An integrated structure and method for fabricating the same

Description of equivalent WO 2004043850 A2

AN INTEGRATED STRUCTURE AND METHOD FOR FABRICATING THE SAME

FIELD O THE INVENTION

0003The present invention relates to an integrated structure including a micro-electromechanical system (MEMS) and a chip for delivery signls to the MEMS.

BACKGROUND OF THE INVENTION

0005The use of micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS) for ultrahigh density data storage- has recently been reported. This approach to data storage utilizes a thermomechanical local probe technique with large arrays of nanometer-scale tips, such as are now used in atomic-force microscope and scanning-tunneling microscope technology. In this technique, a read/write operation is performed by heating a cantilever mechanism, causing a tip to contact a thin film storage medium and either create or detect depressions made therein.

0006Some details of the design of MEMS structures for data storage have been recently published in IBM J. Res. Develop. 44, 323 (2000) and in Sensors and Actuators 80, 100 (2000).

0007An individual cantilever cell is shown schematically in Figure 1A (cross-section view) and Figure IB (plan view) . The MEMS chip 1, typically of silicon, is processed to yield a silicon cantilever 10 with a tip 11 and a heater 13. As shown in Figure 1A, the cantilever structure 10 is formed on a layer at the surface of chip 1, and a cavity is then etched in the bulk silicon behind cantilever 10. Applying electrical power via the through connection 15 causes a temperature increase in the heater and tip, which is in contact with storage medium 12 (typically a thin polymer film on a silicon substrate) . The combination of tip pressure on the storage medium and the tip heating causes the tip to create an indentation in the storage medium, thereby realizing thermomechanical data writing with very high bit areal densities.

0008A conventional 2-dimensional arrangement for controlling the MEMS chip 1 is shown schematically in Figure 2. The MEMS chip 1, which includes a large number of individual cells, is electrically controlled by multiplex drivers 2 having conventional wirebonding connections to the edge of chip 1. There are limitations inherent in the 2- dimensional arrangement of electrical connections. For example, as the number of cells in chip 1 increases, it becomes more difficult to provide electrical isolation between cells; at the same time, higher power is required to address the cell array while the size of individual connections decreases.

0009Accordingly, there is a need for a 3-dimensional integration scheme in which MEMS devices and their control devices (such as CMOS logic chips) may be interconnected, in order to overcome the electrical limitations of the conventional 2- dimensional configuration.

DISCLOSURE OF THE INVENTION

0011The present invention provides a method for integrating a chip having a icromechanical device and a chip having an electronic device. In particular, the present invention provides a method for vertical integration of a chip and a MEMS where the MEMS may make contact with a surface (such as the surface of a film serving as a storage device) and have mechanical motion with respect to that surface in the vertical direction.

0012According to one aspect of the invention, a method is provided for fabricating an integrated structure including a micro-electromechanical system (MEMS) and a chip for delivering signals to the MEMS. The MEMS is formed on a substrate, and has an anchor portion by which it is connected to the substrate. (In one embodiment of the invention, the MEMS is a cantilever which overhangs a cavity in the substrate, and is anchored to the substrate at the anchor portion.) A conductor is formed which extends from the anchor portion of the MEMS through the MEMS substrate. The chip is attached to the MEMS substrate in a direction normal to the substrate surface, so as to make a conductive path from the chip to the MEMS.

0013In one embodiment, the chip may be attached by forming a C4 metal pad on the chip, aligning the C4 metal pad to the conductor, and then bonding the C4 metal to the conductor. In another embodiment, the step of attaching the chip further comprises forming a metal stud on the chip, aligning the stud to the conductor; and bonding the stud to the conductor. In another embodiment, the step of attaching the chip further comprises forming one of a metal stud and a metal pad on the chip; forming the other of the metal stud and the metal pad in contact with the conductor; aligning the stud to the pad; and bonding the stud to the pad.

0014Preferably, the MEMS is formed so as to have a tip extending in the direction normal to said surface. More preferably, the method further comprises the step of providing a storage medium layer in contact with the tip, wherein said layer comprises a storage medium for storing data in accordance with indentations made therein by the tip. Still more preferably, the step of forming the MEMS further comprises : forming a cantilever structure having one end at the anchor portion and another end having a tip extending in the direction normal to the surface; and forming a cavity in the substrate in a portion thereof underlying the cantilever.

