Abstract:
A transfer stamp that can be charged with a spatial pattern of electrostatic charge for picking up selected semiconductor dice from a host substrate and transferring them to a target substrate. The stamp may be bulk charged and then selectively discharged using irradiation through a patterned mask. The technique may also be used to electrostatically transfer selected semiconductor dice from a host substrate to a target substrate.

Description:
[0001]    This patent application claims the benefit of U.S. provisional application No. 61/287,445 filed Dec. 17, 2009. The disclosure of said provisional application is hereby incorporated herein by reference thereto. 
     
    
     TECHNICAL FIELD 
       [0002]    The subject matter of the present invention is directed generally to the manufacture of circuits with transferable semiconductor dice and, more particularly, is concerned with a method and electrostatic transfer stamp for transferring semiconductor dice from a host substrate to a target substrate using electrostatic transfer printing techniques. 
       BACKGROUND ART 
       [0003]    Illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offers an efficient and long-lived alternative to fluorescent, high-intensity discharge and incandescent lamps. Many LED light sources employ high powered LEDs, which pose thermal management problems and other related problems. Another drawback with state of the art LED devices is their high initial cost. 
         [0004]    Small semiconductor dice including those with sizes of 300 um or smaller provide numerous benefits in applications such as broad area lighting, concentrator photovoltaics and electronics. Devices of this scale cannot be transferred from a source wafer to a target substrate utilizing conventional pick and place technology. One technique is transfer printing, for example using composite patterning devices comprising a plurality of polymer layers each having selected values of mechanical properties, such as Young&#39;s Modulus and flexural rigidity; selected physical dimensions, such as thickness, surface area and relief pattern dimensions; and selected thermal properties, such as coefficients of thermal expansion and thermal conductivity; to provide high resolution patterning on a variety of substrate surfaces and surface morphologies. 
         [0005]    There is therefore a need for an innovation whereby small semiconductor dice can be efficiently and effectively transferred from a host substrate to a target substrate. 
       SUMMARY OF THE INVENTION 
       [0006]    The subject matter of the present invention is directed to such an innovation which relates to a method and electrostatic transfer stamp for transferring semiconductor dice directly from a host substrate to a target substrate using electrostatic transfer printing techniques. 
         [0007]    In one aspect of the invention, a method is provided for transferring semiconductor dice from a host substrate to a target substrate wherein the method includes the steps of electrostatically charging regions of a selected one of a transfer stamp or a target substrate, transferring semiconductor dice from a host substrate to the electrostatically charged regions of the selected one of the transfer stamp or target substrate by the application of the electrostatic force of the charged regions to the semiconductor dice when the host substrate is positioned adjacent to the selected one of the transfer stamp or target substrate, and releasing the semiconductor dice onto the target substrate from the transfer stamp, when the selected one is the transfer stamp, by at least removing the electrostatic charge of the electrostatically charged regions or providing an adhesive force between the semiconductor dice and the target substrate that is greater than the electrostatic force between the semiconductor dice and the electrostatically charged regions. 
         [0008]    In another aspect of the invention, a method is provided for transferring semiconductor dice from a host substrate to a target substrate wherein the method includes electrostatically charging regions of a transfer stamp, removing semiconductor dice from a host substrate using the electrostatically charged regions of the transfer stamp, and releasing the semiconductor dice from the transfer stamp onto the target substrate by at least removing the electrostatic charge of the stamp or providing an adhesive force between the semiconductor dice and the target substrate that is greater than the electrostatic force between the semiconductor dice and the transfer stamp. The transfer stamp has spaced apart mesas formed on a light-transmitting substrate and a series of strata overlying the mesas and having a charge transfer outer layer on the mesas defining the electrostatically charged regions. 
         [0009]    In a further aspect of the invention, a transfer stamp is provided for transferring semiconductor dice from a host substrate to a target substrate. The transfer stamp includes a substrate and a series of strata overlying and applied to said substrate, wherein the series of strata includes a charge transfer outer layer having selected regions electrically charged so as to hold a spatially distributed pattern of charge for a duration of time long enough to transfer semiconductor dice from a host substrate to a target substrate. The substrate includes a surface having a plurality of mesas formed thereon, the series of strata at least overlying the mesas. The charge transfer outer layer comprised of spaced apart portions disposed at the mesas. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For clarity, the drawings herein are not necessarily to scale, and have been provided as such in order to illustrate the principles of the subject matter, not to limit the invention. 
           [0011]      FIG. 1  is a schematic view of a portion of an electrostatic transfer stamp with a mesa. 
           [0012]      FIG. 2  is a schematic view of a charged electrostatic transfer stamp being irradiated with light through a mask. 
