Introduction to various processes of quartz MEMS sensor sensitive chips

MEMS (Micro Electro mechanical System) refers to an independent intelligent system formed by integrating mechanical parts, Electronic components, and sensor execution components on a micron-scale chip using micro-electromechanical processing technology according to functional requirements.

MEMS (Micro Electro mechanical System) refers to an independent intelligent system formed by integrating mechanical parts, electronic components, and sensor execution components on a micron-scale chip using micro-electromechanical processing technology according to functional requirements. Because single crystal quartz material has piezoelectric effect, and has excellent temperature, mechanical properties, high quality factors and other characteristics, the quartz MEMS acceleration sensor, pressure sensor and quartz MEMS gyroscope made of single crystal quartz have high precision and high precision. Stability, high resolution, etc., have a wide range of applications in micro-inertial navigation systems, attitude measurement and control, aerospace, automotive electronics, instrumentation and other fields. Its processing technology and equipment manufacturing technology research provide technical support for promoting industrial development. Very important meaning.

1 Quartz MEMS sensor sensitive chip structure

Quartz MEMS sensors are mainly used in vibrating inertial devices, which are inertial devices that measure various motion parameters (including angular velocity, angle, linear acceleration, etc.) of moving objects through the principle of vibration. Quartz MEMS vibrating inertial devices mainly include quartz micromachined vibrating gyroscopes, quartz vibrating beam accelerometers, etc. Its sensitive structure uses quartz crystal, based on the principle of quartz crystal piezoelectric effect, and adopts microelectronic processing technology. It is vibration inertia technology and micromechanical processing technology. The organic combination.

The quartz micromechanical vibrating gyroscope is a Coriolis vibrating gyroscope with a tuning fork structure and a MEMS angular velocity sensor. The sensitive chip structure is a double-ended tuning fork structure, which includes a driving tuning fork, a reading tuning fork and a supporting structure. The working principle of the quartz micromechanical vibrating gyroscope and the structure of the sensitive chip are shown in Figure 1. The driving tuning fork is excited to vibrate at its natural frequency. When the quartz MEMS gyroscope rotates around its vertical axis, the driving tuning fork is subjected to Coriolis force (Coriolis) to produce a vibration perpendicular to the tuning fork plane. This Coriolis motion is transmitted to the reading Out of the tuning fork, the readout tuning fork vibrates perpendicular to the tuning fork plane. The amplitude of vibration is proportional to the speed and external acceleration of the driving tuning fork. The piezoelectric effect is used to detect electrical signals by making electrodes on the readout tuning fork. The readout circuit demodulates to obtain a DC voltage output proportional to the input angular velocity. The sensitive chip structure is an integrated tuning fork structure with quartz crystal as the base material. There are double-ended tuning forks, single-ended tuning forks and other structural forms. The double-ended tuning fork structure separates the driving tuning fork from the read-out tuning fork, which is beneficial to reduce the coupling error, and has high sensitivity, but the volume is large; the single-ended tuning fork structure drives the tuning fork and the read-out tuning fork share a tuning fork, which is small in size, but the signals interfere with each other and the sensitivity Low.

Introduction to various processes of quartz MEMS sensor sensitive chips

Figure 1 Working principle and sensitive chip structure of quartz micromachined vibrating gyroscope

The quartz MEMS accelerometer, also known as the quartz vibrating beam accelerometer, is a feature that detects the acceleration of a moving body based on the characteristic that the resonant frequency of the quartz vibrating beam changes with external force. The working principle of the quartz vibrating beam accelerometer and the structure of the sensitive chip are shown in Figure 2. A pair of matched piezoelectric quartz vibrating beams and masses are supported by a flexible system to form a whole. The quartz vibrating beam and the external circuit together form two self-excited oscillators with different frequencies. If there is acceleration input in the direction of its sensitive axis, one vibrating beam is under tension and the other vibrating beam is under pressure, and the corresponding oscillation frequency increases one by one and the other decreases. , The input acceleration can be measured by finding the frequency difference. The sensitive chip structure of the quartz vibrating beam accelerometer adopts an integrated monolithic structure such as a quartz vibrating beam, a mass block, and a flexible support, and there are usually two forms of a split type and an integrated type. The split structure is composed of a double-ended fixed quartz vibrating beam and a flexible supporting structure. The advantage is that the quartz vibrating beam and the flexible supporting structure are processed separately, and the process is relatively simple, but the assembly process is more complicated. One-piece structure is to complete the production of vibrating beam, flexible support and isolation frame on a quartz substrate. Its advantage is to avoid thermal matching problems caused by different materials, with higher accuracy, smaller volume, and easier integration and assembly. . But its disadvantage is that the chip manufacturing process is difficult and the yield is low.