0015In a preferred embodiment, the step of forming the conductor further comprises: forming a via in the substrate; depositing metal in the via; attaching a carrier plate to the MEMS substrate; thinning the MEMS substrate at a backside surface thereof to expose the metal; and forming a metal pad at the backside surface in contact with the metal in the via. Preferably, the carrier plate is transparent to ablating radiation, and said step of attaching the chip further comprises exposing the plate to ablating radiation, thereby detaching the plate. More preferably, said step of forming the conductor includes coating a backside surface of the MEMS substrate with a first polyimide layer, forming an opening in the first polyimide layer to expose the conductor, and forming one of the metal stud and the metal pad in contact with the conductor, and said step of attaching the chip includes coating the chip with a second polyimide layer, forming an opening in the second polyimide layer in which the other of the metal stud and the metal pad is formed, and performing a lamination process to bond the first polyimide layer and the second polyimide layer. Still more preferably, the method comprises the step of, subsequent to said lamination process formation of the MEMS is completed, including forming a cavity in the MEMS substrate so that the MEMS has a cantilever structure.

0016It is preferable that the MEMS substrate be thinned before the chip is attached; this may conveniently be done by first attaching a carrier plate, and then removing the carrier plate after the attaching process is complete.

0017Preferably, a method is provided for fabricating an integrated structure including a micro-electromechanical system (MEMS) and a chip for delivering signals to the MEMS. In this method, the MEMS is formed on the substrate and has an anchor portion with an opening therein, and a conductor is formed in the opening of the anchor portion. The MEMS substrate is then removed, thereby exposing an underside of the MEMS and the conductor. The chip is then attached to the anchor portion of the MEMS in a direction normal to the substrate surface, so as to make a conductive path from the chip to the MEMS . Before the substrate is removed, it is convenient to deposit a layer overlying the MEMS, and to attach a carrier plate thereto; the carrier plate is removed after the MEMS and the chip are attached. The connection between the chip and the MEMS is through a metal stud formed on the chip, in an opening in a layer overlying the chip. The underside of the MEMS is thus spaced from the chip by a distance corresponding to the thickness of that layer.

0018In a preferred embodiment, the method further comprises the steps of: depositing a second layer on the chip; forming an opening in the second layer; and forming a metal stud in the opening in the second layer, and wherein said attaching step further comprises aligning the metal stud to the conductor at the underside of the MEMS; and performing a lamination process to bond the MEMS to the second layer. Preferably, the method further comprises the steps of: depositing a second layer on the chip; forming an opening in the second layer; forming a metal pad in the opening in the second layer; and forming a metal stud on the underside of the MEMS in contact with the conductor, and wherein said attaching step further comprises aligning the stud to the pad; and performing a lamination process to bond the MEMS to the second layer. More preferably, the carrier plate is transparent to ablating radiation, and the method further comprises the steps of: exposing the carrier plate to ablating radiation, thereby detaching the carrier plate from the first layer; and removing the first layer, after said step of attaching the chip.

0019Suitably, said step of forming the MEMS further comprises forming a cantilever structure having one end at the anchor portion and another end having a tip extending in the direction normal to the surface, and wherein the cantilever is spaced from the chip by a distance corresponding to a thickness of the second layer. More suitably, said step of forming the MEMS further comprises forming a cantilever structure having one end at the anchor portion and another end having a tip extending in the direction normal to the surface, and wherein the cantilever is spaced from the chip by a distance corresponding to a thickness of the second layer. According to a second aspect, the present invention provides a vertically integrated structure is provided in which a micro-electromechanical system (MEMS) and a chip for delivering signals to the MEMS are connected. The structure includes a layer overlying a portion of the chip and having an opening therein; the MEMS having an anchor portion and an end portion extending horizontally therefrom, the anchor portion being attached to the layer and including a conductor aligned to the opening, a metal stud in the opening, the stud contacting the chip and the conductor, and wherein the MEMS is spaced from the chip by a distance corresponding to a thickness of the layer.

0020Preferably, the MEMS comprises a cantilever structure and the end portion includes a tip extending in the vertical direction. More preferably, the structure further comprises a layer extending horizontally and spaced from the chip in the vertical direction, said layer being in contact with the tip.

BRIEF DESCRIPTION OF THE DRAWINGS

0022The present invention will now be described, by way of example only, with reference to preferred embodiments thereof, as illustrated in the following drawings, in which:

0023Figure 1A is a schematic cross-sectional view of a cell of a MEMS chip used for data storage.

0024Figure IB is a plan view of the cell of Figure 1A.

0025Figure 2 is a schematic view of a conventional 2-dimensional arrangement for connecting a MEMS chip to driver chips .

0026Figure 3 is a cross-sectional view of a completed 3-dimensional integrated MEMS/CMOS device where the connection between MEMS and CMOS chips is realized using C4 technology.