           [0013]      FIG. 3  is a schematic view of a selectively charged electrostatic transfer stamp being brought into proximity to a substrate carrying under-etched semiconductor dice. 
           [0014]      FIG. 4  is a schematic view of an electrostatic transfer stamp with semiconductor dice attached. 
           [0015]      FIG. 5  is a schematic view of an electrostatic transfer stamp being brought into contact with a target substrate that is carrying adhesive and electrical interconnects. 
           [0016]      FIG. 6  is a schematic view of a target substrate mounted with semiconductor dice. 
           [0017]      FIG. 7  is a flow diagram of a method for transferring semiconductor dice from a host substrate to a target substrate using an electrostatic transfer stamp. 
           [0018]      FIG. 8  is a schematic view of a host substrate being brought into contact with a target substrate. 
           [0019]      FIG. 9  is a schematic view of a host substrate removed from a target substrate, having transferred semiconductor dice thereto. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0020]    The term semiconductor die (plural: dice) includes light-emitting elements, which are any devices that emit electromagnetic radiation within a wavelength regime of interest, for example, visible, infrared or ultraviolet regime, when activated, by applying a potential difference across the device or passing a current through the device. Examples of light-emitting elements include solid-state, organic, polymer, phosphor-coated or high-flux light-emitting diodes (LEDs), micro-LEDs, laser diodes or other similar devices as would be readily understood. Without limiting the foregoing, micro-LEDs include LEDs with semiconductor die with lateral dimensions of 300 microns or smaller. The output radiation of an LED may be visible, such as red, blue or green, or invisible, such as infrared or ultraviolet. An LED may produce radiation of a spread of wavelengths or multiple discrete wavelengths. An LED may comprise a phosphorescent material such as cerium-activated yttrium-aluminum-garnet (YAG:Ce 3+ ) for converting all or part of its output from one wavelength to another. An LED may comprise multiple LEDs, each emitting essentially the same or different wavelengths. 
         [0021]    While LEDs may be examples of transferable elements that may be transferred by the method of the present invention, other semiconductor dice may also be transferred, for example, integrated circuits, photovoltaic cells (for example single-junction or multijunction cells for concentrator photovoltaic applications), transistors, photodiodes, laser diodes, resistors, capacitors, and non-emitting diodes. Semiconductor dice transferred by the disclosed method may be used in electronic devices or in modules that may be incorporated in electronic devices. For example, a luminaire may comprise elements transferred by the method of the disclosed invention. 
       Exemplary Embodiments 
       [0022]    Referring now to  FIGS. 1-7 , there is shown in  FIG. 1  an electrostatic transfer stamp  10  and in  FIGS. 2-6  a succession of stages in accordance with the basic steps of a method depicted in a flow diagram in  FIG. 7  that uses the electrostatic transfer stamp  10  and electrostatic transfer printing techniques to transfer semiconductor dice  34  from a host substrate  32  to a target substrate  42 , in accordance with the present invention. In an exemplary embodiment shown in  FIG. 1 , the electrostatic transfer stamp  10  includes a transparent, light-transmitting, substrate  12  with a plurality of embossed mesas  14  (only one mesas being seen in  FIG. 1 ) spaced at selected intervals. 
         [0023]    Additionally, the transfer stamp  10 , fabricated by using known patterning techniques, has the following series of strata overlying the transparent substrate  12 . A substantially transparent or translucent electrically conductive layer  16  (for example, indium-tin oxide or graphene) is conformally applied to the surface of substrate  12 . A charge-generating layer  18  is then conformally applied to the transparent or translucent conductive layer  16 . The layer  18  is capable of absorbing light and generating electron-hole pairs. Suitable charge-generating materials for layer  18  may include inorganic materials such as amorphous silicon, arsenic selenide, cadmium sulphide, selenium, cadmium selenide, titanium oxide, and zinc oxide, and inorganic materials such as various perylene, thiapyrrylium, anthranone, squarylium, bisazo, trisazo, and azulenium compounds, as well as organic dyes, as will be known to those skilled in the art of xerographic printing processes. A charge transfer layer  20  is then applied to the layer  18  at the top of each mesa  14  for transporting the generated charge. Suitable charge-transfer materials include various hydrazone, oxazole, triphenlymethane, arylmine, stilbene and enamine compounds. The charge generating and charge transfer layers  18 ,  20  may be incorporated in polymer binders, including polyester resins, polycarbonate resins, acrylic resins, and acryl-styrene resins for application to the substrate  12 . Alternatively the charge generation layer  18  and charge transfer layer  20  may be the same material, such as for example amorphous selenium, selenium alloys, zinc oxide, and cadmium sulphide. 
