Patent Publication Number: US-2022236557-A1

Title: Positional error compensation in assembly of discrete components by adjustment of optical system characteristics

Description:
CLAIM OF PRIORITY 
     This application claims priority to U.S. Patent Application Ser. No. 62/859,830, filed on Jun. 11, 2019, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     This description relates generally to assembling discrete components onto a substrate. 
     SUMMARY 
     In an aspect, an assembly includes a substrate; a coating including a Bingham fluid disposed on a surface of the substrate; and a discrete component partially embedded in or disposed on the coating including the Bingham fluid. 
     Embodiments can include any one or more of the following features. 
     The Bingham fluid includes one or more of a flux, a solder paste, a conductive ink, a semiconducting ink, a gel, and a chemically inert material. 
     The discrete component includes a light emitting diode (LED). 
     The substrate includes a semiconductor wafer. The substrate includes a tape. The substrate includes a rigid substrate. The substrate includes a printed circuit board. 
     In an aspect, an assembly includes a substrate; and a coating including a Bingham fluid disposed on the substrate, a first surface of the coating contacting the substrate, in which the coating is configured such that when a discrete component is incident on a second surface of the coating opposite the first surface, the discrete component is disposed on or partially embedded in the coating. 
     Embodiments can include any one or more of the following features. 
     The Bingham fluid includes one or more of a flux, a solder paste, conductive ink, a semiconducting ink, a gel, and a chemically inert material. The Bingham fluid includes a conductive paste. 
     The substrate includes a printed circuit board. 
     In an aspect, a method includes irradiating a dynamic release structure disposed on a carrier, in which a discrete component is adhered to the dynamic release structure, the irradiating causing the discrete component to be released from the carrier; and receiving the released discrete component into or onto a coating disposed on a surface of a substrate, the coating including a Bingham fluid. 
     Embodiments can include any one or more of the following features. 
     Irradiating the dynamic release layer includes irradiating the dynamic release layer with laser energy. 
     The irradiating induces ablation of at least a portion of the dynamic release layer. 
     The method includes reducing an adhesion of the dynamic release layer prior to the irradiating. 
     Irradiating a dynamic release layer includes irradiating the dynamic release layer through the carrier. 
     The discrete component includes an LED. 
     The second substrate includes a tape. The second substrate includes a printed circuit board. 
     In an aspect, a method includes forming a coating on a surface of a target substrate, the coating including a Bingham fluid; and receiving a discrete component into or onto the coating, the discrete component having been transferred from a carrier by a laser-assisted transfer process. 
     In an embodiment, forming a coating including a Bingham fluid includes disposing one or more of a flux, a solder paste, conductive ink, a semiconducting ink, a gel, and a chemically inert material onto the surface of the target substrate. 
     In an aspect, a method includes determining an alignment error between a discrete component of a discrete component assembly mounted in a laser-assisted transfer system and a target position on a target substrate, the discrete component assembly including the discrete component adhered to a support by a dynamic release layer; based on the alignment error, determining a beam offset characteristic; and providing a signal indicative of the beam offset characteristic to an optical element of the laser-assisted transfer system, the optical element being configured to adjust a position of a beam pattern relative to the discrete component according to the beam offset characteristic. 
     Embodiments can include any one or more of the following features. 
     The method includes by the optical element, adjusting the position of the beam pattern relative to the discrete component according to the beam offset characteristic. The method includes adjusting the position of the beam pattern using a galvanometer laser scanner. 
     Determining an alignment error includes determining one or more of a magnitude of the alignment error and a direction of the alignment error. 
     Determining a beam offset characteristic includes determining an offset between a center of the discrete component and an offset position on the discrete component. Determining an offset includes determining one or more of a magnitude and a direction of the offset between the center of the discrete component and the offset position on the discrete component. 
     The method includes determining a beam offset characteristic to at least partially compensate for the alignment error. The method includes determining a beam offset characteristic such that when the beam pattern is incident on the discrete component assembly according to the beam offset characteristic, the discrete component is transferred to the target position on the substrate. 
     The method includes determining the alignment error using a machine vision system. 
     The method includes irradiating the dynamic release layer with the beam, the position of the beam pattern relative to the discrete component having been adjusted according to the beam offset characteristic, the irradiation causing the discrete component to be released from the carrier substrate. 
     The method includes receiving the discrete component at the target position on the target substrate. 
