Patent Publication Number: US-2022240367-A1

Title: Package structure having solder mask layer with low dielectric constant and method of fabricating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Taiwan Application Serial Number 110102738 filed Jan. 25, 2021, which is herein incorporated by reference in its entirety. 
     BACKGROUND 
     Field of Invention 
     The present disclosure relates to a package structure. More particularly, the present disclosure relates to the package structure having a solder mask layer with a low dielectric constant. 
     Description of Related Art 
     In order to meet the requirement of high-speed and high-frequency transmission of signal, materials applied in printed circuit broads (PCBs) and their corresponding parameters have been developed, such as roughness of metal layer, thickness of substrate, properties of substrate, as so on. Despite of the limitation of process, properties of solder mask layer still can be enhanced. 
     SUMMARY 
     The disclosure provides a package structure having a solder mask layer with a low dielectric constant. The package structure comprises a substrate, a conductive structure on the substrate, and a solder mask layer on the substrate. The solder mask layer comprises bubbles and a solder mask material. The bubbles are disposed within the solder mask layer, and the solder mask material covers the bubbles. 
     The disclosure provides a method of fabricating a package structure having a solder mask layer with a low dielectric constant. The method comprises forming a spherical shell with a solder mask material and the spherical shell is a hollow structure. The method further comprises forming a mixture of the spherical shell and a liquid solder mask material. The liquid solder mask material is the solder mask material in liquid state. The method further comprises forming a solder mask layer on a substrate with the mixture. 
     It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the disclosure as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawings as follows: 
         FIG. 1A  is a cross-sectional view of a package structure according to some embodiments of the present disclosure. 
         FIG. 1B  is a cross-sectional view of a package structure according to some embodiments of the present disclosure. 
         FIG. 2  is a flow diagram of a method for fabricating a package structure according to some embodiments of the present disclosure. 
         FIG. 3  is a schematic diagram of a method for forming a spherical shell according to some embodiments of the present disclosure. 
         FIG. 4  is a flow diagram of a method for forming a spherical shell according to some embodiments of the present disclosure. 
         FIG. 5  is a schematic diagram of forming a spherical shell at one of various process stages according to some embodiments of the present disclosure. 
         FIG. 6A  is a schematic diagram of forming a spherical shell at one of various process stages according to some embodiments of the present disclosure. 
         FIG. 6B  is a cross-sectional view of a spherical shell at one of various process stages according to some embodiments of the present disclosure. 
         FIG. 7A  is a schematic diagram of forming a spherical shell at one of various process stages according to some embodiments of the present disclosure. 
         FIG. 7B  is a cross-sectional view of a spherical shell at one of various process stages according to some embodiments of the present disclosure. 
         FIG. 8A  is a schematic diagram of forming a spherical shell at one of various process stages according to some embodiments of the present disclosure. 
         FIG. 8B  is a cross-sectional view of a spherical shell at one of various process stages according to some embodiments of the present disclosure. 
         FIG. 8C  is an enlarged view of a portion of the spherical shell shown in  FIG. 8B  according to some embodiments of the present disclosure. 
         FIG. 9A  is a schematic diagram of forming a spherical shell at one of various process stages according to some embodiments of the present disclosure. 
         FIG. 9B  is a cross-sectional view of a spherical shell at one of various process stages according to some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the present embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that, although the terms first, second, third etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present disclosure. 
     As used herein, “around”, “about” or “approximately” shall generally mean within 20 percent, preferably within 10 percent, and more preferably within 5 percent of a given value or range. Numerical quantities given herein are approximate, meaning that the term “around”, “about” or “approximately” can be inferred if not expressly stated. 
     Generally speaking, a dielectric constant (Dk) and a dissipation factor (Df) of a solder mask material used in forming a solder mask/solder resist layer are larger than a Dk and a Df of air. Therefore, the formed solder mask layer would degrade a transmission speed of a signal and a transmission quality of the signal. The present disclosure presents a package structure having the solder mask layer with a low Dk by forming bubbles (e.g., air) within the solder mask layer to reduce the Dk and the Df of the solder mask layer. In addition, a method of fabricating the package structure having the solder mask layer with bubbles is present herein as well. 
       FIG. 1A  is a cross-sectional view of a package structure  100  having a solder mask layer with a low Dk according to some embodiments of the present disclosure. The package structure  100  having the solder mask layer with the low Dk includes a substrate  102 , a conductive structure  104  formed on the substrate  102 , and a solder mask layer  106  formed on the substrate  102 . The solder mask layer  106  exposes the conductive structure  104 . The solder mask layer  106  includes bubbles  108  disposed inside the solder mask layer  106 . The substrate  102  can include polymeric or non-polymeric dielectric materials, such as liquid crystal polymer (LCP), bismaleimide-triazine (BT), prepreg (PP), Ajinomoto build-up film (ABF), epoxy, polyimide (PI), other suitable dielectric material, or combinations of foregoing materials. Further, the above-mentioned dielectric materials can include fibers, such as glass fibers or Kevlar fibers, to reinforce the substrate  102 . In some embodiments, the substrate  102  can be formed by photo-imageable dielectric materials or photoactive dielectric materials. 
