Patent Publication Number: US-2022223483-A1

Title: An alignment process for the transfer setup

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of and priority to U.S. Provisional Patent Application No. 62/930,982, filed Nov. 5, 2019 and U.S. Provisional Patent Application No. 62/851,189, filed May 22, 2019, each of which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND AND FIELD OF THE INVENTION 
     The present disclosure relates to an alignment process between a first substrate and the second substrate. In particular, the present invention is directed to an electrical alignment process in combination with optical alignment between the substrates. 
     Many micro devices including light emitting diodes (LEDs), organic LEDs (OLEDs), sensors, solid state devices, integrated circuits, microelectromechanical systems (MEMS), and other electronic components are typically fabricated in batches, often on planar substrates. To form an operational system, micro devices from at least one donor substrate need to be selectively aligned and bonded to a receiver substrate. Various alignment techniques may be employed for wafers to be aligned including an optical alignment. 
     The challenge of alignment over large area substrates is more complicated as some of the techniques used in small substrates cannot be directly transferred to such applications. 
     SUMMARY 
     An object of the present invention is to overcome the shortcomings of the prior art and to replace or enhance conventional optical alignment processes. 
     According to one embodiment, an electrical alignment process may be provided for aligning a first (or temporary) substrate and a second substrate. The method comprises: positioning the first substrate having at least a first alignment mark in close proximity to the second substrate having at least a second alignment mark, measuring an alignment value between the first and second alignment marks of both the first and second substrate; and adjusting the position of the first substrate and the second substrate based on the measured alignment value. 
     In one case, the substrates can be a backplane, an intermediate substrate, a donor substrate, mask, or other form of substrates. In this disclosure, for example, the first substrate may comprise a donor substrate and the second substrate may comprise a receiver substrate. 
     In one case, the alignment marks on the donor substrate and the alignment marks on the receiver substrate facilitates the measurement of an alignment value. The position of the two substrates may be adjusted to either maximize, minimize or equalize the alignment value for each alignment mark. 
     In one embodiment, the alignment marks can be plates forming capacitance if brought close together. In this case, the alignment value (or inductance) between the alignment plates on the donor and the alignment plates on the receiver substrate is maximized. In another case, either substrate may be moved to maximize the capacitance (or other values) between the plates. 
     In another embodiment, the alignment marks can be charged trapped on the surface of one substrate and a plate on the surface of the other substrate. 
     In some embodiments, the alignment marks can be electrical signals on one surface of one substrate and receiver on the surface of the other substrate. 
     In yet another embodiment, the alignment marks can be shield plate on one substrate and receiver plate on the other substrate. The shield plate blocks the signals from a source to reach the receiver substrate. The shield plates are used as alignment marks on one of the substrates blocking between the source and collector plates on other substrates. In this case, the alignment value can be minimized to make the two substrates aligned. 
     In another case, the value is equalized between few alignment corresponding alignment marks between the donor substrate and the receiver substrate. 
     In one embodiment, the plates can be conductive or charge layers. In another embodiment, there can be multiple plates in different orientations forming the alignment mark. 
     In one embodiment, there can be multiple (e.g. four) plates on one substrate and at least one plate on the other substrate. In one case, the value (e.g. capacitance or charge reading) for the multiple plates and the at least one plate on the other substrate is equalized by moving either substrate. 
     In another case, the value between multiple alignment marks (or different sections of a single alignment mark) is equalized. 
     In one embodiment, the electrical alignment mark can be also used as an initial optical alignment mark. 
     The foregoing and additional aspects and embodiments of the present disclosure will be apparent to those of ordinary skill in the art in view of the detailed description of various embodiments and/or aspects, which are made with reference to the drawings, a brief description of which is provided next. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other advantages of the disclosure will become apparent upon reading the following detailed description and upon reference to the drawings. 
         FIG. 1A  shows a series of steps of an alignment process of two wafers in accordance with an embodiment of the present invention. 
         FIG. 1B  shows a perspective view of an example alignment process in accordance with an embodiment of the present invention. 
         FIG. 2A  shows a perspective view of an example alignment process in accordance with an embodiment of the present invention. 
         FIG. 2B  shows a perspective view of an example alignment process in accordance with an embodiment of the present invention. 
         FIG. 3A  shows detector and source signal on one substrate and transfer medium on another substrate. 
         FIG. 3B  shows two detectors and source signal on one substrate and transfer mediums on another substrate. 
     
