Abstract:
A three-dimensional micro- electromechanical (MEM) varactor is described wherein a movable beam and fixed electrode are respectively fabricated on separate substrates coupled to each other. The movable beam with comb-drive electrodes are fabricated on the “chip side” while the fixed bottom electrode is fabricated on a separated substrate “carrier side”. Upon fabrication of the device on both surfaces of the substrate, the chip side device is diced and “flipped over”, aligned and joined to the “carrier” substrate to form the final device. Comb-drive (fins) electrodes are used for actuation while the motion of the electrode provides changes in capacitance. Due to the constant driving forces involved, a large capacitance tuning range can be obtained. The three dimensional aspect of the device avails large surface area. When large aspect ratio features are provided, a lower actuation voltage can be used. Upon fabrication, the MEMS device is completely encapsulated, requiring no additional packaging of the device. Further, since alignment and bonding can be done on a wafer scale (wafer scale MEMS packaging), an improved device yield can be obtained at a lower cost.

Description:
FIELD OF THE INVENTION 
     This invention is generally related to micro-electromechanical (MEM) switches and to a method of fabricating such structures, and more specifically, to a MEM variable capacitor that uses three-dimensional comb-drive electrodes (fins) by bonding a chip to a carrier substrate. 
     BACKGROUND OF THE INVENTION 
     Variable capacitors or varactors are a fundamental part of high-frequency and radio-frequency (RF) circuits. MEM variable capacitors have drawn considerable interest over the last few years due to their superior electrical characteristics. Variable capacitors using MEM technology can be easily implemented in standard semiconductor devices for applications in aerospace, consumer electronics and communications systems. 
     Many researchers have attempted to improve the tuning range of MEMS variable capacitors since the maximum capacitance tuning range achieved by using a parallel plate electrode approach is limited. This is due to the non-linear electrostatic forces involved during actuation. The parallel plate electrodes exhibit a typical “pull-down behavior” at one-third the gap distance, leading to a maximum tuning capacitance of 1.5. Most previous approaches have resulted in increased processing complexity and/or a large number of moving parts, leading to a drastic reduction in reliability. Additionally, packaging the MEMS device and integrating it into CMOS integrated circuit pose great challenges. 
     A. Dec et al., in an article entitled “RF micro-machined varactors with wide tuning range”, published in the IEEE RF IC Symposium Digest, pp. 309-312, June 1998 describe building a MEMS variable capacitor by actuating the movable electrode using two parallel electrodes above and below the movable electrode. The total capacitance tuning range is significantly enhanced as a result of the individual capacitance between the top-movable and movable-bottom being in series. The maximum tuning range achievable using this approach is a ratio of 2:1. A. Dec et al. have reported achieving a tuning range as high as 1.9:1. Even though the tuning range is significantly improved using this approach, the process complexity is increased. 
     The inherent electromechanical aspects involved in present approach are quite different than the parallel plate approach. Comb-drive electrodes are used for actuation while control or signal electrodes sense the motion of the movable electrode. The resulting capacitance tuning range is greatly enhanced since the electrostatic forces are constant in nature. Since this device has three ports (two ports for DC bias and one port for the RF signals), the signal capacitance requires decoupling as is the case in a 2-port varactor device. Moreover, most prior art MEMS devices need to be separately packaged that, at least for MEM devices with moving parts creates certain processing issues that need to be resolved. 
     OBJECTS OF THE INVENTION 
     Accordingly, it is an object of the invention to provide a MEM variable capacitor device that utilized comb-drive electrodes (or fins) for actuation, while the control or signal electrodes sense the motion of the movable beam, leading to a change in capacitance. 
     It is another object to provide a MEM varactor device where the switch contacts are separated by a dielectric to provide electrical isolation between the control signal and the switching signal. 
     It is further an object to provide a MEM varactor device with comb-drive actuation for obtaining large capacitance ratio or tuning range. 
     It is yet another object to configure a plurality of MEM varactor devices in a variety of three dimensional arrangements. 
     It is still another object to provide a MEM varactor with increased drive electrode area for lower drive voltages. 
