CN1220621C - Post-package technology for microelectromechinical system - Google Patents

Abstract

The present invention relates to post-package technology for a micro-electromechanical system which relates to the fields of printed circuit and integrated circuit (IC) manufacture, and microelectronic element package. In the technology, a high temperature is obtained in a limit area to obtain better bond strength; a low temperature is maintained at an element level to protect an MEMS microstructure and a microcircuit; simultaneously, package air tightness can be further enhanced. The present invention comprises the following steps: (1) processing a recess which is compatible with MEMS elements in a bottom board on a cover board; (2) depositing an electrothermal insulating layer; (3) depositing an internal ring and an external ring of conducting heating tapes along the boundary line of the recess; (4) aligning and coinciding the cover board and the bottom board; (5) inputting current to the two rings of conducting heating tapes to realize bonding. A layer of bonding materials can also be deposited for bonding after an electric insulating layer is deposited on the conducting heating tapes. In the present invention, the temperature of the bonding area can reach 1000 DEG C to improve bonding quality; other areas maintain a low temperature; double wall bonding improves the air tightness and yield.

Description

MEMS post-packaging process

technical field

The invention belongs to the field of packaging of micro-electromechanical systems.

Background technique

Micro-Electro-Mechanical systems (MEMS) technology can reduce the size of sensors and actuators to micron and nanometer levels. The biggest difference between MEMS packaging and microelectronic packaging is that MEMS must often be in contact with the outside world. This makes MEMS packaging more expensive, which can account for 70-90% of device costs, while general IC (integrated circuit) packaging costs only account for about 30%. Therefore, MEMS designers must consider package design, process, and reliability at the earliest stages of design. For example, the "Digital Micromirror Module (DMD)" of TI Company of the United States, the accelerometer of AD Company of the United States, and the pressure sensor of Motorola Company of the United States, all achieved great success because of the consideration of packaging from the very beginning. MEMS devices must be designed for packaging and assembly, otherwise the industrialization of MEMS devices will be empty talk. This is also one of the reasons why MEMS devices have been developed for more than ten years but have not been industrialized rapidly. Process and material selection are very important to the performance of the product, but the packaging process often leads to product failure. For each company, packaging is the intellectual property of each company, and neither patents nor articles are rarely seen. Some people think that MEMS packaging should be a part of device manufacturing in the micromachining process, so as to solve the packaging problem of individual devices, but this view cannot solve the problem of MEMS packaging for mass production. Therefore, there is a wide demand for general packaging technology. MEMS post-packaging technology needs to meet several requirements: a) no damage to MEMS microstructure or microcircuit; b) can be widely used in different MEMS processes; c) absorb mature IC packaging technology to save research and development costs; d) In addition, some MEMS devices need to be vacuum-sealed, while others need to be packaged in a low-temperature environment. Because most MEMS have active structures, hermetic packaging is the highest form of packaging. Packaging for such components requires a) a cavity, b) a cap to protect the MEMS device, c) strong bonding for hermetic packaging, d) a wafer-level batch process to reduce manufacturing costs, e) protect the MEMS device A low-temperature process to avoid damage, and f) simplify the assembly process at the same time.

Recently, several new achievements in MEMS post-packaging processes have been reported. In 1997 J.T.Butler, V.M.Bright, and J.H.Comtois, "Advanced multichip module packaging ofmicroelectromechanical systems," in Proc.1997 Int.Conf.Solid-State SensorsActuators, Transducers'97, 1997, pp.261-264. etc. studied An Advanced MCM Packaging Method Using a High-Density Interconnect Process Embedding Bare Dies on a Prefabricated Substrate, S. Vander Groen, M. Rosmeulen, P. Jansen, K. Baert, and L. Deferm, "CMOS compatible wafer scaleadhesive bonding for circuit transfer," in Proc.1997 Int.Conf.Solid-State SensorsActuators, Transducers'97, 1997, pp.629-632. Reported a CMOS circuit conversion technology based on epoxy resin bonding, This method overcomes the surface roughness problem, but resins are not very good materials for hermetic sealing.

In 1996 M.B.Cohn, Y.Liang, R.T.How, and A.P.Pisano, "Wafer-to-wafer transfer of microstructures for vacuum packaging," in Proc.IEEE Solid-State Sensor ActuatorWorkshop, 1996, pp.32-35. The 2μm thick polysilicon cap is used for silicon-gold eutectic bonding in this wafer-to-wafer vacuum packaging process, but the test results show that there are micro-leakages after 50 days. These recent research results indicate a strong need for a general MEMS post-packaging process.

