AI, metal graphics;

[0015] (4) Photolithography and etching are used to form silicon trenches;

[0016] (5) Metal is sputtered onto the surface of the silicon pillar, and the metal is etched to form an electrode;

[0017] (6) Si-metal-Si bonding;

[0018] (7) Photolithography and etching to form silicon pillars.

[0019] There are two ways to implement the array-type MEMS force sensor. One is to dic the MEMS force sensor after it is fabricated and then combine it in an array using a flexible circuit board. The other is to design the number of arrays according to the application requirements during the fabrication of the MEMS force sensor, and interconnect the common electrodes by metal sputtering to reduce the dicing process. The array-type MEMS force sensor formed by this method is smaller in size.

[0020] The advantages of this invention compared to the prior art are:

[0021] This invention discloses a flip-chip force sensor array. The bumps on a flexible board transmit the measured external force to the sensitive area of ​​a MEMS force sensor. The force is then converted into an electrical signal via a Wheatstone bridge, and the signal is transmitted to the flexible board via electrodes. The flip-chip design avoids the risks of poor contact or easy breakage associated with traditional gold wire leads. The silicon pillar design places the force-sensitive film in a suspended state, ensuring stable zero-point output of the MEMS force sensor when no external force is applied. When an external force is applied, the force on the MEMS force sensor is concentrated and highly sensitive. Compared to existing ultrasonic ball bonding technology, the silicon pillar and MEMS force sensor are integrated, reducing the risk of gold ball detachment due to welding, resulting in a more stable and reliable overall structure. The bump design on the flexible circuit board rigidly transmits the measured force to the sensitive area of ​​the MEMS force sensor. Compared to traditional dispensing methods, this results in less attenuation and less waveform distortion during force transmission, leading to higher measurement sensitivity and accuracy. The array design simplifies the sensor fabrication process, reduces the overall size, and provides compensation for the main signal, improving the measurement accuracy of the measured signal. Overall, the inverted force sensor array provided by this invention has advantages such as small size, high accuracy, and high reliability compared with existing MEMS pressure sensors, and is suitable for wearable monitoring fields. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of an inverted force sensor array structure according to the present invention;

[0023] Figure 2(a) is a top view of the MEMS force sensor structure;

[0024] Figure 2(b) is a side view of the MEMS force sensor structure;

[0025] Figure 3(a) is a top view of the flexible circuit board;

[0026] Figure 3(b) is a side view of the flexible circuit board;

[0027] Figures 4(a) to 4(f) This describes the main fabrication process of the MEMS force sensor of this invention.

[0028] In the picture:

[0029] 1-Arrayed MEMS force sensor, 2-Flexible circuit board, 3-MEMS force sensor, 4-Force sensing film, 5-Wheatstone sensitive bridge, 6-Frame, 7-Electrode, 8-Packaging pad, 9-Pad, 10-Bump, 11-Silicon pillar. Detailed Implementation

[0030] Example 1:

[0031] like Figure 1 As shown, the flip-chip force sensor array structure includes an array of MEMS force sensors 1 and a flexible circuit board 2. The array of MEMS force sensors 1 consists of 1×4 MEMS force sensors 3. The MEMS force sensors 3 adopt a crossbeam structure and are composed of a force-sensing film 4, a Wheatstone bridge 5, a frame 6, and electrodes 7. The force-sensing film 4 is located at the center of the crossbeam. The Wheatstone bridge 5 consists of four resistor strips, which are located in the area of ​​maximum stress in the crossbeam. The four electrodes 7 are distributed at the four right-angle positions of the frame 6. The flexible circuit board 2 has m×n MEMS force sensor packaging pads 8. Each packaging pad 8 includes four pads 9 and one bump 10, with the bump 10 located at the center of the packaging pad 8.

[0032] The four electrodes 7 of the MEMS force sensor 3 are formed by deep etching (silicon pillar 11) of silicon wafer and sputtering of metal. Compared with the plane of the force-sensitive thin film 4, the height difference is about 40-50 μm.

[0033] The MEMS force sensor 3 is soldered to the flexible circuit board 2 in an inverted manner;

[0034] The bump 10 of the flexible board encapsulation pad 8 is spherical. The length of the bump 10 is smaller than the length of the force-sensitive film area, and the height is about 45-50μm. The bump 10 is achieved by opening the pad and immersion gold, without any electrical connection. It is used to contact the force-sensitive film 4 and act as a force conductor to transmit the measured force to the force-sensitive film 4.

[0035] Figures 4(a) to (f) illustrate the main fabrication process of a MEMS force sensor according to the present invention, including the following steps:

[0036] (1) Prepare the wafer. The silicon wafer surface is oxidized to generate SiO2, as shown in Figure 4(a);

[0037] (2) Ion implantation, as shown in Figure 4(b);

[0038] (3) Remove adhesive, thermally oxidize, and form an isolation layer;

[0039] (4) Sputter metal AL, pattern the metal, and photolithographically etch a cross beam shape, as shown in Figure 4(c);

[0040] (5) Prepare wafers, perform photolithography and etching to form silicon trenches, as shown in Figure 4(d);

[0041] (6) Metal is sputtered on the front side, and electrodes are formed by photolithography and metal etching, as shown in Figure 4(e);

[0042] (7) Si-metal-Si bonding, photolithography, and etching are used to obtain silicon pillars with a height of about 40-50 μm, as shown in Figure 4(f).

[0043] After the MEMS force sensor is fabricated, it is diced and then arrayed using a flexible circuit board; or, during the fabrication of the MEMS force sensor, the number of arrays is designed according to the application requirements, and the common electrodes are interconnected by metal sputtering, reducing the dicing process. The arrayed MEMS force sensor formed in this way is smaller in size.

Claims

1. An inverted force sensor array, characterized in that, The device includes an array-type MEMS force sensor and a flexible circuit board. The array-type MEMS force sensor consists of m×n MEMS pressure sensors, where m≥1 and n≥1. The MEMS pressure sensor adopts a crossbeam structure and consists of a force-sensing film, a Wheatstone bridge, a frame, and electrodes. The force-sensing film is located at the center of the crossbeam. The Wheatstone bridge consists of four resistor strips with the same configuration, located in the area of ​​maximum stress in the crossbeam. The four electrodes are distributed at the four right-angle positions of the frame. The four electrodes of the MEMS pressure sensor are etched from a silicon wafer to form silicon pillars. The flexible circuit board has m×n MEMS pressure sensor packaging pads distributed on it. Each packaging pad includes four pads and one bump, with the bump located at the center of the packaging pad. The bump is achieved by opening the pad and adding immersion gold, without any electrical connection. It is used to contact the force sensing film and act as an external force conductor to transmit the measured external force to the force sensing film.

2. The inverted force sensor array according to claim 1, characterized in that, The height difference between the silicon pillar and the force-sensing thin film plane is 40–50 μm.

3. The inverted force sensor array according to claim 1, characterized in that, The MEMS pressure sensor is soldered to the flexible circuit board in an inverted manner.

4. The inverted force sensor array according to claim 1, characterized in that, The bumps on the encapsulation pads of the flexible circuit board are spherical, with a diameter less than or equal to the length of the force-sensing film area and a height of 45–50 μm.

5. The inverted force sensor array according to claim 1, characterized in that, There are two ways to implement the array-type MEMS force sensor. One is to dice the MEMS pressure sensor after it is fabricated and then combine it in an array using a flexible circuit board. The other is to design the number of arrays according to the application requirements during the fabrication of the MEMS pressure sensor, and interconnect the common electrodes by metal sputtering.

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