CN102980694A - MEMS piezoresistive pressure transducer without strain membrane structure and manufacture method thereof - Google Patents

Abstract

The invention relates to a MEMS piezoresistive pressure sensor with a strain-free membrane structure, comprising four groups of piezoresistors fabricated on a substrate and forming a Wheatstone bridge, wherein two sets of opposite piezoresistors are arranged along the <100> crystal direction , and the other two groups of opposite varistors are arranged along the <110> crystal direction. The manufacturing steps are: on the front of the substrate, use the doping concentration required by the piezoresistor to perform P-type ion implantation light doping and high temperature thermal annealing; define the P-type heavily doped lead contact area on the front of the substrate by photolithography , heavy doping by ion implantation and high-temperature thermal annealing; making lead holes and metal leads; defining the shape of varistors and contact areas by photolithography, making varistor strips by etching; scribing. The pressure sensor of the present invention has no strain membrane structure, can reduce the chip size of the sensor, and increase the anti-overload capability; its manufacturing method is compatible with the process of standard body silicon piezoresistive pressure sensors, and has low cost and high yield.

Description

MEMS piezoresistive pressure sensor with strain-free film structure and manufacturing method thereof

technical field

The invention belongs to the field of micro-electro-mechanical system (MEMS) sensor design, and relates to a MEMS piezoresistive pressure sensor and a method for manufacturing the pressure sensor on a single wafer by using a MEMS processing technique.

Background technique

MEMS (Micro Electro Mechanical System) is an emerging interdisciplinary high-tech research field. The piezoresistive pressure sensor based on MEMS technology is widely used in the modern market due to its excellent accuracy and reliability and relatively cheap manufacturing cost. Silicon-based piezoresistive pressure sensors have been widely used since the discovery of the piezoresistive properties of silicon materials in the mid-1950s. The working principle of a typical piezoresistive pressure sensor is to make four pressure-sensitive resistors in the stress concentration area by diffusion or ion implantation on a square or circular silicon strained film, and the four resistors are interconnected to form a Wheatstone bridge. When external pressure is applied to the strained silicon film, the piezoresistor area will generate stress due to the bending of the strained film. Through the piezoresistive characteristics of the piezoresistor, the stress will be converted into a change in the resistance value, and finally the resistance value will be converted by the Wheatstone bridge. The change of the output voltage is converted into the output voltage, and the measurement of the pressure can be realized by calibrating the output voltage and the pressure value.

The core composition of the traditional piezoresistive pressure sensor requires a strain film as a stress concentration structure. The pressure range and sensitivity of the piezoresistive pressure sensor are related to the thickness and size of the strain film under the same processing conditions. Due to the existence of the strain film, limited by the fracture strength of the silicon material, the traditional piezoresistive pressure sensor has limited anti-overload ability, and the piezoresistive pressure sensor in the high-pressure application field has the disadvantage of low anti-overload ability.

Contents of the invention

The object of the present invention is to address the above problems, to propose a MEMS piezoresistive pressure sensor with a strain-free membrane structure, and a method for making the pressure sensor. The biggest difference between the pressure sensor with this structure and the typical device structure is that there is no strain film for stress concentration, which can reduce the chip size of the sensor and greatly increase the overload resistance of the pressure gauge; at the same time, the processing method is similar to that of standard bulk silicon The processing technology of the piezoresistive pressure sensor is compatible, the device processing cost is low, and the finished product rate is high.

To achieve the above object, the technical scheme adopted in the present invention is as follows:

A MEMS piezoresistive pressure sensor with a strain-free film structure, including four sets of piezoresistors fabricated on a substrate and forming a Wheatstone bridge, wherein two sets of opposite piezoresistors are arranged along the <100> crystal direction of the substrate , the other two groups of relative piezoresistors are arranged along the <110> crystal direction of the substrate.

Preferably, the piezoresistor is fabricated on a (100) crystal plane of a common single crystal silicon wafer or an SOI (silicon on insulator) silicon wafer.

In the above pressure sensor, the number of piezoresistor strips in each group of piezoresistors is one or a plurality of piezoresistors connected in series.