     It is still a further object to provide a method of fabricating a MEM switch using manufacturing techniques that are compatible with those applicable to CMOS semiconductor devices, which allows fabricating and packaging the MEMS device simultaneously and reduces the number of fabrication steps to a minimum while reducing the cost of processing. 
     SUMMARY OF THE INVENTION 
     MEMS based variable capacitors provide many advantages over conventional solid-state varactors. These devices are operated at higher quality factors leading to low loss during operation. Two types of MEMS varactors are described herein: parallel plate and comb-drive varactors. Most widely investigated MEMS varactors are parallel plate capacitors with a movable electrode and a fixed electrode. The major disadvantage when using these MEMS devices is the limited tuning range of operation obtained upon actuation of these devices. The inherent electromechanical aspects involved restrict the tuning range and lead to snap down of the movable electrode. This is often stated as “pull-down instability effect”. The electrostatic forces acting on the movable electrode are non-linear in nature which cause this effect. In the comb-drive electrodes, the electrostatic forces acting on the movable electrode are linear in nature (directly proportional to the distance) which greatly enhances the tuning range. However, comb-drive electrodes are difficult to process and the change in capacitance obtained is very small (due to less area available). 
     In one aspect of the invention, the MEM switch described includes both of the approaches (parallel plate and comb-drive) that were thus far considered. Greater area is made available during tuning by using a parallel plate type model while incorporating the linear electrostatic forces from the comb-drive approach. The movable and fixed electrodes are processed separately on chip and carrier wafers. The chip side contains the fixed-fixed movable beam. The beam is fabricated with metal “fins” acting as comb electrodes. The carrier side has an actuator (DC electrodes) along the side walls and bottom of the trenches. The RF (signal) electrodes are positioned between the electrodes. The actuator electrodes are connected by way of “through vias” for electrical connection. 
     After completion of processing both the chip and the carrier wafers, the chip side is flipped onto the carrier wafer and precision aligned so as to make electrical connection. The height of the stud on the carrier side determines the air gap between the movable electrode and the fixed electrode. Finally the device can be encapsulated with polymeric material in order to provide controllable environment for the MEMS device during operation. 
     In another aspect of the invention, there is provided a semiconductor micro electro-mechanical (MEM) varactor that includes a first substrate having a movable beam anchored at least at one end of the movable beam to the first substrate, the movable beam having discrete fins protruding therefrom in a direction opposite to the first substrate; and a second substrate coupled to the first substrate having fixed electrodes, each of the fixed electrodes respectively facing one of the discrete fins, the discrete fins being activated by a voltage between the protruding fins and the fixed electrodes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, aspects and advantages of the invention will be better understood from the detailed preferred embodiment of the invention when taken in conjunction with the accompanying drawings. 
     FIG. 1 is a cross-section view of the functional MEM varactor device according to the invention, seen at a cut through the lines A—A shown in FIG.  2 . 
     FIG. 2 is a top-down view of the movable part (top, chip side) of the device shown in FIG. 1, according to the present invention 
     FIG. 3 is a top-down view of the fixed part (bottom, carrier side) of the device shown in FIG. 1, according to the present invention 
     FIG. 3A is a perspective view of the MEM switch, according to the invention. 
     FIG. 4 shows a top-down view of the movable part (top, chip side) of the device with a maze type configuration of the comb-drive electrodes, shown in FIG. 1 according to the present invention. 
     FIG. 5 shows a top-down view of the fixed part (bottom, carrier side) of the device with a maze type configuration for the bottom electrodes, reflecting the FIG. 4 according to the present invention. 
     FIG. 6 shows a top-down view of the movable part (top, chip side) of the device with a another configuration of the comb-drive electrodes, shown in FIG. 1, according to the present invention. 
     FIG. 7 shows a top-down view of the fixed part (bottom, carrier side) of the device with another configuration for the bottom electrodes, reflecting the FIG. 6 according to the present invention. 
     FIG. 8 illustrates another configuration of the MEM varactor device, packaged using the solder bumps for electrical feed through. 
     FIGS. 9 through 21 show the process sequence used for the fabrication of the top chip side of the MEM varactor device, in accordance with the invention. 
