Abstract:
Provided are a method of packaging an MEMS device in vacuum using an O-ring and a vacuum-packaged MEMS device manufactured by the same. The method includes preparing an upper substrate including a cavity and a lower substrate including the MEMS device and loading the upper and lower substrates into a vacuum chamber; aligning the lower and upper substrates by mounting an O-ring on a marginal portion of the MEMS device of the lower substrate; compressing the O-ring between the upper and lower substrates by applying a pressure between the upper and lower substrates; venting the vacuum chamber; and removing the pressure applied between the upper and lower substrates. In this method, the MEMS device can be packaged in vacuum using a simple process without causing outgassing and leakage from a cavity of the upper substrate.

Description:
[0001]     This application claims the priority of Korean Patent Application No. 2004-25198, filed on Apr. 13, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in their entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method of packaging a micro electro mechanical systems (MEMS) device in a vacuum state and a MEMS device manufactured by the same, and more particularly, to a method of packaging an MEMS device in a vacuum state using an O-ring and an MEMS device manufactured by the same.  
         [0004]     2. Description of the Related Art  
         [0005]     In recent years, MEMS have been proposed as leading, innovative system miniaturization technology in the next generation field of electronic components. For example, various MEMS products, such as an accelerometer, a pressure sensor, an inkjet head, and a hard disc head, are being commonly used throughout the world. Also, micro gyroscopes have been produced in large quantities after the production of first micro gyroscopes was launched upon. Nowadays, with development in optical communications technology, various efficient components for wavelength division multiplexing (WDM) optical communications, such as switches, attenuators, filters, and OXC switches, are being studied as a new challenging field of MEMS technology.  
         [0006]     A representative product that derives from MEMS technology is an MEMS gyroscope sensor. A silicon oscillatory gyroscope operates on the principle that when a structure is oscillated in a certain direction due to an electrostatic force and an angular rotation (or an angular velocity) to be detected is given, a Coriolis force acts at a right angle to the oscillation of the structure. At this time, an oscillation acted by the Coriolis force and the extent of an externally applied angular rotation are measured using a variation in capacitance between an inertial body and an electrode.  
         [0007]     Micro gyroscopes can be applied in various fields of subminiature low-price global position systems (GPS), inertial navigation systems (INS), automobile industries including vehicle positive control and driving safety devices such as positive suspension systems, household appliances including a virtual reality, 3-dimensional mouse and a hand trembling preventing device for cameras, military applications including generation weapon systems, missile guidance systems, and intelligent ammunition systems, and other industries including machine control, oscillation control, and robotics.  
         [0008]     In order to improve the sensitivity of an oscillatory gyroscope, it is necessary that an oscillation frequency obtained in a given direction correspond to that obtained in a measured direction and damping be small. That is, when a structure operates, the structure runs into resistance due to a damping effect caused by air flow and viscosity around the structure, or an air attenuation effect, and a value Q (or a quality factor) decreases. For this reason, the structure need to be operated in a vacuum state and packaged in high vacuum.  
         [0009]      FIG. 1  is a cross-sectional view of a conventional oscillatory MEMS gyroscope sensor.  
         [0010]     Referring to  FIG. 1 , the MEMS gyroscope sensor is manufactured using a silicon on insulator (SOI) wafer including a first silicon layer  1 , an oxide layer  5 , and a second silicon layer  10 , which are sequentially stacked. The SOI wafer has a thickness of about 500 μm, and the oxide layer  5  as an insulator has a thickness of about 3 μm. The second silicon layer  10  stacked on the oxide layer  5  is p-type &lt;100&gt; and has a thickness of 40 μm and a resistivity of about 0.01 to 0.02 Ω·cm. The SOI wafer is primarily cleaned, and then a gyroscope structure pattern is formed using a photo-resistor. The resultant structure is sufficiently baked such that the photo-resistor is not carbonized. Thereafter, the second silicon layer  10 , the oxide layer  5 , and the first oxide layer  5  as a sacrificial layer are sequentially and vertically etched using inductively coupled plasma-reactive ion beam etch (ICP-RIE). The photo-resistor is removed using a dry ashing apparatus, and the resultant structure is dipped in an HF solution such that a gyroscope structure  20  is completely released.  