In the above pressure sensor, under the premise of meeting the requirements of the crystal orientation and forming a Wheatstone bridge, the positions of the four groups of piezoresistors can be arranged arbitrarily, and can be connected in series to form an open loop or a closed loop, and the four groups of piezoresistors A metal pad is set at the connection between them as an electrical signal input and output interface. Preferably, each group of varistors is connected end to end to form a parallelogram, more preferably a rhombus, and the two groups of varistors on the opposite side are arranged along the same crystal direction, and metal pads (welding pads) are distributed at the four vertices of the rhombus , adopting this arrangement can minimize the size of the pressure sensor chip.

A method for making the above-mentioned MEMS piezoresistive pressure sensor, the steps comprising:

1) Lightly doped with P-type ion implantation on the front surface of the substrate using the doping concentration required by the piezoresistor, and then activated the implanted ions by high-temperature thermal annealing;

2) Define the P-type heavily doped lead contact area by photolithography on the front of the substrate, perform heavy doping in the contact area by ion implantation, and then activate the implanted ions by high temperature thermal annealing;

3) Make lead holes and metal leads on the front of the substrate;

4) Define the shapes of four groups of varistors and contact areas by photolithography on the front of the substrate, and make varistor strips by etching; the four groups of varistors form a Wheatstone bridge, of which two groups The opposite varistors are arranged along the <100> crystal direction of the substrate, and the other two sets of opposite varistors are arranged along the <110> crystal direction of the substrate;

5) Dicing to make a pressure sensor chip.

In the above method, the order of steps 1) and 2) can be changed, that is, step 1) can be performed first, and then step 2), or step 2) can be performed first, and then step 1). .

In the above method, the etching in step 4) can be performed by dry method or wet method; the dry method is preferably RIE (reactive ion etching) etching, and the wet method is preferably HNA solution (a mixed solution of HF acid, nitric acid, and acetic acid) ) isotropic corrosion. Preferably, when an SOI silicon wafer is used as a device processing substrate, etching is carried out until the buried oxide layer of the silicon wafer is exposed; when an ordinary single crystal silicon wafer is used as a processing substrate, the etching needs to be carried out at least until the implanted ion Below the diffusion depth (0.5μm to 5μm).

The above process can be used to complete the manufacture of MEMS pressure sensors with no strain film structure. The piezoresistors are strip-shaped protrusions distributed on the surface of the device. The piezoresistor strips have three sides that can sense the pressure of the external environment (gas pressure or liquid pressure) ), the four groups of varistors that make up the Wheatstone bridge are along different crystal directions, and the varistors distributed along the <110> crystal direction are subjected to side pressure to increase their resistance value; the pressure along the <100> crystal direction Since the piezoresistive coefficient of the <100> crystal orientation is approximately equal to zero, the resistance value of the sensitive resistor remains unchanged. Under the action of Wheatstone bridge, the change of resistance value is converted into the change of voltage and current. Since the pressure sensor has no strained membrane, there is no deformation of the membrane structure under high-pressure environment, which can overcome the shortcoming of insufficient anti-overload ability of the high-pressure manometer.

The invention provides a high-pressure absolute pressure sensor and a manufacturing method thereof for technicians in the field of MEMS. The pressure sensor (manometer) processed by the method has better performance and higher process reliability. Specifically, the present invention has the following advantages:

1) The MEMS piezoresistive pressure sensor of the present invention can be applied in an extremely high pressure environment;

2) The manufacturing method of the pressure sensor of the present invention has only four photolithography processes, and is compatible with the traditional IC surface processing technology; the process difficulty is relatively low, and it is easy to obtain a higher yield;

3) The structural design of the pressure sensor of the present invention is reasonable, unnecessary steps are reduced during the preparation process, and the difficulty of photolithography is reduced;

4) The device designed in the present invention has no strained membrane structure, which avoids the fluctuation of the thickness of the strained membrane caused by the fluctuation of the process conditions and the fluctuation of the device performance caused by the fluctuation of the strained membrane during the manufacturing process of the traditional strained membrane structure manometer, and improves the performance of the strained membrane. The reliability of the process and the yield of the device are improved.