     FIGS. 22 through 39 show the process sequence used for fabricating the bottom carrier side of the MEM varactor device, according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described more fully, hereinafter with reference to the drawings, in which preferred embodiments are shown. 
     FIG. 1 shows a cross-section of three dimensional MEM varactor device seen through a cut defined by line A—A (see FIG.  2 ). The device is built on two separate substrates  10  and  11  upon which the movable beam  50  and fixed electrodes  51  are respectively fabricated. Added to these fixed electrodes  51 , are a series of driving combs  50 A (electrodes) hovering above fixed electrodes  51  in a direction perpendicular to the movable beam. (The comb-drive structure consists of the combination of protrusions  50  and  50 A). Hereinafter, the movable electrode substrate  10  will be referred to as the “chip side” while the fixed electrode substrate  11  will be referred to as “carrier side”. Metal connections (not shown) to the electrodes are inserted within dielectric  20 A, as it is typically done in the semiconductor fabrication process commonly known as Damascene process. In the preferred embodiment, the metal connections and electrodes are, preferably, copper, with a suitable liner and barrier material. Metal conductors  51 ,  51 A and  51 B are approximately 1000 Å thick. Conductor  51 A is illustrative of a signal electrode wherein the gap distance between electrodes  51 A and  50  determines a change in capacitance. 
     Referring to FIG. 2, the area of comb drive fins  50 A varies significantly, and is typically of the order of 10 μm 2 . The length of movable beam  50  (FIG. 1) is also variable, ranging from 20 μm to over 200 μm. The driving electrodes  51 B (FIGS. 1 and 3) stabilize the motion of the combs  50 A to force them to maintain a perfectly linear and vertical motion to provide the necessary actuation. The attractive force between driving electrodes  51 B and combs  50 A depends on the overlapping areas of the comb-drive lateral surfaces. The area of electrode  50 A ranges from 0.5 to  10 μm   2 , although its dimensions may vary by making it deeper or longer in order to maximize the area of electrode  50 A. The height of electrode  51 B determines the gap distance between the movable beam  50  and fixed electrodes  51 . 
     The width of trench gap  31  provides the necessary space for the electrodes to move up and down when a voltage is applied between electrode  50 A (FIG. 1) and stationary electrode  51 B (FIG.  1 ). When this occurs, electrodes  50 A are attracted towards electrodes  51 . The movable beam  50  is suspended from and loosely attached by a double hinged or fixed-fixed support. The moveable beam is anchored on either side to the dielectric  20 A. The attraction between the comb drive electrodes  50 A and  51 B causes beam  50  to move along the direction of the comb-drive electrodes  51 B. Control electrodes  51 A are separated from the movable beam  50  by an insulating or semi-insulating dielectric material  21 B (FIG.  30 ). Electrodes  51  can be exposed to the trench on one side or set in such a way that a thin layer of dielectric prevents physical contact between the electrodes  50 A and  51 . Preferably, a thin layer of dielectric of the order of 200-500 Å precludes them from touching each other If contact is made, a delta in potential is lost and the drive voltage may fluctuate. Alternately, the moveable beam  50  can be isolated by depositing a thin layer of dielectric on its sides. 
     Still referring to FIG. 2 that shows a top-down view of the MEM varactor “chip side” substrate according to the invention, the comb-drive electrodes are  50 A. The movable electrode is built within the substrate or on a dielectric layer deposited on top of the substrate FIG. 2 illustrates the case where movable electrode  50  is connected to the dielectric on both sides using a double-hinged flexure supports. The movable electrode can be supported by variety of flexure supports providing different spring constant to the beam. Such flexure supports can be single hinged, serpentine, crab-leg, fixed-fixed supports. Metal electrodes  50 A and  50  can be of different or same material, latter is preferable for better electrical connectivity. A cavity  30  is formed in the dielectric or substrate, beneath the movable beam  50  allowing the structure to move freely. The corresponding electrode  50  is formed within the dielectric and over the cavity  30  which is filled with sacrificial material using in conventional semiconductor fabrication techniques such as damascene approach. The electrodes  50 A can be formed over the electrode  50  using through plating approach. When a voltage differential is applied to the electrodes  50  and  51 B, an electrostatic force attracts moveable electrode  50  towards stationary electrode  51 B, causing electrode  50  to deflect or move towards the stationary electrode. When the electrode deforms, the signal electrode(s)  51 A record the change in capacitance due to change in gap distance between the electrodes. 
