PATENT DOCUMENT

Publication Number: US-9653425-B2
Application Number: US-201514836859-A
Country: US
Kind Code: B2

Title: Anisotropic conductive film structures

Abstract:
Anisotropic conductive film (ACF) structures and manufacturing methods for forming the same are described. The manufacturing methods include preventing clusters of conductive particles from forming between adjacent bonding pads and that are associated with electrical shorting of ACF structures. In some embodiments, the methods involve use of multiple layered ACF materials that include a non-electrically conductive layer that reduces the likelihood of formation of conductive particle clusters between bonding pads. In some embodiment, the methods include the use of ultraviolet sensitive ACF material combined with lithography techniques that eliminate conductive particles from between neighboring bonding pads. In some embodiments, the methods involve the use of insulation spacers that block conductive particles from entering between bonding pads. Any suitable combination of the described methods can be used.

Claims:
What is claimed is: 
     
       1. A method of assembling a display assembly, the method comprising:
 arranging an electrically conductive film between a first substrate having first bonding pads and a second substrate having second bonding pads, the electrically conductive film including electrically conductive particles, wherein insulation spacers positioned between the first bonding pads are separated from the first bonding pads by gaps; and 
 compressing the first and second substrates, thereby causing the electrically conductive particles to form an electrically conducting path between the first and second substrates, wherein widths of the gaps are generally smaller than diameters of the electrically conductive particles, thereby preventing the electrically conductive particles from entering the gaps. 
 
     
     
       2. The method of  claim 1 , further comprising arranging a non-electrically conductive film between the first substrate and the second substrate. 
     
     
       3. The method of  claim 1 , wherein a distance between the first bonding pads is about 30 micrometers. 
     
     
       4. The method of  claim 1 , wherein a thickness of the insulation spacers is substantially the same as a thickness of the first bonding pads. 
     
     
       5. The method of  claim 1 , wherein the electrically conductive film includes binding material that flow within the gaps. 
     
     
       6. The method of  claim 2 , wherein the non-electrically conductive film includes non-conductive particles. 
     
     
       7. The method of  claim 2 , wherein the first substrate corresponds to a display component and the second substrate corresponds to an integrated circuit. 
     
     
       8. The method of  claim 2 , wherein the insulation spacers are first insulation spacer, wherein second insulation spacers are positioned between the second bonding pads. 
     
     
       9. The method of  claim 6 , wherein the non-conductive particles are smaller than the electrically conductive particles. 
     
     
       10. A display assembly, comprising:
 a first substrate bonded with a second substrate by an electrically conductive film, the first substrate having first bonding pads and the second substrate having second bonding pads, wherein conductive particles within the electrically conductive film are positioned between the first bonding pads and the second bonding pads such that the first substrate is electrically coupled to the second substrate; and 
 insulation spacers positioned between the first bonding pads and separated from the first bonding pads by gaps, wherein widths of the gaps are generally smaller than the diameters of the conductive particles a thereby preventing the conductive particles from entering the gaps. 
 
     
     
       11. The display assembly of  claim 10 , wherein the first substrate corresponds to a display component and the second substrate corresponds to an integrated circuit. 
     
     
       12. The display assembly of  claim 10 , wherein the second substrate includes second insulation spacers positioned between the second bonding pads. 
     
     
       13. The display assembly of  claim 10 , wherein the insulation spacers are composed of a ceramic material. 
     
     
       14. The display assembly of  claim 10 , wherein a thickness of the insulation spacers is substantially the same as a thickness of the first bonding pads. 
     
     
       15. The display assembly of  claim 13 , wherein an average diameter of the conductive particles is about 3 to 4 micrometers. 
     
     
       16. A display assembly, comprising:
 a first substrate having first bonding pads and insulation spacers between the first bonding pads, wherein the insulation spacers are separated from the first bonding pads by gaps; a second substrate having second bonding pads; 
 a binding material between the first substrate and the second substrate, the binding material binding the first substrate to the second substrate; and 
 conductive particles between the first bonding pads and the second bonding pads, thereby electrically coupling the first bonding pads with the second bonding pads, wherein widths of the gaps are generally smaller than diameters of the conductive particles. 
 
     
     
       17. The display assembly of  claim 16 , wherein the insulation spacers are composed of a ceramic material. 
     
     
       18. The display assembly of  claim 16 , wherein the insulation spacers are first insulation spacer, wherein second insulation spacers are between the second bonding pads. 
     
