CN1242272C - Method for manufacturing micromachined capacitive acceleration sensor by wet etching - Google Patents

Abstract

The invention relates to a method for manufacturing a micro-mechanical capacitive acceleration sensor by a wet method, belonging to the field of micro-electromechanical systems. The method is characterized in that the most important point is that a compensation strip structure sacrificial method and selective corrosion characteristics of (110) monocrystalline silicon in anisotropic corrosion liquid are utilized, a required capacitance acceleration sensor structure is manufactured on (110) monocrystalline silicon by adopting silicon anisotropic wet etching, a movable electrode and a fixed electrode of the capacitance acceleration sensor are simultaneously formed on the (110) monocrystalline silicon, and a gap between capacitance plates is 3-15 microns. The parallel electrodes of the capacitor must be aligned strictly to the (111) crystal orientation. The method provided by the invention simplifies the manufacturing process of the micro-mechanical capacitance type acceleration sensor, improves the yield of the capacitance type acceleration sensor, and ensures that the manufactured acceleration sensor has high sensitivity and output stability.

Description

Method for manufacturing micromachined capacitive acceleration sensor by wet etching

technical field

The invention relates to a method for manufacturing a micromachined capacitive acceleration sensor by wet etching, more precisely, the invention relates to a method for manufacturing a micromachined capacitive acceleration sensor with compensation strip sacrificial technology and silicon anisotropic etching technology as key technologies sensor method. It belongs to the field of microelectromechanical systems.

Background technique

Silicon micro-accelerometers are very important micro-inertial devices that can be used in automobiles, robots, and various guidance and measurement and control systems. According to the sensitive principle, there are roughly the following types of micromechanical acceleration sensors: piezoresistive, piezoelectric, thick film strain gauge, electromagnetic, thermocouple, resonator and capacitive, etc. Among them, the capacitive acceleration sensor can be divided into force balance type and non-force balance type. The movable mass forms a movable electrode of the variable capacitor. When the mass block is displaced by the acceleration, the capacitance formed between the fixed electrode and the movable electrode changes, and the acceleration can be measured by detecting this change with the peripheral circuit. In order to obtain higher measurement sensitivity and reduce the complexity of the peripheral circuit, in the design, the electrode area is increased and the distance between the electrodes is reduced to obtain a higher equivalent capacitance. In order to increase the linearity of the output signal, a differential capacitance measurement structure is often used. Compared with the piezoresistive or piezoelectric type, the capacitive acceleration sensor has the advantages of small temperature effect and good repeatability, and is currently the most developed type of sensor.

There are surface micromachining methods and silicon body micromachining methods for making capacitive acceleration sensors. The advantage of using surface micromachining to make capacitive acceleration sensors is that it is compatible with integrated circuit technology, can integrate signal processing circuits, and has low cost, but there are also disadvantages such as large noise, poor stability, limited range, and small bandwidth. The advantages of using silicon micromachining to make capacitive acceleration sensors are low noise, good stability, high sensitivity, and large damping. The disadvantage is that the volume is slightly larger.

In the past, when silicon micromachining methods were used to manufacture capacitive acceleration sensors, most of them used (100) crystalline silicon wafers, and double-sided etching methods formed capacitive acceleration device structures, such as F.Rudolfet.al, Precision Accelerometers with μg Resolution, Sensors and Actuators, A21-23, (1990) pp297-302. This method has problems such as difficulty in controlling the size of the device structure, difficulty in process processing, and packaging stress. Wet anisotropic etching of silicon wafers with (100) crystal orientation cannot form vertical silicon wafer surfaces and parallel strip structures. Therefore, dry etching is usually required to form vertical silicon wafer surfaces and parallel strip structures. Etching technology (deep reactive ion etching technology DRIE), which requires the use of expensive equipment, increases the cost of device manufacturing, and at the same time, the capacitor plate gap is limited by the aspect ratio of DRIE technology and cannot be very narrow, such as Zhixiong Xiao et.al, Silicon micro-accelerometer with resolution, high linearity and large frequency bandwidth fabricated with twomask bulk process, Sensors and Actuators, A77, (1999) pp 113-119.

Wet anisotropic etching using (110) oriented silicon wafers can form vertical silicon wafer surfaces and strip structures parallel to each other. However, due to the anisotropic characteristics of silicon, usually the result of etching is to form a parallelogram structure instead of a rectangular structure, such as D.R.Ciarlo, A latching accelerometer fabricated by the anisotropic etching of(110) oriented silicon wafers, J.Micromechanics and Microengineering , Volume 2, Number 1, March1992, pp10. This limits the shape design of the device and affects the performance of the device.