As we all know, in the bonding process, "close contact" and temperature are two main factors, and bonding is the key to device packaging. "Intense contact" brings the two surfaces together, and the temperature provides the bonding energy. In 1983, T.R.Anthony, "Anodic bonding of imperfect surfaces", J.Appl.Phys., vol.54, pp.2419-2427, 1983. studied the theoretical study of the influence of surface roughness on the anodic bonding process, and obtained roughness The surface affects bonding parameters such as temperature, time and applied force. Although reflow or mechanical wiping can improve surface planarity, these methods are not suitable for the vast majority of MEMS manufacturing processes.

Regarding temperature, many commonly used bonding methods, such as fusion and anodic bonding, require high temperatures, causing damage to the device and creating thermal stress issues. On the other hand, the temperature must be increased in order to obtain good bond quality. Silicon bonding has previously been used in many types of MEMS devices, such as pressure sensors, micropump biomedical sensors, or chemical sensors, that require mechanical interconnects to bond and bond to silicon. Glass is generally used as a material for anodic bonding in the temperature range around 300-400°C. K.E.Peterson and P.Barth et al., "Silicon fusion bonding for pressure sensors," in 1988 Solid-State Sensor Actuator Workshop, 1988, pp.177-180. Researched different types of silicon fusion bonding at high temperatures exceeding 1000 °C bonded to silicon-silicon dioxide. Trenson, Lee and Coln used eutectic bonding in different applications. Silicon fusion bonding is mostly used in silicon insulator technology, such as silicon-silicon dioxide bonding and silicon-silicon bonding, and its bonding force is very strong. Experiments have proved that this method is feasible. Unfortunately, due to the high temperature above 1000°C, it is not suitable for MEMS post-packaging. Q.-Y. Tong, G. Cha, R. Roman, and U. Gosele, "Low temperature wafer direct bonding," J. Microelectromech. Syst., vol.3, pp.29-35, 1994. H. Takagi, K. Kikuchi, R. Maeda, T.R. Chung, and T. Suga, "Surface activated bonding of silicon wafers at room temperature," Appl. Phys Lett., vol.68, pp.2222-2224, 1996. On low-temperature silicon-silicon Bonding has been reported, and these new methods require special surface treatment, which cannot be applied in MEMS post-packaging.

The discovery of anodic bonding dates back to 1969. Wajjis and Pomerantz found that glass and metal are bonded at a temperature below the melting point of glass by about 200-400 °C with the help of an auxiliary high electric field. This technology was later widely used in integrated circuits in biosensors and sealing small holes in pressure sensors. . Later L.bowman and J.Meindl, "The packaging of implantable integrated sensors," IEEE Trans.Biomed.Eng., vol.BMEe-33, pp.248-255, 1986. M.Esashi, "Encapsulated micromechanical sensors," Microsyst .Technol., vol.1, pp.2-9, 1994. reported the possibility of using a different mechanism to reduce the temperature, unfortunately, the possible contamination of too much alkali metal on the glass and the possible contamination of the microcircuit by the high electric field Damage and the need for a relatively flat bonding surface limit the application of anodic bonding in MEMS post-packaging.

In addition to the solid-type silicon bonding discussed above, there is also a liquid-type bonding. In silicon eutectic bonding, gold is the most commonly used material. Gold and silicon can form a eutectic alloy at 363°C, and 363°C is lower than the melting point of pure gold or pure gold or pure silicon. In order to obtain a good eutectic bonding, the process conditions, such as temperature and time, must be well controlled.

The integrated bulk heating and sealing described above has been successful for MEMS post-packaging, but it has several disadvantages: First, the several high temperature steps performed after the standard surface finishing got damage. Second, the post-package process is very specific and process-dependent. MEMS companies and researchers must integrate post-package processes with their own microfabrication processes. Third, the thickness of the microshells is limited by the film precipitation steps. During the final encapsulation process, the thin microshells are difficult to keep intact in high pressure injection molding.