     FIG. 3 is a top-down view of the carrier side substrate illustrating the driving electrodes  51 B, signal electrodes  51 A and trench gap  31  embodied in insulating material  21  over substrate  11  (FIG.  1 ). 
     The MEM device is also configured such that the drive electrodes can be of variety of configurations to maximize the comb-drive active area for lower drive voltages. FIG. 2 shows the one particular combination of the comb-drive electrodes  51 B. In addition to such a configuration, a maze type configuration shown in FIG.  4  and pin configuration in FIG. 6 can be used. A maze type configuration (FIG. 4) is expected to minimize the lateral pull-down effect of the comb-drive electrodes due to increased stiffness of the electrodes. Other configurations for comb-drive electrodes  50 A are also possible. FIGS. 3,  5  and  7  show the corresponding bottom electrode configurations for FIGS. 2,  4  and  6  respectively. For example, FIG. 3 illustrates the concept where in signal electrodes,  51 A are in between the cavity areas and along the sidewalls of the cavities comb-drive electrodes  51 A are formed. 
     FIG. 8 illustrates another configuration of the MEM varactor device, packaged using the solder bumps  51 C for electrical feed through. In this packaging approach, the carrier substrate is attached to a temporary substrate (not shown) and the bottom substrate  11  is polished or ground to open the electrodes. Thereupon, conventional semiconductor bumping process can be used to make direct electrical connection to the bottom electrodes using solder bumps. The temporary substrate  11 A (not shown) is then removed. Typical height of the solder bumps are of the order of 0.1 to 1 mm. The carrier substrate can be diced and individual components are attached to an organic or ceramic substrate  12  for electrical connections. Using this approach wafer level alignment and bonding of the “chip side” of the device can be done to carrier side of the device providing the advantages of improved yield and lower cost of manufacturing. 
     FIGS. 9 through 21 show the process sequence which can be used for fabrication of the top chip-side of the MEM varactor device using the present invention and FIGS. 22 through 39 show the process sequence which can be used for fabrication of the bottom carrier-side of the MEM varactor device. Step-step process sequence is described briefly below: 
     Chip-side process sequence 
     FIG. 9 shows the first step of the fabrication process wherein insulating or semi-insulating material  20  is deposited on top of the chip-side substrate  10 . Preferably, the thickness of the material  20  is to match the height of a cavity intended to be formed beneath the movable electrode and allowing free motion of the structure. FIG. 10 shows the cavity  30  formed in material  20  formed over the chip-side substrate  10  using conventional semiconductor lithography and patterning techniques. 
     In FIG. 11, sacrificial material or polymer  40  is deposited to fill the cavity formed in the previous step. FIG. 12 illustrates the step of planarizing sacrificial material  40 . An insulating or semi-insulating material  20 A is then deposited over material  40  (FIG.  13 ). The insulating material  20 A is then patterned and etched to form an opening for the formation of the movable electrode and associated connections  30 . Seed layer  50 C is then deposited for further processing (FIG.  14 ). In FIG. 15, conductive material  50  is deposited over the substrate using plating or other similar techniques. The thickness of the metal deposited should be at least equal to the thickness of the movable electrode. In FIG. 16, metal  50  is planarized to form the electrodes over the substrate. Seed metal  50 B, e.g., chrome-copper, is then deposited over the substrate for selectively plate the electrodes (FIG.  17 ). Resist or polymeric material  60  is then deposited and patterned to form a plurality of openings for the selective plating process (FIG.  18 ). In FIG. 19, comb-drive electrodes are plated through the resist or polymeric material. The polymeric material/resist  60  is then removed or stripped from substrate  10  to be followed by seed metal  50 B being etched or removed (FIG.  20 ). FIG. 21 shows the final processing step of the chip-side wherein sacrificial material  40  underneath the movable electrode  50  is etched or removed to form a free standing movable beam. The structure can then be flipped over onto the carrier side as shown in FIG.  1 . 