     In an aspect, a system includes a computing system including one or more processors coupled to a memory and configured to determine an alignment error between a discrete component of a discrete component assembly and a target position on a target substrate, the discrete component assembly including the discrete component adhered to a support by a dynamic release layer, the discrete component assembly being mounted in a laser-assisted transfer system; based on the alignment error, determine a beam offset characteristic; and provide a signal indicative of the beam offset characteristic to an optical element of the laser-assisted transfer system, the optical element being configured to adjust a position of a beam pattern relative to the discrete component according to the beam offset characteristic. 
     Embodiments can include any one or more of the following features. 
     The system includes the optical element. The optical element includes a galvanometer laser scanner. 
     Determining a beam offset characteristic includes determining an offset between a center of the discrete component and an offset position on the discrete component. Determining an offset includes determining one or more of a magnitude and a direction of the offset between the center of the discrete component and the offset position on the discrete component. 
     The one or more processors and memory are configured to determine a beam offset characteristic to at least partially compensate for the alignment error. The one or more processors and memory are configured to determine a beam offset characteristic such that when the beam pattern is incident on the discrete component assembly according to the beam offset characteristic, the discrete component is transferred to the target position on the substrate. 
     The one or more processors and memory implement a machine vision system. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are diagrams of a laser-assisted transfer process. 
         FIGS. 2A-2C  are diagrams of a laser-assisted transfer process. 
         FIG. 3  is a diagram of a system for laser-assisted transfer. 
         FIG. 4  is a diagram of a discrete component and a target substrate. 
         FIGS. 5A-5C  are diagrams of a laser-assisted transfer process. 
     
    
    
     DETAILED DESCRIPTION 
     We describe here approaches to address positional error in laser-assisted processes for transfer of discrete components from supports to target substrates. In some examples, a beam pattern incident on a discrete component assembly can be offset relative to a discrete component to be transferred to a target substrate. The offset of the beam pattern results in a placement error when the discrete component is transferred to the target substrate. The offset of the beam pattern can be tuned such that the placement error compensates for an alignment error between the target substrate and the discrete component assembly, thereby improving precision of the laser-assisted transfer process. The offset of the beam pattern can be performed by optical elements, providing rapid adjustment and enabling high throughput transfer to be achieved. 
       FIG. 1A  depicts a laser-assisted transfer process for high-throughput, low-cost contactless assembly of discrete components  102  onto a substrate  130 . The term discrete component refers generally to, for example, any unit that is to become part of a product or electronic device, for example, electronic, electromechanical, photovoltaic, photonic, or optoelectronic components, modules, or systems, for example any semiconductor material having a circuit formed on a portion of the semiconducting material. In some examples, the discrete components can be light emitting diodes (LEDs). The discrete components can be ultra-thin, meaning having a maximum thickness of 100 μm or less, 50 μm or less, 40 μm or less, 30 μm or less, 25 μm or less, 20 μm or less, 10 μm or less, or 5 μm or less. The discrete components can be ultra-small, meaning having a maximum length or width dimension less than or equal to 300 μm per side, 100 μm per side, 50 μm per side, 20 μm per side, or 10 μm per side. The discrete components can be both ultra-thin and ultra-small. 
     In the laser-assisted transfer process, a discrete component assembly  108  is positioned in a component transfer system  100 . The discrete component assembly  108  includes a support  110 , with a dynamic release structure  112  disposed on a front side of the support  110 . Discrete components  102  are adhered to the support  110  by the dynamic release structure  112 . The support  110  can be a rigid support, such as a glass or rigid polymer substrate; or a flexible support, such as a tape. The dynamic release structure  112  can be a single layer structure or a multi-layer structure, such as a structure having two, three, four, or more than four layers. 
     In the laser-assisted transfer process, a back side of the support  110  is irradiated with radiation, such as laser light, e.g., a beam pattern  116 . The beam pattern  116  can be a single beam (as shown) or a pattern of multiple beams, e.g., from a single light source or from multiple light sources. The support  110  is transparent to the wavelength of the radiation (e.g., the laser energy). An element that is transparent to a given wavelength is an element for which at least some radiation of the given wavelength passes through the element. The radiation passes through the support  110  and is incident on an area of the dynamic release structure  112 , causing ablation of at least a partial thickness of the dynamic release layer in the area on which the laser beam pattern  116  is incident (which we refer to as the irradiated area). The ablation generates confined gas, which expands, generating a stress in the dynamic release structure  112 . The stress causes at least some of the material of the dynamic release structure  112  to deform, forming one or more blisters  118 . For instance, when the beam pattern  116  is a single beam, or a pattern of multiple, closely spaced beams, a single blister  118  is formed. When the beam pattern  116  includes multiple beams that are sufficiently spaced apart, a pattern of multiple blisters  118  is formed. Here, the term blister  118  refers generically to either a single blister or a pattern of multiple blisters. 