     
       19. The display assembly of  claim 16 , wherein the binding material is a resin. 
     
     
       20. The display assembly of  claim 16 , further comprising non-conductive particles.

Description:
FIELD 
     This disclosure relates generally to anisotropic conductive film (ACF) structures and methods for forming the same. In particular embodiments, the ACF structures are used in the manufacture of liquid crystal displays (LCDs). 
     BACKGROUND 
     Anisotropic conductive film (ACF) is an adhesive interconnect system that includes electrically conductive film. The electrically conductive film generally includes conductive particles dispersed within binder material. ACF is commonly used in the manufacture of liquid crystal displays (LCDs) to create the electrical connection between the display components and the integrated circuit (IC) components. In a typical LCD application, ACF is placed between electrodes of a display component and electrodes of an IC component. The display component and IC component are then pressed together such that an electrical and mechanical connection is made. The resulting structure is anisotropic in that there is unidirectional electrical connection between the display component and the IC component (z direction) but no electrical connection between adjacent electrodes of the display component or IC component. 
     A trend in new product designs is to reduce distances between bonding pads between electrodes of display components and IC components. This, however, can create problems using traditional ACF technologies. For example, the smaller distances between electrodes can result in higher probabilities of conductive particles of the conductive film to cluster between the electrodes and cause electrical shorts. What are needed, therefore, are improved anisotropic conductive film structures to accommodate current trends in LCD technology. 
     SUMMARY 
     This paper describes various embodiments that relate to manufacturing methods for anisotropic conductive film (ACF) structures. The systems and methods described can be used to manufacture ACF structures that are resistant to electrical shorting. 
     According to one embodiment, a method of electrically coupling a first contact on a first substrate to a second contact on a second substrate is described. The method includes arranging the first and second substrates on opposing sides of a multi-layer assembly such that the first and second contacts are aligned with each other. The multi-layer assembly including electrically conductive particles isotropically distributed within a first layer, and a non-electrically conductive second layer. The method also includes compressing the multi-layer assembly between the first and second substrates causing the electrically conductive particles to form an electrically conducting path between the first and second contacts. 
     According to another embodiment, a method of electrically coupling first bonding pads of a first substrate to second bonding pads of a second substrate is described. The method includes forming a patterned layer on the first substrate such that the patterned layer overlays the first bonding pads and has a shape in accordance with a pattern of the first bonding pads. The patterned layer includes conductive particles within a binding material. The method also includes positioning the first and second substrates such that the first and second bonding pads are aligned. The method further includes compressing the first and second substrates together such that the conductive particles form an electrically conductive path between the first and second bonding pads. 
     According to a further embodiment, a display assembly is described. The display assembly includes a first substrate bonded with a second substrate by an electrically conductive film. The first substrate has first bonding pads and the second substrate has second bonding pads. Conductive particles are positioned between the first bonding pads and the second bonding pads such that the first substrate is electrically coupled to the second substrate. The first substrate includes insulation spacers positioned between the first bonding pads, the insulation spacers preventing entry of the conductive particles between the first bonding pads. 
     These and other embodiments will be described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements. 
         FIGS. 1A and 1B  show cross-section views of an ACF structure during different stages of manufacture. 
         FIGS. 2A and 2B  show cross-section views of a double layer ACF structure during different stages of manufacture. 
         FIGS. 3A and 3B  show cross-section views of a triple layer ACF structure during different stages of manufacture. 
         FIGS. 4A and 4B  show cross-section views of a triple layer ACF structure having electrically non-conductive particles during different stages of manufacture. 
         FIG. 5  shows a flowchart indicating a process of forming a multiple-layered ACF structure. 
         FIGS. 6A-6E  show cross-section views of an ACF structure with UV sensitive ACF material during different stages of manufacture. 
         FIG. 7  shows a flowchart indicating a process of forming an ACF structure using a UV sensitive ACF material. 
         FIGS. 8A-8H  show cross-section views of an ACF structure having insulation spacers during different stages of manufacture. 