         [0024]    In step  50  of the flow diagram of  FIG. 7 , an electrostatic charge  28  is distributed over the surface of transfer stamp  10 , as shown in  FIG. 2 , by means of a corona discharge, a contact roller with a charge applied to it, or an ionic air curtain. This is referred to as bulk charging of the stamp  10 . The polarity of the charge  28  is chosen to suit the charge generating material  20 , which may be positive-charging or negative-charging. 
         [0025]    In step  52  of the flow diagram of  FIG. 7 , in preparation for transfer of selected semiconductor dice  34  thereto, the non-mesa portion of the charged stamp  10  is then selectively erased of charge by exposing the stamp  10  to optical radiation  22 , as shown in  FIG. 2 , with a wavelength range to which the charge generating material  18  is sensitive. An optical mask  24  with opaque regions  26  is employed to overlie and shield the mesas  14  from having their charge dissipated. As a result of selectively erasing the charge  28 , the electrostatic transfer stamp  10 , as shown inverted in  FIG. 3  compared to  FIG. 2 , in accordance with the present invention now carries partial charge  30  on the mesas  14 , i.e. in the regions where it is desired to pick up semiconductor dice  34 . 
         [0026]    Still referring to  FIG. 3 , the host epiwafer or substrate  32  is shown carrying release-etched semiconductor dice  34  that are suspended by small anchors  36  over gaps  38 . Each die is coated with a dielectric material such as for example SU-8 photoresist (MicroChem Corporation, Newton, Mass.). An electrical charge  40  opposite to the charge  30  of the charged transfer stamp is applied to the host epiwafer  32 . 
         [0027]    In step  54  of the flow diagram of  FIG. 7 , the transfer of the semiconductor dice  34  from the host substrate  32  to the stamp  10  occurs when the stamp  10  is then positioned such that its charged mesas  14  are brought to within a suitably close distance (typically on the order of microns) from one or more semiconductor dice  34 , as also shown in  FIG. 3 . The transfer stamp  10  may in other embodiments be brought into contact with the semiconductor dice  34 . Suitable positioning means include linear and piezoelectric motors actuating a support for the transfer stamp  10 . The separation between the charge transfer layers  20  and the semiconductor dice  34  and the quantities of the electric charges  30 ,  40  are such that, in accordance with Coulomb&#39;s Law, the electrostatic force between the mesas  14  of the transfer stamp  10  and the semiconductor dice  34  is sufficient to fracture the anchors  36  connecting the semiconductor dice  34  to the epiwafer substrate  32  and transfer the semiconductor dice  34  to the top of the mesas. As shown in  FIG. 4 , the semiconductor dice  34  have been transferred to mesas  14  of the transfer stamp  10 . 
         [0028]    Referring to  FIG. 5 , the target substrate  42  to which will be transferred the semiconductor dice  34  on the mesas  14  of the transfer stamp  10  includes an insulating material (such as for example glass, polyethylene terephthalate (PET) or polymethyl methacrylate (PMMA)) upon which solder-coated conductive interconnects  44  and an adhesive material  46  between the interconnects  44  have been deposited. 
         [0029]    In step  56  of the flow diagram of  FIG. 7 , the transfer stamp  10  is positioned, as shown in  FIG. 5 , such that its semiconductor dice  34  may be brought into physical contact with the interconnects  44  of a target substrate  42  and adhesively bonded to the target substrate  42 . As may be appreciated by those skilled in the art, semiconductor dice  34  may comprise other microelectronic components, including but not limited to resistors, capacitors, inductors, diodes, transistors, and integrated circuits. 
         [0030]    If, once the semiconductor dice  34  are brought into physical contact with the interconnects  44  and adhesively bonded to the target substrate  42 , the adhesive force between the semiconductor dice  34  and the target substrate  42  is high enough, then the transfer stamp  10  may be separated from the target substrate  42  without dissipation of the charge. However, if the adhesive force bonding the semiconductor dice  34  to the substrate  42  is less than the electrostatic force binding the dice  34  to the transfer stamp  10 , in step  58  of the flow diagram of  FIG. 7  the transfer stamp  10  may then be optionally bulk erased by exposing it to optical radiation with a wavelength range to which the charge generating material  18  is sensitive. The electrical charge on the mesas  14  will as a result be dissipated, resulting in a reduction of the electrostatic force and in the semiconductor dice  34  being adhesively bonded to the target substrate  42 . 
         [0031]    There may be some natural dissipation of the electrical charges  30 ,  40 , but the materials in the charge generating layer  18  and/or the charge transfer layer  20  are chosen to hold the charge for long enough to effect the transfer of semiconductor dice  34  from the host substrate  32  to the target substrate  42 . 