    
    
     Use of the same reference numbers in different figures indicates similar or identical elements. 
     While the present disclosure is susceptible to various modifications and alternative forms, specific embodiments or implementations have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the disclosure is not intended to be limited to the particular forms disclosed. Rather, the disclosure covers all modifications, equivalents, and alternatives falling within the spirit of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. 
     In LED display applications, where display pixels are single device LEDs, each LED is bonded to a driving circuit which controls the current flowing into the LED device. Here, the driving circuit may be a thin film transistor (TFT) backplane, a system substrate or a display wafer conventionally used in LCD or OLED display panels. Due to the typical pixel sizes (10-50 μm), bonding may be performed at a wafer-level scale. In this scheme, an LED wafer consists of isolated individual LED devices that are transferred and aligned, and bonded to a backplane or a display wafer which is compatible with an LED wafer pixel sizes and pitches. Various alignment techniques may be employed for wafers to be aligned including a conventional optical alignment. 
     The present invention provides an electrical alignment process to replace or enhance the conventional optical alignment process. The electrical alignment process of this disclosure measurement is done through measurement of an alignment value between the substrates. The alignment value may comprise a capacitance value, inductance value, charge value, or another electrical signal value between the substrates. In this case, the alignment marks on the donor substrate and the alignment marks on the receiver substrate may facilitate the measurement of the alignment value. The substrate can be a backplane, an intermediate substrate, a donor substrate, mask, or other form of substrates. 
       FIG. 1A  shows a series of steps of an alignment process between the first substrate and second substrate with an embodiment of the present invention. In one example, the first substrate may be a display substrate/receiver substrate and the second substrate may comprise a cartridge/donor wafer or a mask. The method  100  comprises steps that may be completed in any particular order to achieve a desired state. Here, the first substrate (e.g., a receiver substrate) and the second substrate (e.g. a donor substrate) may be provided. In step  102 , in an initial step of aligning the substrates, the means of alignment (e.g. optical) to course align the donor substrate and the receiver substrate may be used. In the next step  104 , electrical or magnetic alignment using electric signal/parameter (or magnetic signal) may be used for fine alignment. The alignment can be achieved by maximizing the alignment value that can be an electrical signal (e.g. capacitance), or minimizing the electrical signal (e.g. by shielding the signal generated by a source) or equalizing the electrical signals (e.g. where multiple detectors measures the alignment signal associated with multiple alignment marks or different part of one alignment mark). In another case, the alignment value can be magnetic signal. Here, the alignment can be achieved by maximizing the magnetic signal (e.g. induction) or minimizing a magnetic signal (e.g. by shielding the magnetic signal from a source) or equalizing the magnetic signals (e.g. multiple magnetic sources or detectors). Therefore, there can be different type of alignment marks as follow
         1) The alignment marks that create electrical or magnetic components such as capacitance or inductance components. In this case the absolute value of the signal at each moment is measured as alignment value.   2) The alignment marks that shield an electrical signal. In this case the effect of shielding at each moment is measured as the alignment value.
 
Multiple of each alignment mark can be used to enhance the alignment process.
       