         FIG. 9  shows a flowchart indicating a process of forming an ACF structure having insulation spacers. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following descriptions are not intended to limit the embodiments to one preferred embodiment. To the contrary, they are intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims. 
     Described herein are manufacturing processes for forming anisotropic conductive film (ACF) structures with improved performance compared to traditional ACF structures. In particular, the ACF structures provided herein are resistant to shorting associated with clusters of conductive particles forming between adjacent conductive elements within bonded substrates. In some embodiments, the ACF structures are used in display assemblies, such as liquid crystal (LCD) assemblies. 
     The methods described are well suited for ACF structures having condensed bonding pad array patterns, as is the trend in current ACF product designs. In particular, as the number of the electronic components increases with newer product designs, the number of the bonding pads in each unit length in both display and integrated circuit (IC) substrates of LCD assemblies also increases correspondingly. These more condensed bonding pads arrays raise the challenge of creating spatially matched bonding pads of the display and IC substrates without formation of clusters of conductive particles with ACF structure that can crosslink neighboring bonding pads and result in electrical shorting within the ACF. 
     The methods described herein provide solutions for addressing these types of crosslinking and shorting problems. In some embodiments described herein, the ACF structures include multiple layers of material, with at least one non-electrically conductive layer that reduces the likelihood of formation of clusters of conductive particles between neighboring bonding pads. In some embodiments, the ACF structures include ultraviolet (UV) sensitive ACF material that can be combined with lithography techniques during the manufacturing process to form ACF structures that eliminate conductive particles between neighboring bonding pads. In some embodiments, the ACF structures include insulation spacers that block conductive particles from entering between neighboring bonding pads. 
     Methods described herein are well suited for use in the manufacture of consumer electronic products, such as in the manufacture of display assemblies for consumer electronics. For example, the methods described herein can be used in the manufacture of displays for computers, portable electronic devices, wearable electronic devices, and electronic device accessories, such as those manufactured by Apple Inc., based in Cupertino, Calif. 
     These and other embodiments are discussed below with reference to  FIGS. 1A-9 . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. 
     One of the challenges in the manufacture of modern display assemblies, such as LCD assemblies, relates to the reduction in size. In particular, distances between electrical traces and bonding pads within integrated circuits are becoming smaller and smaller, causing limitations when it comes to traditional ACF technologies. This problem is illustrated in  FIGS. 1A-1B , showing manufacturing of ACF structure  100 . 
       FIGS. 1A and 1B  show cross-section views of ACF structure  100  during different stages of manufacture. At  FIG. 1A , electrically conductive film  102  is positioned between first substrate  104  and second substrate  106 . In the case of a display assembly, such as an LCD assembly, first substrate  104  can correspond to an integrated circuit (IC) substrate and second substrate  106  can correspond to a display substrate as part of a display assembly. In some embodiments, the IC substrate is a flexible circuit substrate. First substrate  104  includes first bonding pads  108  that are electrically coupled to an electrical circuit of first substrate  104 . Likewise, second substrate  106  includes second bonding pads  110  that are electrically coupled to an electrical circuit of second substrate  106 . Bonding pads  108  and  110  can be referred to as contact pads or contacts. Distance d corresponds to an average distance between adjacent first bonding pads  108  and second bonding pads  110 . 
     Electrically conductive film  102  includes conductive particles  112  that are dispersed within binding material  114 , which is typically an organic resin. In some embodiments, conductive particles  112  are isotropically distributed within binding material  114 —that is conductive particles  112  are substantially evenly distributed within binding material  114 . Conductive particles  112  generally have an average diameter on the scale of a few micrometers, with electrically conductive film  102  having a thickness on the scale of tens of micrometers. In one embodiment, conductive particles  112  have an average diameter of about 3 to 4 micrometers. 
