Patent Publication Number: US-8980666-B2

Title: Method of fabricating sensors having functionalized resonating beams

Description:
PRIORITY CLAIM AND RELATED APPLICATION 
     This application claims the benefit of priority under 35 U.S.C. Section 119 to European Patent Application Serial No. 10193535.1, filed Dec. 2, 2010, which application is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The global efforts on preserving the health of the planet are focused in some key area like global warming and environmental air quality. One aspect of improving environmental air quality relates to technologies where the gases (e.g., carbon dioxide) that are released from manufacturing processes are captured and sequestered. 
     Capturing and sequestering gases may in the near future involve tightly controlled processes that include continuous real-time monitoring of emissions. The processes and monitoring that are typically associated with capturing and sequestering gases often require high cost-high power sensors. 
     One potential technology that may be utilized to produce low-cost and low-power sensors involves integrated resonant sensing technology. This integrated sensing technology is based on vibrating beams that are functionalized for chemisorptive carbon dioxide capture. The beams are typically doubly clamp (nano)beams, cantilever (nano)beams or even nanowires. The beams change their resonance frequency proportional to the amount of gas adsorbed on the beam. 
     These functionalized resonant sensing beams are typically located on the same chip with an integrated circuit for processing signals from the carbon dioxide sensor. A typical single silicon wafer having diameter of 400 mm may contain hundreds of thousands of on-chip carbon dioxide Nano-Electro-Mechano Sensors and Integrated Circuits (NEMSIC) that communicate with the readout integrated circuit. 
     One of the drawbacks with existing resonant NEMSIC gas sensing systems may relates to an inability to mass produce such systems. One of the difficulties that is associated with mass producing such systems is the low thermal tolerance of the functionalized resonating beams that are typically a primary component in these types of sensors. The low thermal tolerance is due to the fact that the functionalized resonating beams are usually made of organic materials whose chemical stability deteriorates at temperatures higher than 60-70° C. Since many manufacturing methods require temperate above 60-70° C., it becomes difficult to mass produce sensors that include functionalized resonating beams. 
     There is a need for a method of fabricating sensors that include functionalized resonating beams which allows (i) preparation of sensing layers in functionalized resonating beams; (ii) cost effective high volume packaging of the sensors, (iii) compatibility with the thermal limits tolerated by the organic material that is used in functionalized resonating beams; and (iv) the sensing surface to have access to the ambient air in order detect a reversible reaction between a particular gas in the ambient air and the functionalized resonating beams. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow diagram illustrating an example method fabricating a sensor. 
         FIG. 2  illustrates an example substrate wafer that includes multiple chips where each chip has at least one suspended beam and the figure includes an enlarged portion that illustrates one example chip. 
         FIG. 3  illustrates an example chip that includes at least one suspended beam. 
         FIG. 4  is a section view of the chip shown in  FIG. 3  taken along line  3 - 3 . 
         FIG. 5  illustrates the example chip shown in  FIG. 3  where the chip is covered with an adhesive layer. 
         FIG. 6  is a section view of the chip shown in  FIG. 5  taken along line  6 - 6 . 
         FIG. 7  illustrates the example chip shown in  FIG. 3  where a cover has been placed onto the adhesive layer. 
         FIG. 8  is a section view of the chip shown in  FIG. 7  taken along line  7 - 7 . 
         FIG. 9  illustrates a portion of the cover wafer that is used in the chip shown in  FIGS. 7 and 8 . 
         FIG. 10  is a section view of the portion of the cover wafer shown in  FIG. 9  taken along line  10 - 10 . 
         FIG. 11  is a flow diagram illustrating another example method fabricating a sensor. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, electrical, and optical changes may be made without departing from the scope of the present invention. The following description of example embodiments is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims. 
       FIG. 1  illustrates an example method [ 100 ] of fabricating a sensor  10 . The method includes [ 110 ] providing a substrate wafer  12  that includes at least one suspended beam  14  (e.g., a substrate wafer  12  that includes CMOS circuitry or SOI-CMOS circuitry). An example substrate wafer  12  is shown in  FIG. 2  with one chip on the substrate wafer  12  enlarged for purpose of clarity. 
       FIGS. 3-10  also illustrate one chip only for purposes of clarity even though many of steps described herein refer to wafer level processing. It should be noted that even though  FIGS. 3 and 4  show the substrate wafer  12  as having two suspended beams  14 , the substrate wafer  12  may include any number of suspended beams  14 . 
     Referring again to  FIG. 1 , the method [ 100 ] further includes [ 120 ] adding an adhesive layer  16  to the substrate wafer  12  such that the adhesive layer  16  covers portions of the substrate wafer  12  without covering the suspended beams  14 . As an example, the adhesive layer  16  may be added by patterned adhesive polymer bonding. As shown most clearly in  FIGS. 5 and 6 , the suspended beams  14  are exposed through openings  17  in the adhesive layer  16  for subsequent processing. 