     The conductive structure  104  and the solder mask layer  106  are patterned on the substrate  102 . In one embodiment, the solder mask layer  106  exposes and is spaced from the conductive structure  104 . In some other embodiments shown in  FIG. 1B , the solder mask layer  106  exposes and is in physical contact of the conductive structure by covering a portion of the conductive structure  104 . Similarly in  FIG. 1B , the solder mask layer  106  includes bubbles  108  disposed within the solder mask layer  106 . 
     The conductive structure  104  can be formed by metal such as aluminum (Al), gold (Au), silver (Ag), copper (Cu), tin (Sn), other suitable metal, or combinations of foregoing metals. In some embodiments, the conductive structure  104  is formed by Cu. In some embodiments, the conductive structure  104  is a Cu pad. In some embodiments, the conductive structure  104  is a Cu bump. The patterning process used to form the conductive structure  104  can include one or more deposition processes, one or more photolithography processes, one or more etching processes, any suitable processes, or combinations thereof. The deposition process can include an electroplating process, an electroless plating process, a sputter process, an evaporation process, any suitable techniques, or combination thereof. The conductive structure  104  can further electrically connect other elements (not shown) in the following process. 
     The solder mask layer  106  includes the bubbles  108  and a solder mask material  110 . The bubbles  108  are disposed within the solder mask layer  106 . The solder mask material  110  belongs in the solder mask layer  106  excluding the bubbles  108  and covers the bubbles. The bubbles  108  include gas which is within the bubbles  108 . The gas varies with an environment of processes. In some embodiments, the bubbles  108  include air. 
     A diameter of each of the bubbles  108  is less than about 10 μm (micrometer), such as 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 μm. In some embodiments, the diameter of each of the bubbles  108  is less than 5 μm, such as 1, 2, 3, 4, or 5 μm. A diameter average of the bubbles  108  can be statistically obtained. In some embodiments, a ratio of the diameter of each the bubble  108  to the diameter average of the bubbles  108  is in a range of about 0.8 and about 1.2, such as 0.8, 0.9, 1.0, 1.1, or 1.2. Further, a deviation of each of bubbles  108  between the diameter of each bubble  108  and the diameter average of the bubbles  108  can be statistically obtained. In some embodiments, a ratio of the deviation of each bubble  108  to the diameter average of the bubbles  108  is between about 0 and about 0.2. 
     Furthermore, an excess of the bubbles may cast a negative impact on solder mask layer  106 . For example but not limited to, the bubbles  108  interferes a path of light during photolithography processes, or the bubbles  108  deteriorates a stress of the solder mask layer  106  that can withstand. Therefore, a quantity of the bubbles in the solder mask layer  106  is adjusted. In some embodiments, a volume ratio of the bubbles  108  to the solder mask layer  106  (i.e., a combination of the bubbles  108  and the solder mask material  110 ) is in a range of about 5 vol.% and about 50 vol.%, such as 5, 10, 15, 20, 25, 30, 35, 40, 45 or 50 vol.%. In some embodiments, a volume ratio of the bubbles  108  to the solder mask layer  106  (i.e., a combination of the bubbles  108  and the solder mask material  110 ) is in a range of about 5 vol.% and about 10 vol. %, such as 5, 6, 7, 8, 9 or 10 vol. %. 
     The solder mask material  110  of the solder mask layer  106  includes epoxy, PI, or any suitable materials. Moreover, based on process parameters or product design, additives could be included into the foregoing materials, such as, but not limited to curing agents or photoinitiators. In some embodiments, the solder mask material  110  of the solder mask layer  106  can be a thermal curing solder mask ink. In other embodiments, the solder mask material  110  of the solder mask layer  106  can be a radiation curing solder mask ink. The patterning process used to form the solder mask layer  106  can include deposition process, photolithography process, etching process, screen print process, curing process, any suitable process, or combinations thereof. In some embodiments, the patterned solder mask layer  106  is formed by screen print process. In some embodiments of the method presented in the present disclosure (discuss later), the solder mask material  110  is a homogenous material. In some embodiments, a boundary is formed between the bubbles  108  and the solder mask material  110 . Except for the boundary, other heterogeneous boundaries are not observed within the solder mask layer  106 . 
     The volume ratio of the bubbles  108  to the solder mask layer  106  determines an overall Dk and Df of the solder mask layer  106 . In some embodiments, as the Dk and Df of the solder mask material  110  are respectively 3.90 and 0.030, and the Dk and Df of air contained within the bubbles  108  are respectively 1.00 and 0.000, a relation between the volume ratio of the bubbles  108  to the solder mask layer  106  and the Dk and Df of the solder mask layer  106  is summarized in below table. The below table shows a negative relation between the volume ratio of the bubbles  108  to the solder mask layer  106  and the Dk and Df of the solder mask layer  106 . In another words, an increasing volume ratio of bubbles  108  to the solder mask layer  106  has a benefit of decreasing the Dk and Df of the solder mask layer  106 , further forming the solder mask layer  106  with the low Dk. 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
                 Volume ratio of bubbles to 
               