     At  FIG. 1B , first substrate  104  is bonded to second substrate  106  by compressing first substrate  104  and second substrate  106  together under high pressure and temperature conditions. For example, a bonding machine set at temperatures of about 100 to about 200 degrees Celsius can be used. The high temperature and pressure conditions cause the viscosity of electrically conductive film  102  to decrease and become liquefied. As such, binding material  114  spreads within the voids between first substrate  104  and second substrate  106 . In addition, conductive particles  112  move with the flow of binding material  114  and become redistributed within the compressed space between first substrate  104  and second substrate  106 . In this way, some of conductive particles  112 , such as conductive particle  112   a , get trapped between first bonding pads  108  and second bonding pads  110 . In particular, particle  112   a  gets trapped between spatially matched first bond pad  108  and second bond pad  110 . After some time (e.g., a few seconds), binding material  114  cools downs and becomes solid again. The resultant ACF structure  100  includes conductive particles  112  positioned between first bonding pads  108  and second bonding pads  110 , thereby provide electrical conduction between the electrical circuit of first substrate  104  and the electrical circuit of second substrate  106 . 
     In an ideal ACF structure, conductive particles  112  provide electrical conduction between first bonding pads  108  and second bonding pads  110 , but not between adjacent bonding pads (e.g., between adjacent first bonding pads  108  or between adjacent second bonding pads  110 ). This preferential vertical or z-direction conduction is what gives ACF structure  100  its ideally anisotropic electrical conduction characteristic. However, when the distance d between first bonding pads  108  and/or second bonding pads  110  is small, such as in the case of modern fine-pitch bonding structures, there is a higher probability of conductive particles  112  to assembly or conglomerate together in clump  116  between adjacent bonding pads, such as adjacent second bonding pads  110  shown in  FIG. 1B . For example, in some applications the average distance d (sometimes referred to as pitch) is about 30 micrometers or less. In a particular application, the average distance d is about 15 micrometers. 
     If clump  116  is assembled just right, this can provide an electrical path between adjacent second bonding pads  110 , resulting in electrical shorting between adjacent second bonding pads  110  and thereby destroying the anisotropic nature of ACF structure  100 . This type of electrical shorting is sometimes referred to as cross-linking. The smaller the distance d, the higher the probability of forming clump  116  that can cause such cross-linking problems. Note that clump  116  could accumulate between any adjacent bonding pads, such as between adjacent first bonding pads  108  of first substrate  104 . 
     The embodiments described herein provide alternative ACF structures that reduce or eliminate the occurrence of the above-described cross-linking problems associated with fine-pitch bonding structures. In some embodiments, the ACF structures include multiple layers of material, described below with reference to  FIGS. 2A-5 . In some embodiments, the ACF structures include the use of an ultraviolet (UV) sensitive ACF material, described below with reference to  FIGS. 6A-7 . In some embodiments, the ACF structures include use of an insulation spacer, described below with reference to  FIGS. 8A-9 . 
     Multiple Layer ACF 
     As described above, in traditional ACF structures, the conductive particles of the electrically conductive film get trapped between adjacent bonding pads of fine-pitch bonding structures, causing the ACF to have a short circuit. One way to address this problem is by providing one or more extra layers of material other than the electrically conductive film between the substrates, and therefore can be referred to as multiple layered or multi-layered. Some of these embodiments are described below. 
       FIGS. 2A and 2B  show cross-section views of ACF structure  200  that includes two layers during different stages of manufacture. At  FIG. 2A , electrically conductive film  202  is positioned between first substrate  204  and second substrate  206 . First substrate  204  includes first bonding pads  208  and second substrate  206  includes second bonding pads  210 . Electrically conductive film  202  includes conductive particles  212  (e.g., metal and/or graphite) dispersed within binding material  214 . 
     Unlike traditional ACF structures, ACF structure  200  includes non-electrically conductive film  216 , positioned between electrically conductive film  202  and second substrate  206 . Note that in other embodiments, non-electrically conductive film  216  is positioned between electrically conductive film  202  and first substrate  204 . Compared to electrically conductive film  202 , non-electrically conductive film  216  is substantially non-electrically conductive and does not include conductive particles  212 . In some embodiments, non-electrically conductive film  216  is made of a resin material. In some embodiments, the material of non-electrically conductive film  216  is the same as binding material  214  of electrically conductive film  202 . In other embodiments, the material of non-electrically conductive film  216  has a lower viscosity than the viscosity of binding material  214 . 
     At  FIG. 2B , first substrate  204  and second substrate  206  are compressed under high pressure and temperature conditions such that electrically conductive film  202  and non-electrically conductive film  216  liquefy and flow within spaces between first substrate  204  and second substrate  206 . This flow causes some conductive particles  212 , such as conductive particle  212   a , to become trapped between first bonding pads  208  and second bonding pads  210 , thereby providing electrical coupling of an electrical circuit of first substrate  204  and an electrical circuit of second substrate  206 . In addition, non-electrically conductive film  216  takes up part of the space between second bonding pads  210  that electrically conductive film  202  would otherwise take up. 
