Patent Publication Number: US-11648762-B2

Title: Method and system for mass assembly of thin-film materials

Description:
RELATED PATENT DOCUMENTS 
     This application is a divisional of U.S. patent application Ser. No. 17/171,286, filed Feb. 9, 2021, which is a divisional of U.S. application Ser. No. 16/235,251 filed on Dec. 28, 2018, now U.S. Pat. No. 10,926,521, issued Feb. 23, 2021, which are incorporated herein by reference in their entireties. 
    
    
     SUMMARY 
     The present disclosure is directed to a method and system for mass assembly of thin film materials. In one embodiment, sheets of a thin film material are attached to a carrier wafer. The carrier wafer and the attached sheets of thin film material are separated to form chiplet carriers. Each chiplet carrier includes a portion of the sheets of thin film material attached to a portion of the carrier wafer. The chiplet carriers are placed on an assembly surface in a random pattern. The chiplet carriers are arranged from the random pattern to a predetermined pattern, and the portions of the thin film material are transferred from the chiplet carriers to a target substrate. 
     In another embodiment, attaching sheets of thin film material to a carrier wafer that is separable to form a plurality of chiplets. The sheets of thin film material are optically mapped in relation to chiplets. The optical mapping determines an orientation of the portions of the sheets of thin film material relative to the chiplets. The carrier wafer and the attached sheets of thin film material are separated to form chiplet carriers having the portions of the sheets of thin film material attached the respective chiplets. The chiplet carriers are placed on an assembly surface in a random pattern and arranged to a predetermined pattern based on the optical mapping. Portions of the thin film material are transferred from the chiplet carriers to a target substrate. These and other features and aspects of various embodiments may be understood in view of the following detailed discussion and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The discussion below makes reference to the following figures, wherein the same reference number may be used to identify the similar/same component in multiple figures. The drawings are not necessarily to scale. 
         FIG.  1    is a flowchart of a method according to an example embodiment; 
         FIG.  2    is an isometric view showing thin film materials being characterized and separated according to an example embodiment; 
         FIG.  3    is an isometric view of an assembly surface on which chiplet carriers are arranged and oriented according to an example embodiment; 
         FIG.  4    is an isometric view of stacks of thin film material arranged on a target wafer according to an example embodiment; 
         FIGS.  5  and  6    are diagrams of thin film material assembly procedures according to example embodiments and 
         FIG.  7    is a flowchart of a method according to another example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to assembly of thin materials. Certain classes of materials, such as van der Waals materials (also referred to as 2-D materials), may formed as a sheet that can be as thin as one molecule in thickness. One well-known example of a van der Waals material is graphene, which is a single-molecule lattice of carbon atoms. Many van der Waals materials have uncommon properties (e.g., mechanical strength, low thermal and electrical resistance, etc.) and so there is interest in the development of devices that incorporate van der Waals materials. For purposes of this disclosure, the term ‘thin film material’ is used herein to describe any materials (including van der Waals materials) that are thin and delicate, such that the material is easily broken if conventional automated assembly processes are used. Such materials may have a thickness on the order of micrometers and sub-micrometers in some cases. 
     In order to mass-produce devices using thin film materials, automated systems are needed to, among other things, manufacture thin film material (e.g., via exfoliation or chemical layer deposition), divide the thin film material into individual units of the desired size, and move the units into a desired location and orientation, and then manipulate the units to form and assembly (e.g., stack thin film materials together and/or assemble with other components). Methods and apparatuses have been devised to separate the materials into individual units (referred to herein as ‘chiplets’) and move the chiplets into a desired location, e.g., a grid that allows the chiplets to be picked up for further processing. 
     The present disclosure relates to methods and apparatuses used to facilitate massive parallel assembly of thin film materials. In  FIG.  1   , a flowchart outlines a chiplet assembly process according to an example embodiment. The van der Waal (2-D) materials are turned into  100  sheets, e.g., via mechanical or chemical exfoliation, chemical-vapor deposition, etc. The sheets are attached  101  to the top of a chiplet carrier wafer. The sheets may be of irregular sizes and shapes, and so are characterized and recorded  102  with the unique ID of each chiplet carrier that will be formed from the wafer. The chiplets will then be singulated  103  (e.g., divided into regular shapes) into individual chiplet carriers. 
     After singulation  103 , the chiplet carriers are then dispersed  104  using a process compatible with thin film material and arranged into a pattern on an assembly surface. Examples dispersion processes are mechanical sonication or use of a solvent that does not damage the thin film material, and may be part of the singulation process  103 . The chiplets carriers will be used as manipulation handler to arrange the thin film material to the desired pattern and orientations. The thin film material will then be transferred  105  to the final substrate via a stamping process, e.g., using a tacky transfer substrate that separates and picks up a large number of the thin film material portions from the chiplet carriers. 