       FIG. 1B  shows a perspective view of an example alignment process in accordance with an embodiment of the present invention. Here, a first substrate  120  and a second substrate  110  may be provided. There are alignment marks ( 110 - a ,  120 - a ) provided on the substrates. In this case, the first substrate has alignment marks  120 - a  and the second substrate has alignment mark  120 - a  that facilitates the measurement of the alignment value. 
     Alignment Process 
     For faster and more accurate alignment, several alignment marks described before (electrical/magnetic components or electrical/magnetic shielding) can be used across the two substrates for faster alignment value associated with these alignment marks. The alignment can be done based on the absolute alignment value associated with each alignment mark or the rate of change in the alignment values. 
     In one case, the substrates move in respect to each other in different directions (e.g. up, down, left, right) and then, the effect of the moving is measured on each alignment value associated with different alignment marks across the substrates. Based on that, the position of the two substrates may be adjusted to either maximize, minimize, or equalize the alignment values for each alignment mark based on the type of alignment marks. This process can be repeated to achieve a change in the alignment values less than a threshold value. 
     In another case, the alignment value for each alignment mark is extracted at the current position of the two substrates in respect to each other. Based on the values, the misalignment is calculated, and the substrates are positioned based on the calculated misalignment value. This process can be repeated to achieve a change in the alignment values less than a threshold. 
     Alignment Marks 
     In one embodiment, different alignment marks can be used for the aforementioned process. 
     In one case, the alignment marks on the donor substrate and receiver substrate can be metal plates forming capacitance if brought close together. Since the capacitance is mainly dominated by the overlap area between the two plates, the misalignment between the plates will affect the capacitance value which can be extracted as alignment values. 
     In another case, the alignment mark may include trapped charge on the surface of one substrate (e.g., donor substrate) or magnetic material on one of the substrates. The charge (or magnetic field) can be read by an electrode on the other substrate. The electrode can be a plate on the surface of the other substrate (e.g., receiver substrate). The effect of charge on the electrode is a function of the overlap area. Therefore, the detected charge effect can be used as an alignment value. In this case, the alignment value between the alignment plates on the donor and the alignment plates on the receiver substrate is maximized. 
     In one embodiment, the alignment marks can be electrical signals on one surface and receiver on another surface. 
     In another embodiment, the alignment mark can be a shield plate on one substrate and receiver plate on the other substrate. The shield plate blocks the signals from a source to reach the receiver substrate. The shield plates are used as alignment marks on one of the substrates blocking between the source and collector plates on other substrates. In this case, the alignment value can be minimized to make the two substrates aligned. 
       FIG. 2A  shows a perspective view of an example alignment process for an LED transfer machine in accordance with an embodiment of the present invention. Here, the donor substrate and receiver substrates may be placed on the donor alignment plate  202  and the receiver alignment plate  204 , respectively. 
     In one case, the plates  202 ,  204  may be metal plates. These metal plates may be brought close together to form capacitance. These metal plates may be moved to maximize the capacitance (or other electrical values) between the two plates. The plates can be optically aligned initially. Since the capacitance is mainly dominated by the overlap area between the two plates, the misalignment between the plates will affect the capacitance value which can be extracted as alignment values. In case of a low value of capacitance, the position of plates can be shifted to maximize the value. 
     In one embodiment, the plates  202 ,  204  can be conductive or charge layers. In another embodiment, there can be multiple plates in different orientations forming the alignment mark. 
       FIG. 2B  shows a perspective view of an example alignment process in accordance with an embodiment of the present invention. 
     In one embodiment, there can be multiple (e.g. four) plates ( 204 - 1 ,  204 - 2 ,  204 - 3  and  204 - 4 ) on one substrate and at least one plate  202 - b  on the other substrate. In one case, the value (e.g. capacitance or charge reading) for the multiple plates and the at least one plates on the other substrate is equalized by moving either substrate. 
     According to one embodiment, a method of aligning a first substrate and a second substrate may be provided. The method may comprise positioning the first substrate having at least a first alignment mark in close proximity to the second substrate having at least a second alignment mark, measuring an alignment value between the first and second alignment marks of both the first and second substrate; and adjusting the position of the first substrate and the second substrate based on the measured alignment value. 
     According to another embodiment, wherein measuring the alignment value between the alignment marks of both the first and second substrate comprises: extracting the alignment value at a current position of the substrates; and moving the substrates with respect to each other in different directions to measure a change in the alignment value. 
     According to some embodiments, the method may further comprise comparing the alignment value to the change in the alignment value; and adjusting the position of the first substrate and the second substrate based on the comparison. Adjusting the change in alignment value may comprise one of: maximizing, minimizing or equalizing the alignment value for each alignment mark. 
     According to yet another embodiment, the at least one alignment mark on the first substrate and the second substrate may comprise metal plates or capacitive plates. The alignment value may be maximized between the capacitive plates. The at least one alignment mark on the first substrate may comprise charge trapped on a surface of the first substrate and the at least one alignment mark on the second substrate may comprise a plate on the surface of the second substrate. The at least one alignment mark on a first substrate may comprise shield plate on a surface of the first substrate and the at least one alignment mark on the second substrate may comprise a receiver plate on the second substrate. The alignment value may be minimized between the shield plate and the receiver plate. 
     According to further embodiments, the at least one alignment mark on the first substrate may comprise a plurality of multiple plates on the first substrate and the at least one alignment mark on the second substrate may comprise a metal plate on the second substrate and the alignment value for multiple plates and the metal plate may be equalized by moving one of the first substrate or the second substrate. The alignment value may comprise one of: a capacitance value, inductance value, a charge value, or another electrical signal value between the substrates. 
     In one embodiment, the detector and the signal source are on one substrate and a transfer medium is formed on the other substrate. Here, the signal is passed through the transfer medium from the source to the detector. If the source and detectors are aligned with the transfer medium, the signal detected by the detector will be maximized.  FIG. 3A  shows one implementation of this method. Here, the substrate  310  has both the signal source  310 - b  and the detector  310 - a . The mediums  320   a ,  320   b  are in the other substrate  320 . The transfer medium can be a conductive trace. Here, the combination of signal, detector and transfer medium can be in different orientations to assist in different misalignment signals such as offset and rotation. 
     Another implementation demonstrated in  FIG. 3B  has one signal source  310   b  and two detectors  310   a ,  310   c  for each set and transfer mediums  320   a ,  320   b  and  320   c . Here a set comprises at least one signal source and at least one detector on one substrate and a transfer medium on another substrate. Here, the signal is passed to the transfer medium and directed to both detectors. The signal received by the two detectors is equalized and maximized. As a result, each set can offer both offset and rotation misalignment. More sets in different orientations can assist in fine tuning the misalignment information. This information can be used to align the substrates. The size of transfer medium and/or the detector and signal can be used to define the alignment accuracy. For example, if the transfer medium is larger than the detector or signal it can be used for coarse alignment. If the size is the same, it can be used for fine alignment. 
     The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teachings. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.