     In some embodiments, [ 120 ] adding an adhesive layer  16  to the substrate wafer  14  such that the adhesive layer  16  covers portions of the substrate wafer  12  without covering the suspended beams  14  includes curing the adhesive layer before functionalizing the suspended beams  14 . The adhesive layer  16  may be deposited on the substrate wafer  12  by direct printing or photolithographic methods. 
     The adhesive layer  16  may be a thermosetting or thermoplastic polymer layer. Some example thermosetting polymers include benzene-cyclo-butene (BCB) and other materials (e.g., liquid crystal polymer or polyimides). 
     The adhesive layer  16  is preferably a CMOS compatible, low cost, high bond strength polymer adhesive which is thermally curable polymer that becomes mechanically and chemically strong after a thermally activated cross linking polymerization process. As an example, the adhesive layer  16  may have a thickness in the range of 1.5-3 micrometers. 
     When the adhesive layer  16  is formed of BCB, the adhesive layer  16  may be pre-cured in an oxygen free ambient environment at temperatures between 190 and 200° C. for 30 minutes in order to reach a partial polymerization level of about 30-50%. During this stage the solvents and volatile components are removed from the BCB in order to avoid something similar happening during curing. The pre-curing also helps the BCB deform and wet the surface of the substrate wafer  12 . 
     An adhesion promoter may be applied to the substrate wafer  12  in regions where the adhesive layer  16  are to be applied in order to facilitate bonding with the substrate wafer  12 . The adhesive promoter may be applied by mask less direct printing methods. 
     In the illustrated example embodiments, the bond pad areas and sensing and reference beams area are not covered with adhesive layer  16 . All the rest of the chip is covered with the adhesive layer  16  to promote bonding stability and heat transfer outside of the silicon chip. 
     Referring again to  FIG. 1 , the method [ 100 ] further includes [ 130 ] positioning a cover wafer  18  onto the adhesive layer  16  such that the suspend beams  14  are exposed to ambient air through openings  19  in the cover wafer  18 . As shown most clearly in  FIGS. 7 and 8 , the suspend beams  14  are exposed to ambient air through openings  19  in the cover wafer  18 . 
     In some embodiments, [ 130 ] positioning a cover wafer  18  onto the adhesive layer  16  such that the suspend beams  14  are exposed to ambient air through openings  19  in the cover wafer  18  includes fabricating the openings  19  in the cover wafer  18 . As an example, the cover wafer  18  may be formed of silicon such that fabricating the openings  19  in the cover wafer  18  includes anistropic etching of the cover wafer  18 . 
     The openings  19  in the cover wafer  18  in the cover wafer (which create access zones to the suspended beams  14  and possibly bond pads) by a selective masking and deep etching process. As an example, KOH etching or deep reactive ion etching (DRIE) may be used for silicon cover wafer. As another example, DRIE or sand blasting may be used for a glass cover wafer. 
     In some embodiments, [ 130 ] positioning a cover wafer  18  onto the adhesive layer  16  such that the suspend beams  14  are exposed to ambient air through openings  19  in the cover wafer  18  includes bringing the substrate wafer  12  (which includes the adhesive layer  16 ) and the cover wafer  18  to a bonding tool where they are placed on a bonding fixture. The bonding fixture is then transferred to a bonding chamber that initially has a vacuum level of 10-3 mbar. The binding tool applies a bonding pressure of about 0.4 MPa while the wafer stack is heated to about 250° C. for 30-560 minutes to perform the curing process for final BCB based adhesive wafer bonding. 
     Referring again to  FIG. 1 , the method [ 100 ] further includes [ 140 ] functionalizing the suspended beams  14  by contacting the suspended beams  14  with materials through the openings  19  in the cover wafer  18 .  FIGS. 9 and 10  illustrate an example cover wafer  18  that may be used in the method [ 100 ]. 
     In some embodiments, [ 140 ] functionalizing the suspended beams  14  by contacting the suspended beams  14  with materials through the openings  19  may include direct printing of the materials through the openings  19  in the cover wafer  18 . As an example, direct printing of the materials may be done with high positioning accuracy by means of dip pen nanolithography (DPN) tools. 
     Functionalizing the suspended beams  14  after the cover wafer  18  has been bonded to the adhesive layer  16  prevents having the functionalized surface of the functionalized suspended beams  14  exposed to undesirably high temperatures that are used during the bonding process. 
     As used herein, a “functionalized beam” means a beam that is capable of absorbing and/or reacting with a particular gas. It should be noted that a functionalized beam may include functionalized surfaces that are formed as part of a layer or coating that is added to the suspended beams  14 . 