Description of drawings

Fig. 1 is a schematic diagram of the process flow of the strain-free membrane absolute pressure gauge in a specific embodiment, wherein:

Fig. 1 (a) is the schematic diagram of chip substrate;

Figure 1(b) is a schematic diagram of doping by ion implantation;

Figure 1(c) is a schematic diagram of making a heavily doped contact region;

Figure 1(d) is a schematic diagram of making contact holes and metal leads;

Fig. 1 (e) is the schematic diagram of front etching varistor strip;

FIG. 2 is a schematic diagram of layout and varistor distribution at the silicon island structure.

FIG. 3 is a schematic diagram of an open-loop structure formed by a piezoresistor.

In the figure: 1 substrate; 2 surface silicon material after P-type doping; 3- heavily doped contact area; 4 silicon oxide layer; 5- metal contact hole and metal lead; 6- varistor strip; 7—<100 > Crystal varistor strips; 8—<110> crystal varistor strips.

Detailed ways

The present invention will be further described below through specific embodiments and accompanying drawings.

The preparation method of the MEMS piezoresistive pressure sensor in this embodiment is to etch two pairs of piezoresistive strips along the <110> crystal direction and <100> crystal direction on the substrate after P-type ion implantation and doping. The resistance bars are connected end to end to form a rhombus. The steps of the method include:

1) Select (100) crystal plane single crystal silicon wafer or (100) crystal plane SOI silicon wafer as the chip substrate;

2) P-type light doping is completed by ion implantation on the front of the substrate; the doping concentration required for the piezoresistor is completed, and high-temperature thermal annealing activates the implanted ions;

3) The heavily doped contact area is defined by photolithography, and P-type heavy doping is carried out by ion implantation; after completion, high-temperature thermal annealing is used to activate the implanted impurities;

3) Make lead holes and metal leads on the front of the substrate;

4) Defining the shape of the piezoresistive strip by photolithography on the front side of the substrate, and then making the piezoresistive strip by etching;

5) Dicing to make a pressure sensor chip.

A specific preparation example is provided below, as shown in Figure 1, the manufacturing process of the MEMS piezoresistive micro-pressure pressure sensor is:

a) Preparation: select a single crystal silicon substrate as the chip substrate 1, the thickness of the substrate is 400 μm, as shown in Figure 1(a);

b) Using standard piezoresistive technology to fabricate piezoresistor concentration doped single crystal silicon 2 on the silicon wafer, as shown in Figure 1(b), including: ion implantation B ⁺ ; boron advancement;

光刻浓硼区，RIE SiO₂

离子注入B^＋；硼推进；c) Fabricate the heavily doped contact region 3 on the silicon wafer using standard piezoresistive technology, as shown in Figure 1(c), including: thermally oxidized SiO ₂

Photolithographic boron-enriched regions, RIE SiO ₂

Ion implantation B ⁺ ; boron propulsion;

d) Make lead holes (that is, contact holes) and metal leads 5, as shown in Figure 1(d), including:

正面光刻引线孔；RIE SiO₂

溅射Al，厚度为0.8-1.0μm；光刻金属引线；湿法腐蚀Al；进行Al合金化工艺；LPCVD (low pressure chemical vapor deposition) SiO ₂ with a thickness of

Lithographic lead holes on front side; RIE SiO ₂

Sputtering Al with a thickness of 0.8-1.0μm; photoetching metal leads; wet etching Al; performing Al alloying process;

e) Etching the piezoresistive strip 6 on the front side, as shown in Figure 1(e), including:

反应离子刻蚀Si 4μm；压阻条的高度由器件的离子注入后的离子的退火深度决定可以有不同的高度（压阻条的高度必须比离子注入退火后离子的扩散深度大，否则会造成器件短路）；Photolithographic piezoresistive strip area; etching SiO ₂

Reactive ion etching Si 4μm; the height of the piezoresistive strip is determined by the annealing depth of ions after ion implantation of the device and can have different heights (the height of the piezoresistive strip must be greater than the diffusion depth of ions after ion implantation annealing, otherwise it will cause device short circuit);

f) Scribing: This step cuts the entire silicon wafer into small pieces, each small piece is a complete pressure gauge, and each silicon piece can be divided into 100 to 200 pressure gauge pieces according to the size of the designed pressure gauge.

In the above preparation process, the piezoresistive strips are produced after the metal leads are fabricated on the front side, which can avoid the problem of exposure difficulties that may be caused by the height difference of the front piezoresistive strips when the metal leads are fabricated after the piezoresistive strips are fabricated.