     By providing a non-electrically conductive film  216  having a lower viscosity than binding material  214  of electrically conductive film  202 , non-electrically conductive film  216  flows between second bonding pads  210  faster than electrically conductive film  202 , thereby preventing conductive particles  212  from flowing between second bonding pads  210 . Thus, non-electrically conductive film  216  can be referred to as a buffer layer. In some cases, more conductive particles  212  will be trapped between bonding pads  208  and  210 , thereby potentially providing better electrical coupling between substrates  204  and  206 . In addition, since conductive particles  212  are prevented from entering between second bonding pads  210 , or at least reducing the probability of conductive particles  212  entering between second bonding pads  210 , the likelihood of clusters of conductive particles  212  forming between second bonding pads  210  is eliminated or reduced. 
     In some cases, a single layer of non-electrically conductive film is not sufficient to provide adequately reduce the occurrence of cross-linking. For example, in some structures there may still be a likelihood of conductive particles  212  to flow in between first bonding pads  208  of first substrate  204 . 
     To prevent this possibility,  FIGS. 3A and 3B  show cross-section views of triple layer ACF structure  300 , in accordance with some embodiments. At  FIG. 3A , electrically conductive film  302 , first non-electrically conductive film  316 , and second non-electrically conductive film  318  are positioned between first substrate  304  and second substrate  306 . In some embodiments, first non-electrically conductive film  316  and second non-electrically conductive film  318  each have lower viscosity that binding material  314  of electrically conductive film  302 . 
     At  FIG. 3B , first substrate  304  and second substrate  306  are compressed under high pressure and temperature conditions such that electrically conductive film  302 , first non-electrically conductive film  316 , and second non-electrically conductive film  318  liquefy and flow within spaces between first substrate  304  and second substrate  306 . This flow causes some conductive particles  312 , such as conductive particle  312   a , to become trapped between first bonding pads  308  and second bonding pads  310 , thereby electrical coupling the electrical circuit of first substrate  304  and the electrical circuit of second substrate  306 . In addition, first non-electrically conductive film  316  takes up space between second bonding pads  310  and second non-electrically conductive film  318  takes up space between first bonding pads  308 . This reduces the number of conductive particles  312  of electrically conductive film  302  from entering between adjacent first bonding pads  308  or between adjacent second bonding pads  310 , thereby reducing or eliminating crosslink shorting described above. 
     In some embodiments, the non-electrically conductive films include electrically non-conductive particles to enhance performance of the triple layer ACF.  FIGS. 4A and 4B  show cross-section views of a triple layer ACF structure  400  having electrically non-conductive particles  420 , in accordance with some embodiments. It should be noted that that non-conductive particles  420  can also be used in the double ACF layer described above with reference to  FIGS. 2A and 2B . 
     At  FIG. 4A , electrically conductive film  402 , first non-electrically conductive film  416 , and second non-electrically conductive film  418  are positioned between first substrate  404  and second substrate  406 . In some embodiments, first non-electrically conductive film  416  and second non-electrically conductive film  418  each have lower viscosity that binding material  414  of electrically conductive film  402 . Each of first non-electrically conductive film  416  and second non-electrically conductive film  418  includes non-conductive particles  420 , which can be made of any suitable non-electrically conductive material, such as one or more polymer materials and/or certain ceramic materials. 
     At  FIG. 4B , first substrate  404  and second substrate  406  are compressed under high pressure and temperature conditions, similar to the ACF structures described above. Some of conductive particles  412 , such as conductive particle  412   a , are trapped between first bonding pads  408  and second bonding pads  410 , thereby electrically coupling first substrate  404  with second substrate  406 . In addition, non-conductive particles  420  are distributed between adjacent first bonding pads  408  and between adjacent second bonding pads  410 . This further reduces the probability of clusters of conductive particles  412  to form between adjacent first bonding pads  408  and between adjacent second bonding pads  410 , thereby further reducing or eliminating the occurrence of crosslink shorting. If first non-electrically conductive film  416  and second non-electrically conductive film  418  have lower viscosity that electrically conductive film  402 , these films  416  and  418  can flow at a faster rate under the pressure compared to electrically conductive film  402 . In this way, non-conductive particles  420  can have, on average, a higher mobility than conductive particles  412 . 