0027Figure 4 is a cross-sectional view of a completed 3-dimensional integrated MEMS/CMOS device where the connection between MEMS and CMOS chips is realized using vertical stud/via connections . Figures 5A-5F illustrate steps in the fabrication of a MEMS chip having through- wafer connections, in accordance with an embodiment of the invention.

0028Figures 6A-6C illustrate further steps in the fabrication of a MEMS chip, following the steps shown in Figures 5A-5F.

0029Figures 7A-7C illustrate an alternative to the process of Figures 6A-6C in fabricating a MEMS chip.

0030Figures 8A-8C illustrate steps in the 3-dimensional integration of a MEMS chip and a CMOS chip, using C4 connections.

0031Figures 9A-9D illustrate steps in the 3-dimensional integration of a MEMS chip and a CMOS chip, using stud/via connections.

0032Figures 9E-9G illustrate steps in the 3-dimensional integration of a MEMS chip and a CMOS chip, using an alternative stud/via connection process.

0033Figures 10A-10G illustrate steps in the 3-dimensional integration of MEMS devices with a CMOS chip, in accordance with another embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

0035Figure 3 illustrates an embodiment of the invention wherein a memory storage device includes 3-dimensional integration of a MEMS chip 1 and a CMOS chip 2 , and where the connections between the chips are made using C4 technology. The MEMS chip has an array of cantilevers 10 aligned to an array of C4 solder bumps 30 on the CMOS chip. The cantilevers 10 on the MEMS chip make contact with the storage medium 12. Movement of the cantilevers is controlled by the CMOS chip; connection between an individual cantilever and the CMOS chip is through an electrical pathway which includes interconnect 15 through the MEMS chip, pad 33 formed on a polyimide layer 31 on the backside of the MEMS chip, and the C4 solder bump 30. (The solder bump is connected to a pad, not shown, formed on a polyimide layer 32 on the CMOS chip 2. )

0036Figure 4 illustrates another embodiment of the invention, wherein a memory storage device is fabricated by integrating a MEMS chip 1 and a CMOS chip 2 using vertical stud/via connections. The MEMS chip has an interconnect 15 through the chip and a coating 16 of polyimide on the backside of the chip; electrical contact to the MEMS chip is through pads 17 formed in openings in the polyimide coating. The CMOS chip 2 also has a polyimide coating 23 on the backside thereof, with openings to match the locations of pads 17. The CMOS chip has studs 21 in those openings, with solder 22 providing a conductive connection between the pads 17 and the studs. The polyimide layers 16, 23 on the MEMS chip and CMOS chip are in mechanical contact, so that the chips are both physically and electrically integrated.

0037Details of the processes for 3-dimensional integration of MEMS and CMOS chips are given below. It will be appreciated that the MEMS cantilever structure is an example only, and that the invention is applicable to a variety of micro-electromechanical devices.

0038Preparation of MEMS chip

0039In accordance with an embodiment of the present invention, a MEMS chip having an array of silicon cantilevers is prepared as shown in Figures 5A-5F. Figure 5A shows a silicon-on-insulator (SOI) wafer having a buried oxide (BOX) layer 52 on bulk silicon 51, silicon layer 53 overlying the BOX, and a thermal oxide layer 54. Typical thicknesses of layers 52-54 are 400 nm, 4 1m and 500 nm respectively. Oxide layer 54 is then masked and etched so that only a portion 54a remains, which in turn serves as a mask for etching of silicon layer 53 (see Figure 5B) . The masked portion of the silicon layer is processed later to form the nanometer-scale tip. A deep via opening 55 is then etched into the substrate 51; the dimensions of this via are in accordance with available lithography and the design of the MEMS cell (Figure 5C) .

0040A thermal oxide layer 56 is then grown on silicon layer 53 and on the side wall of opening 55. The small unetched portion of layer 53 is partially consumed in the oxidation process, which has the effect of sharpening the silicon to a tip 53t (Figure 5D) . Layers 53 and 56 are then etched to form the cantilever structure including the heater, with the tip 53t still protected by oxide (Figure 5E) .

0041The etched via opening 55 is then filled with metal 57 which will form the through- connection. A contact pad 58 is then deposited over the top of the via and on the adjacent silicon, at the end of the cantilever opposite the tip. Another oxide layer 59 (preferably low-temperature oxide) is deposited as a blanket layer, and an opening 60 is then etched down to the surface of substrate 51 (see Figure 5F) .

0042At this point the MEMS structure is ready to have the cavity etched in the substrate (thereby releasing the cantilever tip end from the substrate) , and to be thinned in preparation for joining to the CMOS device. These steps may be performed in two different ways, as shown in Figures 6A-6C and 7A-7C respectively.