     In  FIG.  2   , an isometric view shows the assembly and characterization of sheets  202 - 205  of thin film material onto a carrier wafer  200  according to an example embodiment. The sheets  202 - 205 , are placed on one surface of the carrier wafer  200 , e.g., such that there is minimal or no overlap between the sheets  202 - 205 . The sheets  202 - 205  may be floated onto the wafer  200  by a fluid layer. The carrier wafer  200  may be made from Si, having an SiO 2  coating on the top. 
     A sensor  206  (e.g., visible light imager, electron imager, infrared imager, ultraviolet imager, atomic force imager, photoluminescent imager, etc.) and processor  208  map the location of the sheets  202 - 205  relative to the wafer  200 . The carrier wafer  200  will later be separated along the gridlines  202   a , e.g., using a dicing tool such as scriber/cleaver, mechanical saw, laser saw, etcher/grinder, etc. The gridlines  202   a  may be pre-etched or otherwise marked on the wafer  202 , or may be virtual, e.g., calculated by the processor  208  based on a predetermined singulation scheme. Note that other shapes besides a rectangular grid may be used to divide the carrier wafer  200  and sheets  202 - 205 , e.g., hexagonal grid, diamond grid, etc. Further, the grid need not be uniform, e.g., may have different regions with different sized grids. 
     The wafer  202  and sheets  202 - 205  will be separated together, as indicated by the dashed lines over sheet  205 . This will result in individual chiplet carriers  205   a - 205   c , each having a rectangular base formed of a chiplet  205   aa ,  205   ba ,  205   ca  (which is a portion of the carrier wafer  200 ) that is partially or fully covered by thin film material  205   ab ,  205   bb , and  205   cb . The size of individual chiplets carriers  205   a - 205   c  (e.g., average of edge dimensions A and B) may be on the order of micrometers to millimeters, although may be smaller or larger in some embodiments. 
     The characterization of the sheets by the camera  206  and processor  208  can uniquely identify each of the chiplet carriers  205   a - 205   c  that are formed from this and other parts of the carrier wafer  200  and sheets  202 - 205 . The processor  208  may give each chiplet carrier  205   a - 205   c  a unique identifier (ID) that can be used for subsequent operations. This unique identifier could be marked on the chiplet carriers  205   a - 205   c , e.g., by laser engraving a barcode on a top or side of the carrier, or by standard photolithography and etching process. 
     As seen in  FIG.  3   , chiplet carriers  300  may be transported to an assembly surface  302 , e.g., via a fluid carrier  304 . On the assembly surface  302 , the chiplet carriers  300  are selected, sorted, oriented and arranged. When first moved onto the assembly surface  302 , the chiplet carriers are generally disordered, e.g., randomly arranged. The assembly surface  302  is part of a micro assembler  301  that utilizes actuators  303 , e.g., electrodes, piezo actuators, magnets, etc., that cause the chiplet carriers  300  to be moved from a disordered (e.g., random) location and orientation at region  306  to ordered locations and orientations at region  308 . 
     The actuators  303  (which may be located elsewhere besides the assembly surface  302 ) may use any combination of electrostatic forces, magnetic forces, acoustics waves, physical forces, fluidic actuation, etc., to move and orient the chiplet carriers  300  to the ordered locations. As indicated by processor  314  and sensor  316 , the micro assembler  301  can automatically sense orientations of the chiplet carriers  300  and cause the actuators to gradually move the chiplet carriers  300  across the assembly surface  302  to the desired locations and orientations, e.g., as previously determined by processor  208  in  FIG.  2   . A description of various micro assembler embodiments can be found in U.S. Patent Publication US2002/0032US01 and U.S. patent application Ser. No. 16/221,803, filed Dec. 17, 2018, which are both hereby incorporated by reference. 
     The ordered locations in this example are a grid pattern that matches that of mildly adhesive protrusions  312  of a stamping head  310 . As seen in  FIG.  4   , the stamping head  310  has picked up one or more stacks of thin film materials, which are then deposited individually or together to form stacks  402  on a target device wafer  404 . Each of the stacks  402  may have differing orientations and/or compositions, as seen by close up views of stacks  402   a - c.    
     In  FIG.  5   , a diagram illustrates a sequence for forming a device using the chiplet carriers according to an example embodiment. A plurality of chiplet carriers  500  are each formed of a thin film material  502  on a chiplet  504 . The chiplet carriers  500  were previously arranged on an assembly surface  506 , e.g., as shown in  FIG.  3   . In this embodiment and as shown in block  510 , a transfer arm  508  has a tacky surface that picks up the chiplet carriers  500  from the assembly surface  506 . As seen in block  512 , the chiplet carriers  500  have been brought into contact with a stamp head  514  that include hemispherical protrusions  516 . Previously deposited portions of thin film material  518  are already assembled on the protrusions  518 , and the transfer arm  508  presses the currently deposited thin film material  502  onto the previously deposited portions of thin film material  518 . The protrusions  516  may be made of a viscoelastic, silicon-based, organic polymer such as polydimethylsiloxane (PDMS). 