Contents of the invention

One of the purposes of the present invention is to provide a method for manufacturing micromachine capacitive acceleration sensors by wet etching. The movable electrode and the fixed electrode of the capacitive acceleration sensor can be formed on (110) single crystal silicon at the same time, and the gap between the parallel capacitor plates can be less than 10 microns. Not only can the manufacturing process be simplified, but also the manufactured micromechanical capacitive acceleration sensor can show higher sensitivity.

The second object of the present invention is to provide a method for protecting the desired monocrystalline silicon structure in the process of wet etching silicon. The thin strip structure is sacrificially etched, so that a rectangular structure is finally formed; the silicon oxide formed by secondary oxidation is used as a sensor The protective mask on the bottom surface of the structure enables the expected device structure to be completely preserved and not damaged after the wet anisotropic etching is completed.

The third object of the present invention is to provide a micromachined capacitive acceleration sensor with a special structure made by the above-mentioned new manufacturing method, which is characterized in that two sets of blocking blocks are used to increase the anti-overload capacity of the micromachined capacitive acceleration sensor. In addition to increasing the sensor's anti-overload capability, the two groups of blocking blocks can also be used for self-testing of the device.

The present invention is implemented through the following processes:

(1) Choose a double-sided polished (110) single crystal silicon wafer, and form a 0.1-0.3 micron silicon oxide layer by the first thermal oxidation, and the thermal oxidation temperature is 1050-1150 °C;

(2) Then the front side is protected by photoresist, and the back side is aligned with the (111) crystal direction to photoetch the pattern of the suspended release area;

(3) use anisotropic alkaline etching solution to corrode silicon to form a depth of 4-10 microns;

(4) Rinse off silicon oxide with HF solution, perform a second thermal oxidation at 1050-1150°C to form a silicon oxide layer of 1-1.4 microns, perform second photolithography on the silicon wafer, and protect the front side with photoresist , the bonding area is etched on the backside;

(5) Carry out double-sided lithography again, the front lithography produces the structural pattern of the acceleration sensor, and the back photoresist protects;

(6) Bond the corroded surface of the photoetched silicon wafer with the Pyrex glass sheet or with another silicon wafer, the bonding temperature is 380-470°C, and the voltage is 800-1100V;

(7) Put the silicon wafer into the anisotropic etching solution after bonding, and conduct corrosion penetration at 60°C. The thin strip structure is sacrificially etched to form a capacitive acceleration sensor structure, and the silicon wafer is taken out for cleaning;

(8) bleach silicon oxide with HF solution, release the accelerometer structure;

(9) Evaporate the metal aluminum film to form electrodes; complete the device packaging with the flip-down packaging method.

Below in conjunction with accompanying drawing, illustrate the present invention in detail.

The specific process of making the acceleration sensor is as follows:

Select double-sided polished (110) crystal silicon wafer 1, and perform thermal oxidation at 1050-1150° C. to form silicon oxide 2 with a thickness of 0.1-0.3 microns ( FIG. 3 ). Then, the front side is protected by photoresist, and the back side is aligned with the <111> crystal direction to photocut the pattern of the suspended release area of the capacitive acceleration sensor (FIG. 4). Etch silicon with silicon anisotropic etchant such as potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH), etch to a certain depth, which is controlled according to needs, generally within 4-10 microns. Rinse off silicon oxide with HF solution, and perform a second thermal oxidation at 1050-1150°C to form silicon oxide of 1-1.4 microns. The silicon wafer is subjected to secondary photolithography, the front side is protected by photoresist, and the back side is photoetched to form the bonding region 3 (FIG. 5). Then double-sided photolithography is carried out, and the structural pattern of the acceleration sensor is photo-etched on the front side, and the photoresist on the back side is protected. Anodically bond the photoetched silicon wafer to Pyrex 7740 glass 4, the bonding temperature is 380°C-470°C, and the bonding voltage is 800V-1100V (Figure 6). Or it can be bonded to another silicon wafer. The bonding of silicon wafers to glass wafers is anodic electrostatic bonding, and the bonding of silicon wafers to silicon wafers is high-temperature bonding. Then put the bonded silicon chip into potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH) silicon anisotropic etching solution for etching, and the etching temperature is 60°C. After the silicon wafer is corroded and penetrated to form the acceleration sensor structure, the silicon wafer is taken out for cleaning (FIG. 7). Rinse off the silicon oxide with HF solution to release the accelerometer structure. The metal aluminum film 5 is evaporated to form an electrode (FIG. 8). Finally, the package of the acceleration sensor is completed with flip-down packaging (Figure 9).