Contents of the invention

The invention proposes a micro-electro-mechanical system post-packaging process, the purpose of which is to obtain high temperature in a restricted area to obtain better bonding strength, and maintain a low temperature at the wafer level to protect MEMS microstructures and microcircuits. At the same time, double-wall bonding is used to further improve the airtightness of the package, thereby improving the yield of MEMS devices.

The post-packaging process of micro-electro-mechanical systems of the present invention comprises the following steps in sequence: (1) processing pits on the cover plate corresponding to the position of the micro-electro-mechanical system components on the bottom plate and the corresponding size space, (2) depositing electric heat insulation on the cover plate layer to cover the cover plate and the inner surface of the pit, (3) deposit two inner and outer conductive heating bands on the cover plate along the boundary line of the pit, and electrically insulate between the two circles of conductive heating bands, and the interval is 5-30 microns, (4 ) Align the cover plate and the base plate so that the pit just encapsulates the corresponding MEMS device, (5) input current to the two coils of conductive heating tape separately or simultaneously, and realize bonding between the base plate and the cover plate.

The MEMS post-packaging process is further characterized in that: after depositing the electric thermal insulation layer on the cover plate, first deposit the conductive material lead-out line on the cover plate along the boundary line of the pit, and then deposit a layer on it The electrically insulating material is then deposited with conductive heating tape.

In the MEMS post-packaging process, after the conductive heating tape is deposited on the cover plate, a layer of electrically insulating material can be deposited to cover the conductive heating tape, and then a layer of bonding material can be deposited along the tracks of the two turns of the conductive heating tape, and then Implement next steps.

In the microelectromechanical system post-packaging process, the pits on the cover can be chemically etched in the same phase, out of phase, or machined; The deposition method of the tape and the bonding material can be one of CVD, sputtering or evaporation; the bonding method can be two or more of melting, eutectic and solder.

In the MEMS post-packaging process, the cover plate can be made of silicon wafer, glass or ceramic material, and the electrothermal insulating layer can be made of silicon oxide material; the conductive heating belt can be one of gold and polysilicon; the electrical insulating material can be nitrogen One of silicon dioxide, silicon dioxide or a mixture of the two; the bonding material can be one of silicon dioxide, gold, and polysilicon.

In the MEMS post-packaging process, when solder is used as the bonding method, the solder is indium.

In order to improve the productivity of MEMS packaging and reduce the cost of MEMS packaging, wafer-level packaging is generally used. Firstly, a rectangular array of MEMS chips is processed on the bottom plate, and the pits of the same array are etched on the cover plate by etching in the same phase or out of phase, and then a layer of electric insulation layer is deposited on it, and then along the peripheral boundary track of the pits Precipitate a layer of conductive heating belt on the line, deposit a layer of electric insulation material on it, and then deposit another layer of conductive heating belt at a certain distance (such as 3 μm) from the outer boundary track of the pit to form an inner and outer nested micro heating bring. Align the bottom plate and the cover plate closely, input the same or different currents to the two micro heating strips, and form double-wall bonding under slight pressure.

In the process proposed by the present invention, the bottom plate material is generally silicon, glass or ceramics, while the cover plate material can be silicon, glass, or ceramic materials. When the cover plate material is silicon, an electrically insulating layer must be deposited on the cover plate to prevent current from diffusing into the cover plate.

In various bonding processes, except for direct bonding of wafers such as anodic bonding and melting and eutectic, other bonding processes require intermediate layer materials, namely bonding materials, according to the temperature and metal required by the bonding process The effect on the bonding quality is used to select the material of the micro-heating tape. Different bonding materials are used for different midrange layer bonding processes. If the solder bonding uses solder indium, if the bonding material is conductive and the cover is also conductive, a layer of electrical insulation must be deposited on the cover to insulate the bonding material.