Introduction to various processes of quartz MEMS sensor sensitive chips

Figure 2 Working principle of quartz vibrating beam accelerometer and sensitive chip structure

2 Quartz MEMS sensor sensitive chip processing technology

The sensitive chip structure of the quartz MEMS sensor uses quartz crystal material. The usual processing methods of quartz include mechanical processing, laser processing, dry etching and wet etching. Mechanical processing and laser processing are not suitable for the complex and fine structure of quartz MEMS sensor sensitive chips due to limited processing quality and dimensional accuracy; the dry etching structure is well controlled, and a high-aspect-ratio structure with a flat surface of the quartz crystal can be obtained, but its processing cost is high , Low efficiency, the current dry deep etching equipment for quartz is not mature enough; wet etching is suitable for quartz MEMS sensors with small processing size, high dimensional accuracy, batch processing, high efficiency, and low cost. Process technology for processing complex microstructures of sensitive chips. The process flow of the quartz MEMS sensor sensitive chip is shown in Figure 3. After the quartz wafer is cleaned and dried, the wafer is coated on both sides to form a wet-etched protective film, and then through the double-sided photolithography process, the sensitive chip structure is formed on the metal film of the wafer, and then the quartz wafer is wet-etched Etch to form the required three-dimensional chip structure, and then form electrodes on the chip structure to form a complete sensitive chip. Since the quartz wet etching solution is a highly corrosive HF solution, which is corrosive to most materials including photoresist, metal is generally used as the mask layer. The chemical properties of gold are stable and do not react with HF acid. And the dense gold mask layer can well prevent the penetration of the corrosive liquid, but the adhesion between gold and quartz is poor, so long-term immersion in the corrosive liquid can easily fall off with the quartz, making the mask ineffective. Therefore, a layer of chromium or titanium with strong adhesion to quartz is usually sputtered or vapor-deposited between gold and quartz to effectively prevent the mask layer from failing.

Introduction to various processes of quartz MEMS sensor sensitive chips

Figure 3 Process flow of processing sensitive chip of quartz MEMS sensor

3 Wet etching process

Under normal pressure, quartz has variants with different properties as the temperature changes, including five types of low-temperature quartz (α quartz), high-temperature quartz (β quartz), phosphorite, cristobalite, and quartz glass. Among them, quartz glass is a glass melt that transforms crystalline silicon dioxide into an amorphous form, also called fused quartz, which is isotropic and does not have a piezoelectric effect. Quartz crystal usually refers to low temperature quartz (α quartz). α quartz crystal has a typical piezoelectric effect, good insulation and significant anisotropy. It is suitable for the processing of irregular and complex quartz structures, such as sharp corners, cavities, high aspect ratio sidewalls, cantilever beams, etc. It is an important crystal material for the complex three-dimensional structure of quartz MEMS sensor sensitive chips.

Due to the directional arrangement of the atomic structure of quartz crystals, the atomic arrangement and atomic density of different tangential crystal planes are different, causing different chemical reactions (etching rates) on different crystal planes, showing anisotropic characteristics. Quartz MEMS sensor sensitive chip wet etching process, using the anisotropic etching characteristics of quartz crystal, that is, through the anisotropic chemical reaction between the chemical etching solution and the etched crystal to remove the etched part to realize the micro-nano of the sensitive chip Graphic structure.

In order to obtain the expected stable etching structure and good quartz surface processing quality, it is necessary to strictly control the etching time and the etching rate. In order to achieve this goal, the etching rate of each crystal plane is generally changed by selecting the appropriate etching solution ratio and controlling the temperature and concentration of the etching solution, reducing the amount of lateral corrosion, and achieving the expected morphology and structure.

Usually, the corrosive solution is HF solution with appropriate amount of NH4F solution, or saturated NH4HF2 solution, the temperature range (40~90 ℃) ± 1 ℃. Chemical reaction equation:

SiO2+6HF→H2SiF6+2H2O

SiO2+4NH4F+2H2O→SiF4↑+4NH3・H2O

Usually wet etching is divided into three processes: ①The chemical etching solution diffuses to the wafer surface; ②The etching solution chemically reacts with the wafer material; ③The reacted product diffuses from the wafer surface into the solution and is discharged with the solution. Generally, the higher the temperature of the solution, the faster the diffusion; the higher the concentration of the solution, the stronger the corrosiveness and the higher the corrosion rate. HF+NH4F+H2O solution reacts with quartz crystal to produce SiF62- ions, and bubbles generated during the reaction will also be adsorbed on the surface of quartz crystal, forming a micromask to hinder the diffusion of HF solution. Therefore, the wet etching equipment structure technology, chemical liquid temperature, chemical liquid flow field, chemical liquid concentration and bubble removal of the quartz MEMS sensor sensitive chip are the key manufacturing technologies for wet etching equipment.

4 Key manufacturing technology of wet etching equipment

4.1 The overall structure technology of wet etching equipment

The overall structure of the wet etching equipment is shown in Figure 4, which is mainly composed of corrosion tanks and cleaning tanks installed in a clean and enclosed environment, including corrosion-resistant racks, tanks, exhaust systems, control systems, and water and gas pipeline systems. Due to the extremely corrosive nature of HF solution, the safety of the equipment is of utmost importance. The frame is usually plastic coated with a steel structure skeleton, and the shell is welded and formed by a PP (polypropylene) sheet with good corrosion resistance and strength. The tank body is made of PVDF material (polyvinylidene fluoride) with high cleanliness, HF corrosion resistance and high temperature resistance to ensure that the tank body is not deformed during long-term etching. In addition to overload, over temperature, exhaust air duct wind pressure detection, pipeline area acid leakage detection and other conventional safety protections, due to the relatively high HF concentration, HF gas concentration detection alarms are set up in the operation area and pipeline area, etc. Protection to ensure the safety of equipment and personnel.