In order to form a vertical silicon wafer plane and parallel capacitive plates, the lithographic pattern of the parallel electrode of the acceleration sensor capacitor must be strictly aligned with the <111> crystal direction, that is, the plane of the finally formed parallel capacitive plates is parallel to and (110) the surface of the silicon chip Vertical (111) plane. The movable electrode and the fixed electrode of the capacitive acceleration sensor can be formed on (110) single crystal silicon at the same time, and the gap between the parallel capacitive plates is 3-15 microns.

In order to form the required rectangular acceleration sensor structure, the thin strip structure 8 (Fig. 10) is used on the side of the capacitor plate perpendicular to the gap between the capacitor plates, so that during the etching process in the silicon anisotropic etching solution, due to the anisotropy of the silicon Anisotropic characteristics, the thin strip structure is sacrificially etched, so that the final rectangular sensor structure can be formed. The exact length of the sacrificial strip structure is determined according to the chamfering corrosion rate of the etching solution. Usually, the ratio of the length of the sacrificial strip structure to the depth of the etched silicon wafer is 1.2-2:1, and the width of the sacrificial strip is generally 5 microns to 50 microns. The pitch is 20 microns to 100 microns.

In order to protect the bottom surface of the movable electrode of the acceleration sensor from being corroded in the process of anisotropic wet etching of the structure of the acceleration sensor, the silicon oxide formed by the second oxidation is used as a mask for the structure of the anisotropic wet etching of the acceleration sensor. It is used to protect the bottom surface of the movable mass electrode 9 (Fig. 7).

In order to improve the anti-overload ability of the capacitive acceleration sensor, a structure of the capacitive acceleration sensor is designed. By adopting the method for manufacturing the acceleration sensor, while the fixed acceleration electrode 14 and the movable mass electrode 9 are formed by corrosion, two sets of blocking blocks 12 and 13 can be formed at the two ends of the movable electrode at the same time. The gap between the blocking blocks 12, 13 and the movable mass electrode 9 is smaller than the gap between the fixed electrode 14 and the movable mass electrode 9, so that the blocking blocks can improve the anti-overload capability of the capacitive acceleration sensor. The two groups of blocking blocks 12, 13 cooperate with relevant circuits to make the capacitive acceleration sensor a capacitive acceleration sensor with a self-check function (Fig. 1).

According to the structure of the capacitive acceleration sensor provided by the method of the present invention, the fixed electrodes are located on both sides of the movable mass electrode, one end of the elastic cantilever beam is connected with the fixed anchor point, and the other end is connected with the movable mass electrode; two sets of blocking blocks They are respectively located on two sides of the movable mass block, and are symmetrical to the elastic cantilever beam.

In short, the present invention provides a method for fabricating a micromachine capacitive acceleration sensor, which is easy to manufacture and low in cost compared with previous methods. The invention also provides a capacitive acceleration sensor structure, which has two independent blocking blocks, which can be used for sensor overload protection and self-check. In addition, due to the use of single crystal silicon, the performance of the acceleration sensor is stable, and different cantilever lengths can be designed to change the sensitivity of the sensor according to different application needs, which has great flexibility.

Description of drawings

FIG. 1 is a schematic diagram of an acceleration sensor structure formed after a silicon wafer is corroded and penetrated.

FIG. 2 is a schematic cross-sectional view of the acceleration sensor shown in FIG. 1 . (a) AA section (b) BB section.

Fig. 3 is a schematic diagram of forming a suspended release area pattern on a silicon substrate.

FIG. 4 is a schematic diagram of corroding silicon to form a certain suspension depth.

FIG. 5 is a schematic diagram of etching silicon oxide to form a mask for protecting device structures.

Figure 6 is a schematic diagram of bonding.

FIG. 7 is a schematic diagram of the silicon wafer being corroded and penetrated to form an acceleration sensor structure.

Fig. 8 is a schematic diagram after rinsing silicon oxide with HF solution, evaporating metal aluminum film, and forming electrodes.

FIG. 9 is a schematic diagram of the package of the acceleration sensor completed by flip-down packaging.

Fig. 10 is a schematic structural diagram of an acceleration sensor made according to the present invention.