     Generally, the steps shown in block  510  and  512  can be repeated for a number of steps. In one arrangement, when the newly added material  502  is moved over the protrusions  516 , the ambient temperature may be elevated, e.g., around 45° C. After the material  502  contacts the protrusions  516  (or any intervening material  518 ), the temperature is rapidly cooled, e.g., to around 39° C. After cooling, the stamp head  514  will be separated from transfer arm using a smooth, swift motion. 
     As seen in block  520 , the thin film material stacks  502 ,  518  are built on the array of protrusions  516 . At this stage, the stacks  502 ,  518  are ready to be pressed into a target substrate  522  (e.g., intermediate or final substrate). Before being pressed together, stacks  502 ,  518  and substrate  522  are the previous temperature from block  512 , e.g., around 39° C. As seen in block  524 , the stacks  502 ,  518  are pressed together with the target substrate  522 . After the pressing together, the ambient temperature is increased, e.g., to around 135° C., to facilitate separating the stacks  502 ,  518  from the protrusions  516 . As seen in block  526 , the stamp head  514  is slowly removed, leaving the material stacks  502 ,  518  behind on the target substrate  522 . The target substrate can then be further processed, e.g., depositing additional material layers, etching  502 ,  518  the stacks and additional materials, etc. 
     In  FIG.  6   , a diagram illustrates a sequence for forming a device using the chiplet carriers according to an example embodiment. A plurality of chiplet carriers  600  are formed on an assembly surface  606 . Each chiplet carrier  600  includes a thin film material  602  on a chiplet  604 . In this embodiment, a stamp head  614  that includes hemispherical protrusions  616  is brought into contact with the chiplet carriers  600  removing the chiplet carriers  600  from the assembly surface. As seen in block  612 , the chiplets  604  are brought into contact with a tacky removal substrate  608  that facilitates removing the chiplets  604  from the thin film material  602 , the latter remaining on the stamp head  614 . The operations in block  612  may be performed at a somewhat low temperature, e.g., about 39° C., to help ensure the thin film material portions  602  do not separate from the protrusions  616 . 
     The operations in blocks  610  and  612  can be repeated multiple times, resulting in a stack of thin film materials  602 ,  618  on the stamp head  614  as seen in block  620 . At block  620  the stacks  602 ,  618  are ready to be pressed into a target substrate  622  (e.g., intermediate or final substrate). Before being pressed together, stacks  602 ,  618  and substrates  614 ,  622  may be at the previous temperature from block  612 , e.g., 39° C. As seen in block  624 , the stacks  602 ,  618  are pressed together with the target substrate  622 . After the pressing together, the ambient temperature is increased, e.g., to 136° C., to facilitate separating the stacks  602 ,  618  from the protrusions  616 . As seen in block  626 , the stamp head  614  is slowly removed, leaving the material stacks  602 ,  618  behind on the target substrate  622 . The target substrate can then be further processed, e.g., depositing additional material layers, etching  602 ,  618  the stacks and additional materials, etc. 
     The systems and methods above can facilitate high-throughput assembly of devices that utilize thin film materials. Large numbers (e.g., on the order of 1000&#39;s) of stacks/device can be assembled together. High throughput enables repeatable and consistent manufacturing. The methods and systems described herein can be used with other parallel assembly techniques, e.g., robotic pick and place, lithography, etc. Some processes may be modified, e.g., to use robotic pick-and-place machines on some aspects of the assembly. Using chiplet as a thin film material handler allows the specialized, sensitive 2-D process be detached from the other conventional chip assembly processes. The 2-D assembly processes described herein can be generalized beyond thin film material assembly. For example, small objects such as quantum dots, quantum wires, nano crystals, etc., can be carried with sacrificial chiplets and parallel-assembled in a similar manner as thin film materials. 
     In  FIG.  7   , a flowchart shows a method according to an example embodiment. The method involves attaching  700  sheets of thin film material to a carrier wafer. The carrier wafer and the attached sheets of thin film material are separated  701  to form chiplet carriers. Each chiplet carrier includes a portion of the sheets of thin film material attached to a portion of the carrier wafer. The chiplet carriers are arranged  702  on an assembly surface from a disordered pattern to a predetermined pattern. 
     Operations  703 - 706  are an example of transferring the portions of the thin film material from the chiplet carriers in parallel to a target substrate. A stamp is pressed  703  against the chiplet carriers on the assembly surface. The stamp has an array of mildly adhesive protrusions in the predetermined pattern so that each of the protrusion contacts the portion of thin film material. The stamp is separated  704  from the chiplet carriers so that the portions of the thin film material are removed from the chiplets by the protrusions. As indicated by block  706 , steps  703 - 704  can be optionally repeated for multiple layers of thin film material. After all layers are on the stamp, the stamp is pressed  705  onto a target substrate such that the thin film material is deposited on the target substrate. 
     Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein. The use of numerical ranges by endpoints includes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within that range. 
     The foregoing description of the example embodiments has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. Any or all features of the disclosed embodiments can be applied individually or in any combination are not meant to be limiting, but purely illustrative. It is intended that the scope of the invention be limited not with this detailed description, but rather determined by the claims appended hereto.