     Carrier-side process sequence 
     FIG. 22 illustrates the first step for processing the carrier-side (‘bottom half’) of the MEM varactor device. Substrate  11  is patterned and etched to fabricate a plurality of deep vias  31  to form the bottom electrodes. Insulating material or dielectric  21  is then conformally deposited over the vias (FIG.  23 ). Conductive material  51 , preferably metal, is then embedded within the vias and planarized to form the bottom electrodes, as it is commonly done in a Damascene process (FIG.  24 ). Dielectric or insulating material  21 A is deposited over the bottom electrodes (FIG.  25 ), the thickness of which determining the depth of the trenches for the comb-drive electrodes, the thickness of which should be at least equal or greater than the height of the comb-drive electrodes  50 A previously fabricated on the chip-side (FIG.  21 ). Dielectric material  21 A is then patterned and etched to form openings  31 A over the bottom electrodes (FIG.  26 ). Conductive material or metal  51 F is then deposited over the patterned dielectric (FIG.  27 ). Resist or polymeric material  41  is blanket deposited over the structure and patterned to expose portions of the metal  51 F (FIG.  28 ). 
     In FIG. 29, the exposed metal is etched except in the areas where the resist/polymer covers the metal, forming the drive electrodes  51 B and signal electrodes  51 A. The resist or polymeric material is then removed. Insulating material  21 B is deposited over the openings to cover the electrodes  51 A and  51 B (FIG.  30 ). Resist or polymer material is then patterned and the dielectric material  21 B is etched at the bottom of the openings (FIG.  31 ). Resist or polymer  41 A is again blanket deposited and patterned to selectively open the regions at the interconnections or contact pads  51  on either side of the device (FIG.  32 ). Thereafter, seed metal  51 E, e.g., chrome-copper, is deposited over polymer  41 A (FIG.  33 ). The thickness of the polymer  41 A is a critical parameter since it determines the height of the raised contact electrodes and the initial gap distance between top and bottom electrodes. FIG. 34 shows the step wherein metal  51 D (of the same material as  51 ) is deposited over the contact pads and is planarized. FIG. 35 shows the final processing step for the carrier-side substrate, wherein the resist or polymeric material  41 A is removed or stripped to expose raised contact pads  51 D. Once the carrier and the chip side substrates are completed, the chip side substrate can be diced and the chip-side device (movable beam) can be flipped over onto the carrier-side substrate, aligned and bonded to form the final device as seen in FIG.  1 . Since individual “chipside” devices can be bonded onto the carrier-side substrate simultaneously, yield and manufacturing costs can be lowered (wafer level packaging). Also, once the final device is formed, no additional packaging is necessary, as the device is completely surrounded by the substrate on either side. For applications requiring hermetic sealing of the MEMS device, vacuum lamination of the final device can be done using polymer resulting in good polymer encapsulation of the device. 
     In another embodiment, the MEMS varactor device can also interconnected using an alternate fabrication method. After the carrier substrate with the raised contact pads (FIG. 34) is formed, polymeric material is left on top of the substrate and another temporary substrate  11 A is attached to the top of the polymer (FIG.  36 ). Substrate  11  is then polished or ground to open the bottom of the electrodes for the interconnections (FIG.  37 ). Solder bumps of material  51 C are attached to the bottom of the substrate using conventional bumping fabrication methods (FIG.  38 ). Typical material used for solder bumping is lead-tin, tin-silver. Insulating or semi-insulating polymeric material is deposited at the bottom of substrate  11  to ensure mechanical stability. 
     FIG. 39 shows the last processing step for the carrier-side of the device with solder bump interconnections. Herein, glass substrate  11 A and polymeric material  41 A are removed to expose raised contact pads  51 D. The substrate can then be diced and attached to another organic or ceramic substrate  12  to provide the interconnections. Individual ‘chip-side’ devices can be flipped over and bonded to the carrier substrate to form the final device as shown in FIG.  8 . 
     While the invention has been described in conjunction with a preferred embodiment, it is to be understood that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the aforementioned description. Accordingly, it is intended to all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims. All matters set forth herein or shown in the accompanying drawings are to be interpreted in an illustrative and non-limiting sense.