     In some embodiments, [ 140 ] functionalizing the suspended beam by contacting the suspended beams  14  with materials through the openings  19  in the cover wafer  18  includes drying the deposited material. As an example, drying the deposited material may include changing the material from a liquid to a solid. 
     The technology associated with functionalizing the suspended beams  14  will depend in part on the type of gas that is to be detected by the sensor  10 . As discussed above, only the regions of the chip where the suspended beams  14  are located is exposed for the whole sequence of chemical processes that are needed for functionalization. All the other parts of the chip are protected by the substrate wafer  12 , adhesive layer  16  and the cover wafer  18 , which are chemically resistant to processes used for functionalization (e.g., silicon functionalization). 
     As shown in  FIG. 1 , the method [ 100 ] may further include [ 150 ] dicing the cover wafer  18  and the substrate wafer  12  to form the sensor  10 . It should be noted the cover wafer  18  and the substrate wafer  12  may be diced by any known method. 
       FIG. 11  illustrates an example method [ 200 ] of fabricating a sensor  10 . The method includes [ 210 ] providing a substrate wafer  12  that includes at least one suspended beam  14  (e.g., a substrate wafer  12  that includes CMOS circuitry or SOI-CMOS circuitry). 
     The method [ 200 ] further includes [ 220 ] functionalizing the suspended beam and [ 230 ] adding an adhesive layer  16  to the substrate wafer  12  such that the adhesive layer  16  covers portions of the substrate wafer  12  without covering the functionalized suspended beams  14 . As shown most clearly in  FIGS. 5  and  6 , the suspended beams  14  are exposed through openings in the adhesive layer  16  for subsequent processing. 
     In some embodiments, [ 220 ] functionalizing the suspended beams  14  includes direct printing of the materials onto the suspended beams. As an example, direct printing of the materials may be done with high positioning accuracy by means of dip pen nanolithography (DPN) tools. 
     In some embodiments, [ 220 ] functionalizing the suspended beams includes drying the deposited material. As an example, drying the deposited material may include changing the material from a liquid to a solid. The technology associated with functionalizing the suspended beams  14  will depend in part on the type of gas that is to be detected by the sensor  10 . 
     Referring again to  FIG. 11 , the method [ 200 ] further includes [ 240 ] positioning a cover chip  18  onto the adhesive layer  16  such that the suspended beams  14  are exposed to ambient air through openings  19  in the cover chip  18 . 
     In some embodiments, [ 240 ] positioning a cover chip  18  onto the adhesive layer  16  such that the suspended beams  14  are exposed to ambient air through openings  19  in the cover chip  18  includes fabricating openings  19  in a cover wafer (see, e.g.,  FIG. 9 ) and dicing the cover wafer to form the cover chip  18 . 
     Positioning a cover chip  18  onto the adhesive layer  16  allows only known good glass dies to be bonded to known good dies on the substrate wafer  12 . As an example, only good cover chips  18  may be picked and placed process on the silicon sensor chips. Appropriate pick and place procedures may serve to facilitate proper alignment before bonding good cover chips  18  to respective silicon sensor chips. 
     In some embodiments, the adhesive layer  16  may be cured by using ultraviolet light (UV) at room temperature in order to preserve chemical properties of functionalized suspended beams  14 . When a UV curable adhesive is used as part of the adhesive layer  16 , the chip  18  may be made of glass so that the chip  18  is transparent to UV. 
     One example UV curable adhesive may be a thermosetting polymer, such as NEA 121 UV adhesive from Norland product or SU8 epoxy)\. In some embodiments, adhesive curing may take place in UV for 10 minutes where the UV is provided by a UV lamp that illuminates at 14 mW/cm2. One example lamp is model no. SB-100P manufactured by Spectronics). 
     As shown in  FIG. 11 , the method [ 200 ] may further include [ 250 ] dicing the substrate wafer  12  to form the sensor  10 . It should be noted the cover the substrate wafer  12  may be diced by any known method. 
     In the method [ 200 ] described herein, the suspended beams  14  are functionalized before the cover chip  18  is bonded to the adhesive layer  16 . The method allows for easier separation of known good chips from the rest of the chips that are on substrate wafer  12  after wafer level testing is done on the substrate wafer  12 . 
     Since the suspended beams  14  are functionalized before the cover chip  18  is placed on the adhesive layer  16 , the packaging process needs to preserve the quality of the functionalized (i.e., organic) layer. Therefore, the cover chip  18  should be bonded to the substrate wafer  12  at room temperature. 
     The Abstract is provided to comply with 37 C.F.R. §1.72(b) to allow the reader to quickly ascertain the nature and gist of the technical disclosure. The Abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.