The materials that can be used as micro-heating wires and as the intermediate layer at the same time are polysilicon and gold. For silicon-silicon/glass fusion bonding, polysilicon acts as both a micro-heating wire and as a bonding material. The local silicon-glass fusion bonding process is as follows: firstly, silicon dioxide and heavily phosphorus-doped polysilicon are deposited on the silicon device substrate. Borosilicate glass (Pyrex) was pressed onto the polysilicon microheater. Input a current of 71 mA to the polysilicon micro-heater to generate a high temperature of 1300 ° C, and form a fusion bond under slight pressure. For silicon-gold eutectic bonding, gold is used as both micro-heating wire and bonding material. The local eutectic bonding process is as follows: the silicon wafer is first thermally oxidized to form an oxide layer as an electric thermal insulation layer, chromium is deposited on it as an adhesive layer, and gold is deposited on it as a micro-heating wire. Under a pressure of 1Mpa, a clean silicon cap ( Cap) is pressed on top of the substrate to form a eutectic bond. These two direct bonding processes are simple, but there is a problem: in these two processes, the atoms of the micro-heating wire are easily diffused or fused to the cover plate so that the resistance changes, and the temperature changes, which affects the bonding quality consistency. Therefore, it is necessary not only to increase the current density to maintain the high temperature unchanged. For MEMS devices with higher airtightness requirements, the intermediate layer bonding process such as solder bonding can be preferred.

For the solder bonding process, a 1 μm thick thermal oxide layer is first grown on the substrate. Next, silicon oxide is grown on the cover plate by thermal oxidation as an electric thermal insulation layer, and phosphorus-doped polysilicon micro heaters are then deposited on it to form micro heaters. LPCVD silicon oxide is deposited on top as an electrically insulating layer. The solder is then deposited again. Since the micro-heating wire is separated from the middle layer, damage to the micro-heating wire is avoided.

The present invention provides a local heating process, which increases the temperature of the bonding area by local heating, but maintains a low temperature adjacent to the bonding area, thereby reducing the damage of high temperature to MEMS internal circuits and temperature sensitive materials. Its local heating effect is realized by generating heat after inputting current through the resistive micro-heating wire. The insulating layer can reduce the high temperature of 1000°C to 80°C. The entire device is thus kept at a low temperature. At the same time, in order to prevent current from flowing into the bottom plate and the cover plate, the thermal insulation layer is also an electrical insulation layer.

In order to improve the airtight packaging effect of MEMS devices and improve the service life and yield of products, the present invention adopts a double-wall bonding process, that is, two identical or different bonding processes such as fusion-melt bonding and fusion-melt bonding are adopted simultaneously. Solder bonding, etc. Although the width of the two micro-heating lines is increased by a few microns, due to the improved bonding quality, this process is suitable for MEMS devices with higher airtightness requirements, especially military standard MEMS products.

The locally heated double wall bonding process of the present invention brings several opportunities: first, better and faster temperature control can be obtained; second, high temperature is used to improve bond quality; third, new bonding mechanisms that require high temperature , such as brazing can be applied to MEMS, therefore, the local heating double-wall bonding process will be potentially applied in a wide range of MEMS devices, and is expected to promote the development of the MEMS packaging field.

Description of drawings

Fig. 1 shows embodiment 1 of the present invention,

Fig. 2 is embodiment 2 of the present invention.

Detailed ways

The implementation state of the present invention will be further described below in conjunction with FIG. 1 and FIG. 2 .

Embodiment 1: The two materials of gold and polysilicon are used as both heating material and bonding material.

Firstly, a 1 μm thick thermal oxide silicon dioxide layer 2 is grown on the silicon base plate 1 . Then manufacture the microstructure 3 of the resonator on it, then process a pit 5 on the cover plate 4 corresponding to the mems device position of the bottom plate, and then deposit a thermal oxide silicon dioxide layer 6 with a thickness of 1 μm, and then place it along the boundary line of the pit Phosphorus-doped polycrystalline silicon with a thickness of 1.6 μm is deposited on the cover plate as the inner conductive heating belt 7 , and LPCVD silicon oxide with a thickness of 0.45 μm is deposited on the inner and outer boundary lines of the cover plate as the electrical insulating layer 8 . Then deposit 0.15 μm chromium 10 on the outer boundary line to enhance the adhesion of gold. Then deposit gold 12 with a thickness of 1 μm on the outer boundary line, and press the cover plate on the bottom plate. Input currents of 45mA and 15mA to the two micro-heating wires respectively to form bonds. The temperature of the two micro heaters is measured non-contact by an infrared thermometer up to 800°C and 370°C, the applied pressure is 0.2Mpa, and the bonding is completed within 1.5 minutes. A tensile test is carried out on the bonded part to check the bonding of the interface strength. Experimental results show the formation of strong fusion and eutectic double-wall bonding.

Example 2: Fabrication and packaging of microresonators using locally heated fusion and solder double-wall bonding processes.