     The size of non-conductive particles  420  can vary depending on design requirements. In some embodiments, the non-conductive particles  420  have an average diameter less than the average diameter of conductive particles  412  so as not to interfere with the electrical conduction that conductive particles  412  provide. For example, insulating particle  420   a , which is positioned between first bonding pads  408  and second bonding pads  410 , has a smaller diameter than conductive particle  412   a  and therefore does not interfere with electrical conduction through conductive particle  412   a.    
       FIG. 5  shows flowchart  500  indicating a process of forming a multiple-layered ACF structure, in accordance with some embodiments. At  502 , a multiple-layered film is positioned between a first substrate and a second substrate. The first substrate has first bonding pads and the second substrate has second bonding pads. The multiple-layered film includes an electrically conductive film and a non-electrically conductive film. The electrically conductive film has conductive particles within a binding material. In some embodiments, the multiple-layered film includes two non-electrically conductive films positioned adjacent to opposing sides of the electrically conductive film. In some embodiments, the one or more non-electrically conductive films include non-conductive particles. 
     At  504 , the first and second substrates are pressed together such that the non-electrically conductive film prevents formation of conductive particle clusters between adjacent first bonding pads or adjacent second bonding pads. As described above, conductive particle clusters positioned between adjacent bonding pads are associated with forming electrical shorts within the ACF structure. 
     UV Sensitive ACF Material and Lithography Assisted ACF Bonding 
     A further way to address the crosslink shorting problems described above is by using an ultraviolet (UV) light sensitive ACF material, where aspects of ACF techniques are combined with aspects of photoresist lithography techniques.  FIGS. 6A-6E  show cross-section views of ACF structure  600  formed using UV sensitive ACF material, in accordance with some embodiments. 
     At  FIG. 6A , first substrate  604  having first bonding pads  608  is provided. First bonding pads  608  provide electrical access to one or more electrical circuits of first substrate  604 . First substrate  604  can correspond to an IC substrate or a display substrate as part of a display assembly (e.g., LCD assembly). At  FIG. 6B , first substrate  604  is coated with UV sensitive ACF  603 . UV sensitive ACF  603  includes conductive particles  612  embedded within UV sensitive binding material  615 . In some embodiments, UV sensitive binding material  615  is a UV sensitive resin, such as a negative or positive photoresist material. 
     At  FIG. 6C , a portion of UV sensitive ACF  603  is exposed to UV light. In particular, mask  607  has a pattern of openings  609  corresponding to the pattern of first bonding pads  608 . Openings  609  allow underlying portions of ACF  603  to be exposed to UV light, while opaque portions  611  of mask  607  block other portions of UV sensitive ACF  603  from UV light. In the instant embodiment, UV sensitive binding material  615  is a positive type of photoresist such that portions of UV sensitive ACF  603  exposed to UV light become more soluble in a subsequently applied developer solution. Alternatively, UV sensitive binding material  615  can be a negative type of photoresist that becomes insoluble when exposed to UV light—in which case, mask  607  would have an opposite pattern with openings  609  corresponding to opaque portions and opaque portions  611  corresponding to openings. 
     At  FIG. 6D , UV sensitive ACF  603  is exposed to a developer solution such that portions of UV sensitive ACF  603  above first bonding pads  608  remain and most or all of UV sensitive ACF  603  between first bonding pads  608  are removed, including the conductive particles  612  in the removed portion of UV sensitive ACF  603 . In this way, after the UV patterning and development, conductive particles  612  are completely removed from the spaces between first bonding pads  608 . In some embodiments, first substrate  604  is ready for bonding with a second substrate. In other embodiments, a separate binding material is applied on and/or around UV sensitive ACF  603  to assist subsequent bonding. 
     At  FIG. 6E , second substrate  606  is positioned over and pressed with first substrate  604  under high pressure and temperature conditions, similar to the ACF structures described above. It should be noted, however, that UV sensitive binding material  615  might have different liquefying temperatures than more traditional types of binding material. Therefore, the pressure and temperature conditions should be adjusted accordingly. Since conductive particles  612  are already positioned over first bonding pads  608  prior to the bonding, most or all of conductive particles  612  become trapped between first bonding pads  608  and second bonding pads  610 . Conductive particles  612  thereby electrically couple first substrate  604  with second substrate  606  with substantially no conductive particles  612  between adjacent first bonding pads  608  or second bonding pads  610 . 