     The blister  118  exerts a mechanical force on the discrete component  102 . When the mechanical force exerted by the blister  118  is sufficient to overcome the adhesion between the discrete component  102  and the dynamic release structure  112 , the mechanical force exerted by the blister  118  (in combination with gravity) propels the discrete component away from the support  110  (e.g., in a downward direction) for transfer to a target substrate  130 . 
     Further description of laser-assisted transfer processes can be found in U.S. Patent Publication No. US 2014/0238592, the contents of which are incorporated here by reference in their entirety. 
     A transfer vector  150  indicates the direction of travel of the discrete component  102  upon its release from the support  110  and points to the ultimate placement of the discrete component  102  on the target substrate  130 . The direction of the transfer vector  150  is a function of the position of the laser beam pattern  116  with respect to the discrete component  102 . When the incident laser beam pattern  116  is centered with respect to the discrete component  102 , as shown in  FIGS. 1A and 1B , the resulting blister  118  (e.g., the single blister or the pattern of multiple blisters) is also centered with respect to the discrete component  102 , and the transfer vector  150  is normal to the surface of the dynamic release structure  112  (referred to as a normal transfer vector). As a result, the discrete component  102  lands on the target substrate  130  at a position directly below the position of the discrete component  102  in the discrete component assembly  108 . 
     An objective of the laser-assisted transfer process can be to transfer the discrete component  102  to a specific target position  140  on the target substrate  130 . The target position  140  can be the position, on the target substrate  130 , at which the center of the discrete component  102  is intended to be positioned. For instance, when the target substrate  130  is a device substrate, such as a printed circuit board, the target position  140  can be a position on the target substrate  130  at which interconnection elements, such as bond pads, will be aligned with interconnection elements on the discrete component  102 . 
     To align the target position  140  on the target substrate  130  with the position of the discrete component  102  in the discrete component assembly  108 , the target substrate  130  can be moved relative to the discrete component assembly  108 , or the discrete component assembly  108  can be moved relative to the target substrate  130 , or both. The relative movement can be implemented using a mechanical adjustment mechanism such as a linear translation stage, an actuator, or another suitable mechanical adjustment device. For instance, the coordinates, in a global x-y coordinate system, of the center of the discrete component  102  in the discrete component assembly  108  can be adjusted to match the global x-y coordinates of the target position. 
     The mechanical adjustment of the relative positioning of the discrete component assembly  108  and the target substrate  130  can sometimes be subject to error, such as deviations along the x-axis, the y-axis, or both, e.g., resulting from imprecision in the mechanical adjustment device. Such error can result in an alignment error between the x-y coordinates of the discrete component  102  and the x-y coordinates of the target position  140 . Other sources of error can also contribute to an alignment error between the x-y coordinates of the discrete component  102  and the x-y coordinates of the target position  140 , e.g., in addition to or instead of errors due to the mechanical adjustment. For instance, a difference between the expected position of the discrete component  102  in the discrete component assembly  108  and the actual position of the discrete component  102  in the discrete component assembly  108  can arise during prior processing, such as during singulation of the discrete components  102  or the placement of the discrete components  102  onto the support  110 . If the x-y coordinate alignment between the target position  140  and the position of the discrete component  102  in the discrete component assembly  108  is based on the expected position of the discrete component  108 , this difference resulting from prior processing can result in an alignment error between the x-y coordinates of the discrete component  102  and the x-y coordinates of the target position  140 . In some examples, such alignment errors can be at least partially addressed by fine mechanical adjustment of the relative positions of the target substrate  130  and the discrete component assembly  108  prior to initiating the laser-assisted transfer process. 
     Referring to  FIGS. 2A-2C , in some examples, the beam pattern  116  can be offset from the center of the discrete component  102  during the laser-assisted transfer process, meaning that the beam pattern  116  is incident at a position that is offset from a center of the discrete component. The offset of the beam pattern  116  can be used to at least partially compensate for the alignment error between the x-y coordinates of the discrete component  102  and the x-y coordinates of the target position  140 . 
     When the beam pattern  116  is offset from (e.g., not centered on) the discrete component  102 , the resulting blister  118  (e.g., the single blister or the pattern of multiple blisters) is also not centered with respect to the discrete component  102 . This offset causes the transfer vector  150  to diverge from the surface normal by an angle α (referred to as an angled transfer vector). The angle α of the transfer vector  150  is related to the magnitude d and direction of the offset of the beam pattern  116  (sometimes referred to as the beam offset) from the center of the discrete component  102 . With a normal transfer vector (e.g., as shown in  FIG. 1A ), the discrete component  102  would be transferred to the target position  140  that is directly below the position of the discrete component  102  in the discrete component assembly. The angled transfer vector causes the discrete component  102  to be transferred to an offset position  134  on the target substrate  130  that is shifted relative to the target position  140 . 