         [0032]    Finally, in step  60  of the flow diagram of  FIG. 7  the semiconductor dice  42  are electrically bonded to their solder-coated interconnects  44  for example by using a suitable heat source, such as for example an infrared or laser light source, or by conductively or convectively heating the substrate  42 . As a result, as shown in  FIG. 6 , the target substrate  42  is mounted with semiconductor dice  34 , that are attached to it with adhesive  46  and electrically connected to interconnects  44  thereon. 
       Alternative Embodiments of Method and/or Transfer Stamp 
       [0033]    In a first alternative embodiment, semiconductor dice  34  are removed from their host epiwafer substrate  32  by means of laser liftoff techniques rather than by fracturing the anchors  36 . 
         [0034]    In a second alternative embodiment, semiconductor dice  34  are connected to their host epiwafer substrate  32  by anchors  36  that are preferentially resistant to fracturing, dependent on the distribution of electrostatic force applied thereto. The distribution of electrostatic charge applied to charge transfer layer  20  may then be selected such that only semiconductor dice  34  with a specific orientation may be successfully removed by fracturing of their anchors  36 . 
         [0035]    In a third alternative embodiment, the mask  24  may, in addition to exposing the inter-mesa regions of the transfer stamp  10 , optionally expose charge transfer layers  20  on selected mesas  14  in a preselected pattern to effect the erasure of their charge as well. In a preferred embodiment as an example, the mask  24  may shield only the mesas  14  that are located in a rectangular array at every m th  row and n th  column of a square array of mesas. A square array of semiconductor dice  34  on a host epiwafer substrate  32 , which may for example have a center-to-center spacing of 100 microns, may then be repetitively transferred to multiple target substrates  42  with a resulting center-to-center spacing of m×100 microns in one direction and n×100 microns in the orthogonal direction. 
         [0036]    In a fourth alternative embodiment, substrate  12  of the transfer stamp  10  is planar, without mesas  14 , and transfer of semiconductor dice  34  from their host epiwafer substrate  32  to the transfer stamp  10  is effected by means of electrically charging selected regions of charge transfer layer  20 . 
         [0037]    In a fifth embodiment, the target substrate includes mesas upon which semiconductor dice  34  are deposited using the substantially planar transfer stamp of the previous alternative embodiment. 
         [0038]    In a sixth alternative embodiment, erasure of the electrostatic charge  30  from charge transfer layer  20  is effected by selective activation of a mechanically or electrically scanned LED array or laser scanner, as will be known to those skilled in the art of xerographic printing processes. 
         [0039]    In a seventh alternative embodiment, the charge transfer layer  20  is selectively discharged by the localized application of conducted thermal energy or ionized air. 
         [0040]    In an eighth alternative embodiment, the anchors  36  connecting semiconductor dice  34  to their host epiwafer substrate  32  are broken by mechanical means (including one or more of the application of constant or varying force upon the transfer stamp  10 , ultrasonic vibration of the transfer stamp  10  or host epiwafer  32 , and shock waves or supersonic shock waves propagated through the transfer stamp by means of ultrasonic transducers) while the dice are simultaneously held in place for transfer by electrostatic forces. 
         [0041]    In a ninth alternative embodiment, semiconductor dice  34  are transferred to a target substrate  32  without an adhesive coating on the target substrate, wherein the dice are electrically bonded to the interconnects  44  before the transfer stamp  10  is removed. 
         [0042]    In a tenth alternative embodiment, the mesas  14  are coated with an electret material possessing a quasi-permanent dipole polarization. Suitable materials include waxes, polymers, and resins that are melted and then cooled to solidification in a static electric field, and ferroelectric materials. The electret materials provide a permanent electric field. 
         [0043]    In an eleventh alternative embodiment, the semiconductor dice  34  are directly transferred from the host substrate  32  to a target substrate  70  as indicated in  FIGS. 8 and 9 . Semiconductor dice  34  are attached to host substrate  32  by anchors  36 . The target substrate  70  with electrostatically charged mesas  72  is brought into contact with host substrate  32 , whereupon selected semiconductor dice  34 A are detached from anchors  36  and transferred to mesas  72  when the host substrate  32  and target substrate  70  are separated. The semiconductor dice  34 B that are not selected remain attached to the host substrate  32 . 
         [0044]    In a twelfth alternative embodiment, the host substrate  32  may not be directly charged, and charges in the semiconductor dice may be induced as the charged transfer stamp  10  is brought into proximity with the host substrate  32 . 
         [0045]    In the description herein, embodiments disclosing specific details have been set forth in order to provide a thorough understanding of the invention, and not to provide limitation. However, it will be clear to one having skill in the art that other embodiments according to the present teachings are possible that are within the scope of the invention disclosed, for example the features described above may be combined in various different ways to form multiple variations of the invention.