         [0011]     In order to package a lower substrate  25  including the gyroscope structure  20 , an upper substrate  30  is prepared. The upper substrate  30  is formed of Corning Pyrex 7740 glass, whose coefficient of thermal expansion is relatively close to that of silicon, and has a thickness of about 350 μm. The glass upper substrate  30  has a cavity  35  inside and a via hole  37  in a top surface as shown in  FIG. 1 . The cavity  35  is required to protect the gyroscope structure  20  and create a vacuum state. The via hole  37  serves as a path for connecting the gyroscope structure  20  and an external electrical interconnection. The cavity  35  and the via hole  37  of the glass upper substrate  30  are formed using sandblasting.  
         [0012]     The lower substrate  25  including the gyroscope structure  20  and the upper substrate  35  including the cavity  35  are aligned and loaded into a vacuum chamber. The degree of vacuum in the chamber is set to about 5×10 −5  Torr, and then anodic bonding is carried out. During the anodic bonding, a voltage is applied to the upper and lower substrates  35  and  25  while raising the temperature of the chamber. After the anodic boding is finished, the upper and lower substrates  35  and  25  are unloaded from the chamber, and an electrical interconnection  40  is formed by depositing Al on the glass upper substrate  35 . After that, the bonded upper and lower substrate  35  and  25  are diced into individual chips.  
         [0013]     In the foregoing wafer-level vacuum packaging process, the conventional MEMS gyroscope sensor is completed. However, in this case, a variation in degree of vacuum of a package affected by environmental conditions and time is not sufficiently reliable.  
         [0014]     When a gyroscope is used, a value Q is varied. If a value Q or a frequency varies, sensitivity and precision, which are performance factors of the gyroscope, are directly affected. When a gyroscope is used, a reduction in value Q means a variation in degree of vacuum of a gyroscope package. In other words, a pressure in a cavity is increased than an initial pressure so that damping of air increases, thus lowering the value Q.  
         [0015]     Generally, the rise in the pressure of the cavity results from outgassing or leakage, which occurs in the cavity.  
         [0016]     The leakage is caused by holes or micro cracks formed in an interfacial surface between bonded substrates or defects of materials after a bonding process is finished.  
         [0017]     The outgassing refers to emission of gases from a cavity during or after a bonding process. During the bonding process, if a high voltage is applied, not only oxygen ions emitted from a glass substrate or an interface between bonded substrates, but also gases contained in contaminants remaining on an inner surface of a package or on the surfaces of materials are continuously outgassed into the cavity with a rise in temperature.  
         [0018]     By analyzing outgassing resulting from an SOI wafer and a glass wafer, it can be seen that gases emitted from the wafers contain H 2 O for the most part, CO 2 , C 3 H 5 , and other contaminants. Because the glass wafer emits an about 10-fold larger amount of gas than the SOI wafer, the glass wafer becomes a major cause for the outgassing from a cavity. A very large amount of H 2 O is outgassed from the glass wafer. Particularly, it is demonstrated that after the glass wafer is processed using sandblasting, an about 2.5-fold larger amount of gas is outgassed than before.  
         [0019]     Accordingly, a new method of packaging an MEMS device in vacuum, which solves leakage and outgassing, is required.  
       SUMMARY OF THE INVENTION  
       [0020]     The present invention provides a method of packaging a micro electro mechanical systems (MEMS) device in vacuum without causing gas leakage and a vacuum-packaged MEMS device manufactured by the same.  
         [0021]     Also, the present invention provides a method of packaging an MEMS device in vacuum, which includes neither a baking process nor anodic bonding so that no outgassing occurs, and a vacuum-packaged MEMS device manufactured by the same.  
         [0022]     According to an aspect of the present invention, there is provided a method of packaging an MEMS device in vacuum. In this method, an upper substrate including a cavity and a lower substrate including the MEMS device are prepared and loaded into a vacuum chamber. The lower and upper substrates are aligned by mounting an O-ring on a marginal portion of the MEMS device of the lower substrate. The O-ring is compressed between the upper and lower substrates by applying a pressure between the upper and lower substrates. Thereafter, the vacuum chamber is vented so that the upper and lower substrates can be packaged in vacuum due to a difference between vacuum and atmospheric pressure. After that, the pressure applied between the upper and lower substrates is removed.  
         [0023]     A sealant, such as a torr-seal, may be filled between the upper and lower substrates outside the O-ring. In order to maintain airtightness, outer portions of the upper and lower substrates may be clamped using a clamp.  
         [0024]     After the MEMS device is packaged using wafer-level vacuum packaging, the upper and lower substrates may be diced into individual chips. Also, the upper and lower substrates between which the MEMS device is embedded may be connected by an electrical connection and molded using a molding compound. The molding compound may be formed of one selected from the group consisting of metals, ceramics, glass, and thermosetting resins.  