In the picture:

1-Silicon 2-Silicon Oxide

3-Silicone bonding area 4-Pyrex glass

5-Metal aluminum 6-Package cover

7-solder 8-sacrifice compensation strip

9- Movable mass electrode of capacitive acceleration sensor

10- Elastic cantilever beam

11-Anchor point for fixing the movable electrode of the capacitive acceleration sensor

12-The first group of anti-overload blocking blocks 13-The second group of anti-overload blocking blocks

14-Capacitive acceleration sensor fixed electrode

Detailed ways

The substantive features and remarkable progress of the present invention will be further explained through the following description of the method of manufacturing a micromachine capacitive acceleration sensor by wet etching, but the present invention is by no means limited to the embodiments.

Example 1

(1) Choose a double-sided polished (110) crystal silicon wafer 1, and perform thermal oxidation at 1100°C to form a 0.2 micron silicon oxide layer 2;

(2) Then the front side is protected by photoresist, and the back side is aligned with the <111> crystal direction to photocut the pattern of the suspended release area of the capacitive acceleration sensor;

(3) Corroding silicon with KOH anisotropic etching solution, with an etching depth of 6 microns;

(4) Rinse off silicon oxide with HF solution, and perform a second thermal oxidation at 1100° C. to form silicon oxide of 1.2 microns;

(5) Carry out secondary photolithography to the silicon wafer, the front photoresist is protected, and the back photoetches the bonding area 3;

(6) Carry out double-sided lithography again, the front lithography produces the structure pattern of the acceleration sensor, and the back photoresist protects;

(7) Anodically bond the photolithographic silicon wafer to Pyrex 7740 glass 4, the bonding temperature is 420°C, and the bonding voltage is 1000V;

(8) Put the bonded silicon wafer into the tetramethylammonium hydroxide etching solution for corrosion penetration, and the corrosion temperature is 60°C;

(9) After the silicon chip is corroded and penetrated, and the thin strip structure is corroded, an acceleration sensor structure is formed, and the silicon chip is taken out for cleaning;

(10) Rinse off silicon oxide with HF solution to release the acceleration sensor structure. Evaporating the metal aluminum film 5 to form an electrode;

(11) Finally, the package of the acceleration sensor is completed with an inverted package.

The structure of the capacitive acceleration sensor manufactured through the above process is shown in Figure 1. The gap between the parallel capacitor plates is 10 microns.

Claims (3)

1. A method for manufacturing a micromachine capacitive acceleration sensor by wet etching, comprising single crystal silicon wafer selection, oxidation, corrosion process steps, characterized in that: (1) Choose a double-sided polished (110) single crystal silicon wafer, and form a 0.1-0.3 micron silicon oxide layer by the first thermal oxidation, and the thermal oxidation temperature is 1050-1150 °C; (2) Then the front side is protected by photoresist, and the back side is aligned with the (111) crystal direction to photoetch the pattern of the suspended release area; (3) use anisotropic alkaline etching solution to corrode silicon to form a depth of 4-10 microns; (4) Rinse off silicon oxide with HF solution, perform a second thermal oxidation at 1050-1150°C to form a silicon oxide layer of 1-1.4 microns, perform second photolithography on the silicon wafer, and protect the front side with photoresist , the bonding area is etched on the backside; (5) Carry out double-sided lithography again, the front lithography produces the structural pattern of the acceleration sensor, and the back photoresist protects; (6) Bond the corroded surface of the photoetched silicon wafer with the Pyrex glass sheet or with another silicon wafer, the bonding temperature is 380-470°C, and the voltage is 800-1100V; (7) Put the silicon wafer into the anisotropic etching solution after bonding, and conduct corrosion penetration at 60°C. The thin strip structure is sacrificially etched to form a capacitive acceleration sensor structure, and the silicon wafer is taken out for cleaning; (8) bleach silicon oxide with HF solution, release the accelerometer structure; (9) Evaporate the metal aluminum film to form electrodes; complete the device packaging with the flip-down packaging method. 2. The method for manufacturing a micromachine capacitive acceleration sensor by wet etching according to claim 1, wherein the silicon anisotropic etching solution is potassium hydroxide or tetramethylammonium hydroxide. 3. The method for manufacturing a micromachine capacitive acceleration sensor by wet etching according to claim 1, characterized in that after the bonding, anisotropic etching is carried out, and the thin strip structure is sacrificially etched, and the width of the sacrificial thin strip is 5-50 Micron, the spacing of the thin strips is 20-100 microns; the ratio of the length to the depth of the etched silicon wafer is 1.2-2:1.

Images