0043(1) Figure 6A shows the structure of Figure 5F first covered by a polyimide layer 61 and then having a carrier plate 62 bonded to the polyimide. The carrier plate is used to facilitate handling of the MEMS substrate after the substrate is thinned. It is preferable that the carrier be transparent to ablating radiation (e.g. a glass wafer) so that it can be conveniently removed afterwards. The MEMS substrate is then thinned in a grinding or polishing operation, so that the electrical through-connec ion is exposed on the backside surface 51b of the substrate (Figure 6B) . A polyimide layer 63 is then deposited on the backside of the substrate, and has openings 64 formed therein to expose the metallization 57. Metal pads 65 are then formed in the openings to contact metal 57 and complete the through-connection (Figure 6C) . The cavity etch is performed through opening 60 after the carrier 62 and polyimide layer 61 are removed. This structure is suitable for integration with a CMOS chip using stud/via connections, as described in more detail below.

0044(2) Figure 7A shows the structure of Figure 5F, after the cavity etch is performed in the substrate 51 (using cavity opening 60 and layer 59 as a mask) . The oxide layer 59 and BOX layer 52 are then etched; in particular, BOX layer 52 is etched at its underside through cavity 70, so that the remaining portion of silicon layer 53 becomes a cantilever overhanging cavity 70 (Figure 7B) . The top surface of the MEMS structure (including the cavity) is then covered with a polyimide layer 71, and a carrier plate 72 is bonded thereto. As noted above, the carrier plate is preferably transparent (e.g. a glass wafer) . After the carrier is attached, the substrate is thinned so that metal 57 is exposed on the backside surface 51b of the substrate. This backside surface is then coated with a polyimide layer 73, which has openings 74 formed therein; metal pads 75 are formed in the openings to contact metal 57 (Figure 7C) . This MEMS structure is suitable for integration with a CMOS device using C4 connections, as follows. MEMS/CMOS integration using C4

0045A process for MEMS/CMOS integration using C4 connections is shown in Figures 8A-8C. In Figure 8A, CMOS substrate 81 is shown with a metal pad 82 on the surface thereof and a polyimide layer 83 overlying the substrate surface. (It is understood that the electronic devices for controlling the MEMS chip have already been fabricated and are part of the CMOS substrate 81; those CMOS devices will not be discussed in detail.) As understood by those skilled in the art, a C4 connection is prepared by forming an opening in the polyimide layer 83 to expose pad 82, depositing a seed layer 84 in the opening, and plating C4 metal through a mask. The mask and excess seed layer are then removed, and a reflow process is performed to yield the C4 bonding pad 85.

0046The MEMS chip (shown with the cavity 70 already formed, in accordance with Figure 7C) is then aligned to the CMOS chip (using carrier 72) and bonded to the C4 pads. Specifically, pad 75 on the backside of the MEMS chip is bonded to the C4 pad 85, so that an electrical and thermal through-connection is made from the CMOS chip through the MEMS chip and to the cantilever structure (see Figure 8B) . Details of the C4 bonding process are known in the art .

0047The carrier 72 is then detached from polyimide layer 71 (preferably by laser ablation of the interface between the carrier and polyimide layer 71) and removed from the front side of the MEMS chip. Polyimide layer 71 is then removed, yielding the structure shown in Figure 8C. It should be noted that in this embodiment, there is a gap 88 between opposite polyimide layers 73 and 83 on the MEMS and CMOS chips respectively.

0048MEMS/CMOS integration using stud/via connections

0049Figure 9A shows a CMOS substrate 91, with electronic devices assumed to be already fabricated therein. Wiring connections 92 to the top surface of substrate 91 are shown in Figure 9A. The substrate is covered by a polyimide layer 93, with openings formed therein to make connection to the CMOS devices . As shown in Figure 9A, the openings are preferably tapered to facilitate alignment to the MEMS chip. Studs 94 are built in the openings, making electrical contact with wiring 92. The studs are capped with solder 95 for making connection to the metal pads on the MEMS chip.

0050Figure 9B shows the MEMS chip (processed in accordance with Figure 6C) aligned and bonded to the CMOS chip. With presently available techniques, this alignment may be performed accurately to less than 1 l . Polyimide layer 63 on the backside of the MEMS chip is brought into contact with polyimide layer 93 on the CMOS chip. Bonding pad 65 on the MEMS chip, which is in contact with the metallized via 57, is aligned to the stud 94. A high-temperature lamination process is then performed in which solder 95 is caused to flow, filling the opening in the polyimide layer and bonding to pad 65. It should be noted that in this embodiment, the opposing polyimide layers 63, 93 are in contact with no gap between the chips. One or both of polyimide layers 63, 93 may be coated with an adhesive before the lamination process is performed.