         [0025]     The MEMS device may be one selected from the group consisting of a gyroscope, an accelerator, an optical switch, an RF switch, and a pressure sensor and be used for a system on a package (SoP).  
         [0026]     According to another aspect of the present invention, there is provided a vacuum-packaged MEMS device including an upper substrate including an MEMS device; a lower substrate including a cavity; and an elastic O-ring interposed between marginal portions of the upper and lower substrates.  
         [0027]     The vacuum-packaged MEMS device may further include a sealant, such as a torr-seal, filled between the upper and lower substrate outside the O-ring. Also, a molding compound may be molded outside the upper and lower substrates between which the MEMS device is embedded. The molding compound may be one of metals, ceramics, glass, and thermosetting resins. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]     The above object and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0029]      FIG. 1  is a cross-sectional view of a conventional oscillatory micro electro mechanical systems (MEMS) gyroscope sensor;  
         [0030]      FIG. 2  is a cross-sectional view of an MEMS gyroscope vacuum-packaged according to an embodiment of the present invention;  
         [0031]      FIGS. 3A through 6A  are perspective views illustrating a method of packaging an MEMS device according to an embodiment of the present invention;  
         [0032]      FIGS. 3B through 6B  are cross-sectional views illustrating the method of packaging an MEMS device shown in  FIGS. 3A through 6A ; and  
         [0033]      FIG. 7  is a perspective view illustrating a method of packaging a plurality of MEMS devices in a vacuum state on a wafer level according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]     The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. In the drawings, the thicknesses of layers or regions may be exaggerated for clarity. The same reference numerals are used to denote the same elements throughout the specification.  
         [0035]     In the embodiments of the present invention, an upper substrate including a cavity and a lower substrate including a micro electro mechanical systems (MEMS) device are bonded using an O-ring. Specifically, the upper and lower substrates are spaced a predetermined distance apart from each other by the O-ring in a vacuum chamber and compressed. Then, the vacuum chamber is vented so that the upper and lower substrates can be bonded due to a difference between vacuum and atmospheric pressure. In this process, conventional anodic bonding is not required. Therefore, no outgassing occurs, a process is simple and economical, and no leakage occurs so that high vacuum can be maintained.  
         [0036]      FIG. 2  is a cross-sectional view of an MEMS gyroscope vacuum-packaged according to an embodiment of the present invention.  
         [0037]     Referring to  FIG. 2 , a gyroscope structure  120  is formed by an ordinary method in a silicon on insulator (SOI) lower wafer  125  including a first silicon layer  100 , an oxide layer  105 , and a second silicon layer  110 , which are sequentially stacked. On the lower wafer  125  in which the gyroscope structure  120  is formed, an upper wafer  130  is packaged in vacuum by interposing an O-ring  150 . Preferably, the upper wafer  130  includes a cavity  135  inside, and a sealant  155 , such as a torr-seal, is filled outside the O-ring  150  interposed between the upper and lower wafers  125  and  130 .  
         [0038]      FIGS. 3A through 6A  are perspective views illustrating a method of packaging a MEMS device according to an embodiment of the present invention, and  FIGS. 3B through 6B  are cross-sectional views illustrating the method of packaging an MEMS device shown in  FIGS. 3A through 6A . In the embodiments of the present invention, a variety of MEMS devices, for example, a gyroscope, an accelerator, a pressure sensor, an optical switch, and a radio-frequency (RF) switch, can be packaged in vacuum. Preferably, an oscillatory MEMS device can be packaged in vacuum.  
         [0039]     Referring to  FIGS. 3A and 3B , a lower substrate  225  including an MEMS device  220  and an upper substrate  230  including a cavity are prepared. The upper substrate  230  may be formed of silicon, and the cavity can be formed by performing wet or dry etching using ordinary photolithography.  
         [0040]     Thereafter, the lower and upper substrates  225  and  230  are loaded into a vacuum chamber (not shown). In order to secure an ultrahigh vacuum state, an exhausting process is performed by operating a pump installed in the chamber. In the vacuum chamber, a pressurizing unit including a pressurizing plate ( 260  of  FIGS. 5A and 5B ) is installed to enable high-vacuum exhaust and pressurize the upper and lower substrates  230  and  225 .  
         [0041]     Thereafter, an O-ring  250  is mounted on the lower substrate  225  such that the MEMS device  220  is surrounded by the O-ring  250 . The O-ring  250  may be formed of one of various elastic materials and preprocessed at a temperature of about 230° C. before being put on the lower substrate  225 .  
         [0042]     Referring to  FIGS. 4A and 4B , the upper substrate  230  is aligned on the lower substrate  225  on which the O-ring  250  is located.  