Introduction to various processes of quartz MEMS sensor sensitive chips

Figure 4 Overall structure diagram of wet etching equipment

The process etching tank is the core structural unit of the wet etching equipment. In order to achieve uniform controllability of the corrosion morphology, it is essential that the chemical solution spreads uniformly to the wafer surface. The schematic diagram of the structure of the etched tank is shown in Figure 5, which is mainly composed of the tank, the sealing tank cover, the wafer rotation mechanism, the injection and discharge interface and so on. The tank body adopts a 360° circulating overflow structure on all sides. The chemical liquid injection adopts the method of uniform small hole injection and uniform flow hole plate in the symmetrical chambers on both sides of the bottom. The solution circulation adopts the air bag pump to minimize the pulsation of the solution. The pressure and flow rate of the injection pump can be adjusted to realize a uniform flow field of the chemical liquid from the bottom to the surroundings. Large-caliber discharge pipes are used at the bottom and overflow of the chemical liquid to ensure that at the end of the process, DIW (deionized water) is quickly flushed to stop the chemical corrosion, and the corrosion-cleaning integrated structure is realized.

Introduction to various processes of quartz MEMS sensor sensitive chips

Figure 5 Schematic diagram of the structure of the etching tank

4.2 Wet etching cell concentration control technology

Generally, the higher the concentration of the chemical solution, the stronger the corrosiveness, the stronger the lateral corrosivity, and the more difficult it is to control the morphology. Quartz crystal wet etching usually has a solution temperature of 40~90 ℃, and the solution volatilizes quickly. If the volatilization of the solution cannot be effectively controlled, the concentration of the solution rises quickly, and the morphology of the corrosion process cannot be effectively controlled. The high-precision concentration controller can accurately detect and control the HF ion concentration, but the implementation is complicated and the cost is high. As shown in Figure 6, the condensate seal tank cover is adopted, and the tank cover is in an arched structure. Cooling water of 10~15 ℃ is passed into the tank cover to effectively condense the solution volatilized during the corrosion process and effectively introduce the process through the herringbone structure of the condensation cover. Slot. The groove cover is made of stainless steel, and the whole is sprayed with fluorine for corrosion resistance, and the sealing ring is made of fluorine.

Introduction to various processes of quartz MEMS sensor sensitive chips

Figure 6 Condensate seal tank cover

Combined with the precise liquid level detection mechanism of the process tank and the automatic timing and quantitative water replenishment system, the concentration of the corrosion process does not rise.

Usually after a certain ratio of solution (concentration) is tested, during the entire corrosion process, the fluoride ion will decrease, the concentration will decrease, the corrosion efficiency will be reduced, the corrosion time will be prolonged, and uncontrollable factors will increase. Therefore, a large-volume corrosive liquid is usually used, that is, the process tank and the large-volume liquid storage tank are connected through a circulating pump to reduce the rate of concentration drop. The solution heating adopts the on-line heating of the liquid storage tank input heating element pipeline, combined with the wafer rotation mechanism, to achieve the uniformity of the solution temperature of the etching tank.

4.3 Wafer rotation motion control technology

A certain amount of SiF4 gas will be generated during the etching process of the quartz crystal, which will form bubbles in the solution. These bubbles will form a micro-mask and affect the diffusion of the acid. The bubbles can be effectively discharged through the lifting or rotating movement of the wafer. In addition, the rotation of the wafer realizes that each point on the wafer can appear in each position in the tank to the maximum extent, so that it can be perfectly combined with the control of the solution temperature, flow field, and concentration. For this reason, the method of wafer rotation + revolution is adopted. The wafer rotation mechanism is shown in Figure 7. It is mainly composed of wafer loading fixture and drive shaft. Typical parameters are as follows: rotation mechanism and fixture material: PVDF material; fixture rotation method: revolution + rotation ( Rotation speed ratio: 1/4); speed: 0~10 r/min (continuously adjustable), adjustment accuracy ≤ 1 r.

Introduction to various processes of quartz MEMS sensor sensitive chips

Figure 7 Schematic diagram of wafer rotation mechanism

5 concluding remarks

Wet etching equipment is one of the process carriers for the manufacture of quartz MEMS sensor sensitive chip structure. The key technologies such as the overall structure technology of the equipment, the tank solution concentration and flow field control, and the wafer rotation motion control are the realization of the quartz crystal topography structure etching process. An important guarantee for performance. More than 20 sets of wet etching equipment using these key technologies have been applied. After nearly 10 years of actual operation, it has proved that the equipment manufacturing technology has stable performance, safety and reliability, strong process adaptability, and can be extended to other wet etching processes. Fields, such as deep trench etching of monocrystalline silicon and aluminum pattern etching in MEMS.

The Links:   LC215WUE-TCA1 NL128102AC28-01E

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