       FIGS. 2B and 2C  show top views of the discrete component assembly  108  with the attached discrete component  102 , with the beam pattern  116  being offset from the center of the discrete component  102 . The dashed boxes  132  depict the position of the discrete component  102  on the target substrate  130  following the transfer, with the dashed boxes being centered on the offset position  134 . The distance between the target position  140  and the offset position  134  is the placement error c induced by the offset of the beam pattern  116 . The placement error c can be along the x-axis, as shown in  FIG. 2B ; along the y-axis (not shown); can have a component along each of the x-axis and the y-axis, as shown in  FIG. 2C . 
     The magnitude of the placement error c is related to the magnitude d of the beam offset and the height g of a gap  135  between a bottom surface of the discrete component  102  in the discrete component assembly  108  and a top surface of the target substrate  130 . For instance, the magnitude d of the beam offset can have a direct linear relationship with the placement error c given the gap height g. The spatial direction in which the transfer vector  150  points is opposite to the direction of the beam offset. For instance, in  FIG. 2A , the beam pattern  116  is offset to the left of the figure while the transfer vector  150  points toward the right of the figure. 
     As discussed above, an alignment error can occur between the x-y coordinates of the discrete component  102  and the x-y coordinates of the target position  140 . To compensate for such error, the beam pattern  116  can be purposely offset such that the placement error c matches the alignment error between the x-y coordinates of the discrete component  102  and the x-y coordinates of the target position  140 . Compensating for alignment error by introducing an offset to the beam pattern  116  can help to achieve precise alignment for the laser-assisted transfer process, and can enable rapid achievement of the alignment, thereby facilitating high throughput transfer processes. 
     In an example, referring to  FIG. 3 , an alignment error a (not shown) may be present between the x-y coordinates of the discrete component  102  and the x-y coordinates of the target position  140  in a laser-assisted transfer system  300 . The alignment error a can be due to, e.g., imprecision in the mechanical alignment mechanism, positioning error of the discrete component  102  onto the support  110 , or other reasons. To compensate for the alignment error, the beam pattern  116  can be offset to give rise to an angled transfer vector  150  that results in a placement error c equal to the alignment error a. The discrete component  102  is then transferred directly to its target position  140 , despite the alignment error a between the x-y coordinates of the discrete component  102  in the discrete component assembly  108  and the x-y coordinates of the target position  140 . 
     The laser-assisted transfer system  300  includes a light source  302 , such as a laser. A light beam  306  from the light source  302  can be incident on one or more optical elements  304 , such as lenses, diffractive optical elements (e.g., beam splitters), that can form the light beam  306  from the light source  302  into the beam pattern  116 . The beam pattern  116  can be a single beam of light (as shown) or can include multiple beamlets, which are beams of light that have a smaller size (e.g., diameter) than the light beam  306 . 
     A light detector  310 , such as a camera, is positioned to capture images of the relative positions of the discrete component  102  and the target position  140  on the target substrate  130 . Images  312  from the light detector  310  are provided to a computing system  314  including one or more processors coupled to a memory and configured to process the images  312  to determine the alignment error a. For instance, the computing system  312  can implement a machine vision approach to identify the discrete component  102  and the target position  140  and to determine the magnitude and direction of the separation between a center of the discrete component  102  and the target position  140 . 
     Based on the alignment error a, the computing system  312  determines a beam offset characteristic for the beam pattern  116  such that when the beam pattern  116  is offset by the beam offset characteristic, the discrete component  102  will be transferred to the target position  140 . The beam offset characteristic indicates the magnitude d of the beam offset and the direction of the beam offset. In some examples, the beam offset characteristic can indicate other parameters, such as a number or arrangement of beamlets in the beam pattern  116 . 
     The computing system  314  provides a signal  316  indicative of the beam offset characteristic to the laser-assisted transfer system  300 , e.g., to control one or more optical elements  318  to adjust the position of the beam pattern  116  according to the beam offset characteristic. In an example, the one or more optical elements  318  can be a scanner, such as a galvanometer laser scanner, that can adjust the position of the beam pattern  116 . 