Figure 2 is a schematic diagram of the overall structure of the pressure sensor, which is mainly composed of piezoresistive strips and metal pads. The piezoresistors are strip-shaped protrusions distributed on the substrate, and the combined shape is arranged in a rhombus, and the piezoresistive strips in the rhombus are respectively arranged along the <100> and <110> crystal directions. Metal pads and heavily doped contact areas are located at the 4 endpoints of the rhombus.

In the pressure sensor of the present invention, the number of piezoresistive strips is not limited to the number in Figure 2, and the piezoresistive strips in each direction can be composed of multiple piezoresistive strips in the same direction in series; the connection mode of the piezoresistive strips is not limited to that shown in Fig. In the rhombus connection mode in 2, the piezoresistive strips can be arranged arbitrarily under the premise that the crystal direction along each group of piezoresistive strips remains unchanged and the series connection mode of the piezoresistive strips remains unchanged.

The Wheatstone bridge in the above embodiment selects the series structure of piezoresistive strips in closed-loop mode as an example. Those skilled in the art should understand that within the scope of not departing from the essence of the present invention, piezoresistive strips in open-loop mode can also be used. Barrier series structure. The open-loop method means that the circuit does not form a loop, that is, the circuit has one more metal pad. When the circuit is working, it is only necessary to connect the disconnected loop through an external circuit. The purpose of using an open loop is to facilitate testing. Figure 3(a) and Figure 3(b) illustrate two open-loop connections.

The outstanding feature of the present invention is that piezoresistors of the Wheatstone bridge are directly connected in series, which can reduce the size of the pressure sensor chip to the greatest extent.

The above-mentioned embodiments are only used to illustrate the principles of the present invention and not to constitute limitations. Those skilled in the art should understand that within the scope of not departing from the essence of the present invention, certain modifications can be made for the structure and size of the piezoresistive strips in the present invention. Changes and modifications, the protection scope of the present invention should be based on the claims.

Claims (10)

1. A MEMS piezoresistive pressure sensor with no strain film structure is characterized in that it comprises four groups of piezoresistors that are fabricated on the substrate and form a Wheatstone bridge, wherein two groups of piezoresistors are opposite to each other along the surface of the substrate. The <100> crystal direction is arranged, and the other two sets of relative piezoresistors are arranged along the <110> crystal direction of the substrate. 2 . The pressure sensor according to claim 1 , wherein the substrate is a single crystal silicon wafer or an SOI silicon wafer, and the piezoresistor is fabricated on the (100) crystal plane of the substrate. 3 . 3. The pressure sensor according to claim 1 or 2, characterized in that each group of piezoresistors comprises one or a plurality of piezoresistor strips connected in series. 4. The pressure sensor according to claim 1 or 2, characterized in that: the four groups of piezoresistors are connected in series to form an open loop or a closed loop. 5. The pressure sensor according to claim 4, wherein the four groups of piezoresistors are connected in series to form a rhombus. 6. The pressure sensor according to claim 4, characterized in that: metal pads are provided at the connections between the four groups of piezoresistors. 7. A method of making the pressure sensor according to claim 1, the steps comprising: 1) Lightly doped with P-type ion implantation on the front surface of the substrate using the doping concentration required by the piezoresistor, and then activated the implanted ions by high-temperature thermal annealing; 2) Define the P-type heavily doped lead contact area by photolithography on the front of the substrate, perform heavy doping in the contact area by ion implantation, and then activate the implanted ions by high temperature thermal annealing; 3) Make lead holes and metal leads on the front of the substrate; 4) Define four groups of varistors and the shape of the contact area by photolithography on the front of the substrate, and make varistor strips by etching; the four groups of varistors form a Wheatstone bridge, and two groups of varistors are opposite to each other The varistors are arranged along the <100> crystal direction of the substrate, and the other two sets of opposite varistors are arranged along the <110> crystal direction of the substrate; 5) Dicing to make a pressure sensor chip. 8. The method according to claim 7, characterized in that: step 1) is performed first, and then step 2); or step 2) is performed first, and then step 1). 9. The method according to claim 7, characterized in that: the etching in step 4) is performed by a dry method or a wet method. 10. The method according to claim 9, characterized in that: the dry method is RIE etching, and the wet method is isotropic etching with HNA solution.

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