     Compared with the traditional ACF processes, use of UV sensitive ACF  603  and the above-described pre-bonding process can not only prevent the issue of crosslinking in neighboring bonding pads  608  and  610  by eliminating conductive particles  612  in between them, but also accurately deliver conductive particles  612  on top first bonding pads  608 , thereby effectively managing the locations of conductive particles  612  instead of randomly distributing conductive particles  612  across the whole of substrates  604  and  606  as in the traditional ACF procedures. This can allow for more overall conductive particles  612  to be trapped between bond pads  608  and  610 , and thereby increase the electrical conductivity between substrates  604  and  606  compared to traditional ACF techniques. 
       FIG. 7  shows flowchart  700  indicating a process of forming an ACF structure using a UV sensitive ACF material, in accordance with some embodiments. At  702 , a UV sensitive ACF is applied on a first substrate having first bonding pads. In some embodiments, the first substrate corresponds to a display substrate or IC substrate of a display assembly for an electronic device. The UV sensitive ACF includes conductive particles within a UV sensitive binding material. The UV sensitive binding material can include a positive or negative type of photoresist material. 
     At  704 , a portion of UV sensitive ACF is exposed to UV light in accordance with a pattern of the first bonding pads. A mask having openings and opaque regions can be used to expose appropriate portions of the UV sensitive ACF. At  706 , a portion of the UV sensitive ACF between first bonding pads of the first substrate is removed. If the UV sensitive ACF includes a positive type photoresist, the removed portion of UV sensitive ACF will correspond to the UV exposed portion. If the UV sensitive ACF includes a negative type photoresist, the removed portion of UV sensitive ACF will correspond to the portion blocked from UV exposure. After removal, the UV sensitive ACF will have a patterned shape in accordance with the first bonding pads, with conductive particles positioned over the first bonding pads. In some embodiments, the one or more additional layers of binding material are applied onto the UV sensitive ACF prior to bonding at  708 . 
     At  708 , the first substrate is bonded with a second substrate having second bonding pads. The first bonding pads are aligned with the second bonding pads such that the conductive particles electrically couple the first substrate with the second substrate. Since the conductive particle are not positioned between adjacent first bonding pads or adjacent second bonding pads, clusters of conductive particles do not form between adjacent first bonding pads and adjacent second bonding pads, thereby preventing formation of electrical shorts within the ACF structure. 
     Insulation Spacer Incorporated ACF Bonding 
     A further way to address the crosslink shorting problems described above is by providing insulation spacers within the ACF structures.  FIGS. 8A-8H  show cross-section views of ACF structure  800  having insulation spacers being formed, in accordance with some embodiments. 
     At  FIG. 8A , first substrate  804  having first bonding pads  808  is provided. First bonding pads  808  provide electrical access to one or more electrical circuits of first substrate  804 . First substrate  804  can correspond to an IC substrate or a display substrate as part of a display assembly (e.g., LCD assembly). At  FIG. 8B , first substrate  804  is coated with photoresist  803 . Photoresist  803  can include any suitable photoresist material or materials, such as resins that are UV sensitive (e.g., negative or positive photoresist material). 
     At  FIG. 8C , photoresist  803  is exposed to UV light. Mask  807  is positioned over first substrate  804  such that openings  809  and opaque portions  811  of mask  807  are arranging in a pattern in accordance with the locations of bonding pads  808 . Openings  809  allow underlying portions of photoresist  803  to be exposed to UV light, while opaque portions  811  block other portions of photoresist  803  from UV light. In the embodiment shown in  FIGS. 8A-8H , photoresist  803  is a positive type of photoresist such that those portions exposed to UV light become more soluble in a subsequently applied developer solution. In other embodiments, photoresist  803  is a negative type of photoresist that becomes insoluble when exposed to UV light—in which case, mask  807  would have an appropriate pattern. 
     At  8 D, photoresist  803  is exposed to a developer solution such that most of photoresist  803  between first bonding pads  808  is removed and portions above first bonding pads  608  remain. At  8 E, insulation material  813  is deposited onto first substrate  804  and photoresist  803 . As shown, first portion  813   a  of insulation material  813  can be deposited on exposed substrate  804  and second portion  813   b  of insulation material  813  is deposited on photoresist  803 . Insulation material  813  can be made of any suitable electrically insulating material that can be deposited, using for example spraying, sputtering or other suitable deposition technique. In some embodiments, insulation material  813  includes ceramic material (e.g., aluminum oxide or other oxide material). The amount of insulation material  813  deposited on substrate  804  can be chosen to achieve a desired thickness T. In some embodiments, thickness T of insulation material  813  is substantially the same as the thickness of first bonding pads  808 . In other embodiments, thickness T is chosen such that insulation material  813  extends above a top surface of bonding pads  808 . 