Firstly, a 1 μm thick thermal oxide silicon dioxide layer 2 is grown on the silicon base plate 1 . Then manufacture the microstructure 3 of the resonator on it, then process a pit 5 on the cover plate 4 corresponding to the mems device position of the bottom plate, and then deposit a thermal oxide silicon dioxide layer 6 with a thickness of 1 μm, and then place it along the boundary line of the pit Precipitate 1.45 μm thick phosphorus-doped polysilicon on the cover plate as the inner ring conductive heating belt 7, then deposit 0.45 μm thick phosphorus-doped polysilicon on the outer boundary line of the cover plate as the outer ring conductive heating belt 9, and then deposit 0.45 μm thick on the cover plate. μm thick LPCVD silicon oxide is used as an electrical insulating layer 8, and 0.15 μm chromium 10 is deposited on the outer boundary line to enhance the adhesion of solder indium. Then deposit 1 μm thick indium solder bonding material 11 on the outer boundary line, and then LPCVD silicon dioxide bonding material on the inner boundary line to be flush with the indium on the outer ring, and press the cover plate on the bottom plate. Input currents of 60mA and 20mA to the two micro-heating wires respectively to form bonds. The temperature of the two micro-heaters is measured up to 1000°C and 300°C through non-contact measurement, the applied pressure is 0.2Mpa, and the bonding is completed within 2 minutes. A tensile test is performed on the bonding site to check the bonding strength of the interface. Experimental results show the formation of strong fusion and solder double-wall bonds.

Claims (8)

1. A micro-electro-mechanical system post-packaging process, the sequence includes the following steps: (1) Process pits on the cover plate corresponding to the position of the micro-electromechanical system components on the bottom plate and their corresponding size spaces, (2) Precipitate an electric insulation layer on the cover plate to cover the cover plate and the inner surface of the pit, (3) Precipitate two circles of conductive heating bands inside and outside on the cover plate along the boundary line of the pit, and electrically insulate between the two circles of conductive heating bands, and the interval is 5-30 microns, (4) Align the cover plate and the bottom plate so that the pit just encapsulates the corresponding MEMS device, (5) Input current to the two coils of conductive heating belts separately or simultaneously to realize bonding between the bottom plate and the cover plate. 2. The micro-electro-mechanical system post-packaging process as claimed in claim 1, characterized in that: after the electrothermal insulation layer is deposited on the cover plate, the conductive material lead-out line is deposited on the cover plate along the boundary line of the pit, and then on the cover plate. A layer of electrically insulating material is deposited on it, and then a conductive heating tape is deposited. 3. The micro-electro-mechanical system post-packaging process according to claim 1 or 2, characterized in that: after the conductive heating tape is deposited on the cover plate, a layer of electrical insulating material is deposited to cover the conductive heating tape, and then conduction heating along two circles The belt tracks respectively deposit a layer of bonding material, and then carry out subsequent steps. 4. The micro-electro-mechanical system post-packaging process as claimed in claim 2, characterized in that the pits on the cover adopt chemical in-phase etching, out-of-phase etching or machining; The deposition method of the wire, the electrical insulating material and the conductive heating belt is one of CVD, sputtering or evaporation; the bonding method is a combination of two or more of melting, eutectic and solder. 5. The micro-electro-mechanical system post-packaging process as claimed in claim 3, characterized in that the pits on the cover adopt chemical in-phase etching, out-of-phase etching or machining; The deposition method of wire, electrical insulating material, conductive heating tape and bonding material is one of CVD, sputtering or evaporation; the bonding method is two or more of fusion, eutectic, solder made. 6. MEMS post-encapsulation process as claimed in claim 4, is characterized in that cover plate is silicon chip, glass or ceramic material, and electrothermal insulating layer is silicon oxide material; Conductive heating band adopts gold or polysilicon; Electrical insulating material is One of silicon nitride, silicon dioxide, or a mixture of the two. 7. MEMS post-packaging process as claimed in claim 5, characterized in that the cover plate is a silicon wafer, glass or ceramic material, and the electrothermal insulating layer is a silicon oxide material; the conductive heating band adopts one of gold and polysilicon; The electrical insulating material is one of silicon nitride, silicon dioxide or a mixture of the two; the bonding material is one of silicon dioxide, gold and polysilicon. 8. The MEMS post-packaging process according to claim 4, 5, 6 or 7, wherein when solder is used as the bonding method, the solder is indium.