         [0043]     Referring to  FIGS. 5A and 5B , the lower and upper substrates  225  and  230  are compressed in a vacuum state by use of the pressurizing plate  260  of the pressurizing unit. Once the upper and lower substrates  225  and  230  are compressed, the O-ring  250 , which is elastic, is compressed and closely adhered to the upper and lower substrates  230  and  225 .  
         [0044]     Referring to  FIGS. 6A and 6B , while the upper and lower substrates  230  and  225  are being compressed by interposing the O-ring  250 , the vacuum chamber is vented to an atmospheric pressure. Once the vacuum chamber is under the atmospheric pressure, the upper and lower substrates  230  and  225  are closed bonded to each other due to the atmospheric pressure.  
         [0045]     Thereafter, the pressure applied between the upper and lower substrates  230  and  225  by the pressurizing plate  260  is removed. At this time, the upper and lower substrates  230  and  225  are packaged in vacuum due to a difference between vacuum inside the upper and lower substrates  230  and  225  and the atmospheric pressure outside the same.  
         [0046]     The vacuum-packaged upper and lower substrates  230  and  225  are unloaded from the vacuum chamber. A sealant  270 , such as a torr seal, can be filled outside the O-ring  250  between the upper and lower substrates  230  and  225 .  
         [0047]     In some cases, adhesion between the upper and lower substrates  230  and  225  can be reinforced by using a clamping unit (not shown), such that a high degree of vacuum is maintained.  
         [0048]     Also, outer portions of the upper and lower substrates  230  and  225  between which the MEMS device  220  is embedded may be molded using a molding compound. In this molding process, airtightness of the MEMS device  220  can be maintained, components can be protected from surrounding conditions, such as temperature and humidity, any damage or transformation caused by mechanical oscillation and shocks can be avoided. The molding compound may be one selected from the group consisting of metals, ceramics, glass, thermosetting resins (particularly, thermosetting epoxy resins).  
         [0049]      FIG. 7  is a perspective view illustrating a method of packaging a plurality of MEMS devices  320  in a vacuum state on a wafer level according to another embodiment of the present invention.  
         [0050]     Referring to  FIG. 7 , a lower wafer  325  including the plurality of MEMS devices  320  and an upper wafer  330  including cavities corresponding to the MEMS devices  320  can be packaged in a vacuum state on a wafer level by aligning an O-ring structure including a plurality of O-rings  350  that surround the MEMS devices  320 , respectively. In this case, the MEMS devices  320  may be a variety of MEMS devices, for example, a gyroscope, an accelerator, an optical switch, an RF switch, and a pressure sensor.  
         [0051]     After being packaged in vacuum on the wafer level, the lower and upper wafers  325  and  330  may be diced into respective chips so that time can cost can be saved. Before or after the package is diced into the respective chips, a sealant, such as a torr-seal, may be filled between the lower and upper wafers  325  and  330  outside the O-ring structure.  
         [0052]     Also, after the upper and lower wafers  330  and  325 , which are diced into the respective chips and between which the MEMS devices  320  are embedded, are connected to each other by an electrical interconnection and molded using a molding compound, they can be used for a system on a package (SoP). The SoP refers to a technique of integrating a system on chip (SoC) including conventional multifunctional semiconductor devices with modules such as MEMS sensor devices, RF integrated circuits (ICs), and power devices. This SoP technique reduces the cost of development in each module and the packaging cost.  
         [0053]     The vacuum-packaged MEMS device according to the present invention facilitates the SoP and enables easier constitutions of an SoP telemetric sensor that integrates ultra-precise MEMS sensor technology, SoC technology, and telematics.  
         [0054]     According to the present invention, an upper substrate and a lower substrate can be easily bonded to each other, and the MEMS device can be packaged in vacuum using a simple process.  
         [0055]     Also, a vacuum-packaged MEMS device with excellent reliability and a long life span can be manufactured so that it can resist mechanical stress, such as shock and oscillation, and environmental stress, such as temperature, humidity, and thermal shock.  
         [0056]     Further, the MEMS device can be reliably packaged in vacuum without causing leakage or outgassing from a cavity, and a plurality of MEMS devices can be packaged in vacuum on a wafer level, thus reducing the cost and time.  
         [0057]     Moreover, the vacuum-packaged MEMS device according to the present invention facilitates SoP techniques and enables easier constitutions of an SoP telemetric sensor that integrates ultra-precise MEMS sensor technology, SoC technology, and telematics.  
         [0058]     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.