0051After the bonding process, the carrier 62 is removed from polyimide layer 61, preferably by laser ablation. Polyimide layer 61 is then removed to expose the cavity etch opening 60. The cavity etch processes are then performed to remove silicon from behind the cantilever structure (Figure 9C) ; oxide layers 52, 56, 59 are then etched to expose the cantilever and cantilever tip. The completed MEMS/CMOS integrated structure is shown in Figure 9D.

0052An alternative stud/via connection process is shown in Figures 9E-9G. In this process the studs are located on the MEMS chip instead of CMOS chip. As shown in Figure 9E, the backside of the MEMS substrate (shown here processed as in Figure 6B) is covered by a polyimide layer 96 and has openings formed therein. Studs 97 are then built in the openings and capped with solder 98 for making connection to the CMOS chip. The CMOS chip 91 has a polyimide layer 121 formed thereon, with openings for- making electrical connections to the CMOS devices. Bonding pads 99 are formed in these openings, making electrical contact with wiring 92 (Figure 9F; compare Figure 9A) . The MEMS chip and CMOS chip are then brought into contact with pad 99 aligned to stud 97, as shown in Figure 9G (compare Figure 9B) . A high-temperature lamination process is performed as described above, so that solder 98 bonds to pad 99. One or both of polyimide layers 96, 121 may be coated with an adhesive before the lamination process is performed. After the bonding process, processing of the MEMS/CMOS integrated structure is completed as described above.

0053As shown in Figures 8C, 9D and 9G, the above-described processes yield a 3- dimensional integrated MEMS/CMOS structure in which electrical signals travel in the z- direction through the MEMS chip (that is, in the same direction in which the cantilever tip is actuated) . In another embodiment of the invention, electrical and thermal contact is made from the CMOS chip directly to the cantilever, without a through-wafer connection.

0054Cantilever/CMOS direct contact

0055In this embodiment of the invention, the cantilever structure is anchored to a stud/via arrangement directly connected to the CMOS chip. Fabrication of the cantilever begins with an SOI wafer as previously discussed with reference to Figure 5A. The oxide layer 54 is masked and etched so that portions 54a and 54b remain, as shown in Figure 10A; the underlying silicon layer 53 is then etched. The unetched silicon areas are then processed to form the cantilever tip and anchor ends.

0056The cantilever tip 53t is sharpened by thermal oxidation of the silicon layer 53; the silicon layer is then patterned to form the cantilever, as shown in Figure 10B. In particular, the anchor portion 100 of the cantilever has a via opening 101 formed therein, so that a portion of the underlying BOX layer 52 is exposed. A metal layer 102 is then formed in the via opening. This structure is then coated with a polyimide layer 103, and a carrier plate 104 is attached thereto (see Figure 10C) .

0057The silicon substrate 51 is then removed by a grinding, polishing, wet etch or plasma etch process. The BOX layer 52 is also removed, by either a wet etch or plasma etch process. The resulting structure is shown in Figure 10D.

0058The CMOS chip is prepared as shown in Figure 10E. As noted above, it is assumed that the CMOS substrate 105 has electronic devices formed therein with wiring connections at the surface of the substrate. An inorganic layer 106, 1 lm to 2 1m thick, is deposited on the substrate surface and patterned to form a structure having an opening exposing the wiring connection. A stud 107 is formed in the opening with solder 108 on top of the stud (as discussed above with reference to Figure 9A) .

0059The MEMS chip and CMOS chip are then brought into alignment, with the anchor portion 100 of the cantilever contacting the top surface 106a of the patterned inorganic layer 106. A high-temperature lamination process is then performed in which the solder 108 is caused to flow, thereby establishing electrical and thermal contact between stud 107 and the metal pad 102 in the anchor portion 100 of the cantilever. The resulting structure is shown in Figure 10F. The carrier plate 104 is then detached from the polyimide layer 103 (preferably by laser ablation, as discussed previously) and removed. Finally, the polyimide layer 103 is stripped away, preferably with a plasma process. As shown in Figure 10G, the result of this process is an integrated structure where an array of separate MEMS devices are connected in the z-direction to a CMOS chip, without the need for through-connections in a MEMS substrate. Whereas in the previous embodiments it was necessary to etch a cavity 70 in the substrate to provide a range of vertical motion for the tip, in this embodiment a spacing 110 is established between the chip and the cantilever in accordance with the thickness of layer 106.

0060While the present invention has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the invention is intended to encompass all such alternatives, modifications and variations which fall within the scope of the invention and the following claims.