     At  FIG. 8F , photoresist  803  and second portion  813   b  of insulation material  813  are removed using, for example, a photoresist removal washing process. This leaves insulation material  813  positioned in the spaces between bonding pads  808 . Thus, insulation material  813  can be referred to as insulation spacers. Note that in some embodiments, insulation spacers  813  are not deposited in between every neighboring bonding pad  808 , but instead every few bonding pads  808 , such as every two neighboring bonding pads  808 . 
     At  8 G, electrically conductive film  802  is applied on first substrate  804 , e.g., on insulation spacers  813  and first bonding pads  808 . Electrically conductive film  802  includes conductive particles  812  within binding material  814 . An average diameter of conductive particles  812  can be chosen to larger than an average spacing  815  between adjacent insulation spacers  813  and first bonding pads  808 . Note that in some embodiments spacing  815  is substantially zero and first bonding pads  808  contact adjacent spacers  813 . However, in practicality the lithography process described above with reference to  FIG. 8C  may include tolerance such that spacing  815  may not be zero. 
     At  8 H, second substrate  806  is positioned over and pressed with first substrate  804  under high pressure and temperature conditions, similar to the ACF structures described above. Second substrate  806  has been processed similar to first substrate  804  such that second insulation spacers  816  are positioned between second bonding pads  810  of second substrate  806 . Second substrate  806  is bonded with first substrate  804  such that conductive particles  812  become trapped between first bonding pads  608  and second bonding pads  610 , thereby electrically coupling first substrate  804  and second substrate  806 . Insulation spacers  813  prevent conductive particles  812  from entering between first bonding pads  808 . In particular, since average spacing  815  between adjacent insulation spacers  813  and first bonding pads  808  is smaller than the average diameter of conductive particles  812 , conductive particles  812  cannot enter. Similarly second insulation spacers  816  prevent conductive particles  812  from entering between second bonding pads  810 . In this way, insulation spacers  813  and second insulation spacers  816  prevent clustering of conductive particles  812  between first bonding pads  808  or between second bonding pads  810 , thereby preventing crosslink shorting of ACF structure  800 . 
       FIG. 9  shows flowchart  900  indicating a process of forming an ACF structure having insulation spacers, in accordance with some embodiments. At  902 , a patterned photoresist layer is created over a pattern of bonding pads on a first substrate. This can be accomplished using photolithography techniques described above. At  904 , an insulation material is deposited over the photoresist and exposed surfaces of the first substrate. In some embodiments, the insulation material includes an oxide material. 
     At  906 , removing the photoresist and a portion of the insulation material deposited on the photoresist forms insulation spacers between the bonding pads. At  908 , an electrically conductive film is applied on the bonding pads and the insulation spacers. In some embodiments, an average space between adjacent insulation spacers and bonding pads is less than an average diameter of conductive particles. 
     At  910 , the first substrate is bonded with a second substrate having insulation spacers positioned between corresponding bonding pads. The bonding pads of the first substrate are aligned with the bonding pads of the second substrate such that the conductive particles electrically couple the first substrate with the second substrate. The insulation spacers of the first substrate prevent conductive particles from entering between adjacent bonding pads of the first substrate. Likewise, the insulation spacers of the second substrate prevent conductive particles from entering between adjacent bonding pads of the second substrate. In this way, clusters of conductive particles do not form between bonding pads of the first substrate and between bonding pads of the second substrate, thereby preventing formation of electrical shorts within the ACF structure. 
     Note that suitable combinations of any of the methods described above can be combined to form an ACF structure. For example, in some embodiments, UV sensitive ACF material is combined with formation of insulation spacers. Likewise, in some embodiments, multiple layer ACF is used in combination with formation of insulation spacers. Further, in some embodiments, multiple layer ACF is used in combination with UV sensitive ACF material. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not target to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings.

Metadata:
Filing Date: 20150826
Publication Date: 20170516
Grant Date: 20170516
Priority Date: 20150826
Inventors: ZHANG BO
KIM SANG HA
LIU CYRUS Y.
SUNG KUO-HUA
Assignee: APPLE INC
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Family ID: 58104295