     By implementing the beam offset characteristic by a scanner, compensation for the alignment error can be achieved quickly, enabling the transfer time per discrete component to be low and thus increasing the throughput of the laser-assisted transfer process. 
     Referring to  FIG. 4 , in some examples, a target substrate  430  that receives a discrete component  102  in a laser-assisted transfer process can include a coating including a die catching material  402  disposed on its surface. The die catching material (DCM)  402  can be a material that receives discrete components as they are transferred from the carrier substrate and keeps them in their targeted positions while reducing post-transfer movement of the discrete components on the target substrate. We sometime refer to the process of receiving of discrete components and keeping them in their targeted positions as catching the discrete components. The DCM can be a viscous fluid that slows the speed of the discrete component  102  when the discrete component  102  is incident on the coating  402 . 
     In some cases, once a discrete component  102  has been caught by the DCM of the coating  402 , the discrete component  102  can “float” in the coating. Floating of a discrete component  102  is the slow motion shifting of the discrete component  102  from its original placement position by an arbitrary distance. Floating can be caused by surface tension, thermal drift, leveling of the fluid after component impact, Brownian motion, or other factors, or a combination of multiple factors. Floating of a received discrete component can be undesirable, e.g., in that floating can contribute to overall placement error in the transfer process. For instance, component floating may cause the placement error to exceed established criteria thereby rendering the placed discrete component unusable for its intended application. In some examples, such as in applications involving tightly spaced discrete components, component floating may cause an incoming discrete component to make contact with a previously placed discrete component, thereby adding further placement error to the previously placed discrete component or creating other undesirable characteristics in the set of discrete components. 
     In some examples, the rheological properties of the DCM of the coating  402  can be tuned for a specific application, e.g., for a discrete component of a specific size or mass, to improve the accuracy and reliability of the laser-assisted transfer process. 
     Referring to  FIGS. 5A and 5B , in some examples, a target substrate  530  includes a coating  502  including a Bingham fluid disposed on a surface of a substrate  504 . A Bingham fluid is a viscoplastic material that behaves as a rigid body at low stress but flows as a viscous fluid at and above its yield stress. Bingham fluids are also sometimes referred to as Bingham liquids, Bingham plastics, structured liquids, or viscoplastic solids. The Bingham fluid can include a gel (e.g., alcogels, hydrogels, organogels, or other types of gels), a synthetic polymer, a tacky paste, a flux, a solder paste, a suspension, a conductive or a semiconductive ink (e.g., an organic ink), a chemically inert material, or a combination of any two or more of them. 
     When a discrete component  102  is transferred to the target substrate  530  in a laser-assisted transfer process, the discrete component  102  is caught by the coating  502  including the Bingham fluid and is partially embedded into the coating (as shown in  FIG. 5B ) or disposed on top of the coating. By partially embedded, we mean that the bottom surface  136  of the discrete component  102  is below a top surface  506  of the coating  502 , while a top surface  508  of the discrete component  102  is above the top surface  506  of the coating  502 . Once the discrete component  102  is partially embedded in the coating  502 , the rheological characteristics of the Bingham fluid prevent the discrete component  102  from spontaneously shifting its position or floating. The component placement precision and accuracy can thus be enhanced through the use of a coating including a Bingham fluid as a die catching material. 
     The substrate  504  can be a tape, such as a wafer tape. The substrate  504  can be a rigid substrate, such as a glass substrate or a semiconductor substrate. The substrate  504  can be a device substrate such as a printed circuit board (PCB). In some examples, the substrate  504  can be a semiconductor substrate, such as a semiconductor wafer or portion thereof, intended for use in multi-chip stacks. 
     When the substrate  504  is a device substrate, the coating  502  including a Bingham fluid can facilitate interconnection of the discrete component  102  to electrical contacts, such as bond pads, on the substrate  504 . For instance, the coating  502  including the Bingham fluid can be a conductive paste, such as a conductive solder paste. Referring to  FIG. 5C , in some examples, a processing step can be carried out after the laser-assisted transfer of the discrete component  102  to bring the bottom surface  136  of the discrete component  102  into contact with electrical contacts on the substrate  504 . In an example, the coating  502  including the Bingham fluid can be a solder flux that is removed during a soldering process. In an example, the assembly of the substrate  504  with the discrete component  102  partially embedded into the coating  502  is subjected to pressure, e.g., from a thermode, which squeezes out the Bingham fluid from between the discrete component  102  and the substrate  504 , thereby enabling electrical contact between the discrete component  102  and the substrate  504 . 
     A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, some of the steps described above may be order independent, and thus can be performed in an order different from that described. 
     Other implementations are also within the scope of the following claims.