               
                   
                 solder mask layer (vol. %) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 0 
                 10 
                 20 
                 30 
                 40 
                 50 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Dk of solder mask layer 
                 3.90 
                 3.61 
                 3.32 
                 3.03 
                 2.74 
                 2.45 
               
               
                 Df of solder mask layer 
                 0.030 
                 0.027 
                 0.024 
                 0.021 
                 0.018 
                 0.015 
               
               
                   
               
            
           
         
       
     
     Referring to  FIG. 2 , a flow diagram of a method  200  for fabricating the package structure  100  according to some embodiments of the present disclosure. Operations in the method  200  can be performed in a different order or not performed depending on specific applications. The method  200  may not produce a complete package structure  100 . Accordingly, it is understood that additional processes can be provided before, during, and/or after the method  200 , and that some other processes may be briefly described herein. 
     The method  200  begins with operation  202  and the process of forming a spherical shell with a solder mask material. The spherical shell is a hollow structure. The spherical shell has been cured and therefore it is in solid state. The curing process of spherical shell includes a thermal curing process, a radiation curing process, any other suitable process, or combination of foregoing processes. In some embodiment of the radiation process, ultraviolet (UV) can be applied. Due to the hollow structure, an interior space of the spherical shell includes gas, varying with an environment of processes. In some embodiments, the interior space of the spherical shell includes air. A gas-state space confined within the spherical shell substantially equals to the bubbles  108 . Thus, the spherical shell is basically referred to as a former status of the bubbles  108  in  FIG. 1A  and  FIG. 1B . The spherical shell can be formed by a method  300  and a method  400  (discuss later) according to some embodiments of the present disclosure. 
     Next, in operation  204 , a mixture of the spherical shell and a liquid solder mask material is formed and the liquid solder mask material is the solder mask material in liquid state. Therefore, the liquid solder mask material and the solder mask material used to form the spherical sphere are the same. In some embodiments, stirring is performed to achieve a well-mixing mixture of the spherical shell and the liquid solder mask material. As above discussion, since the quantity of the bubbles in the solder mask layer  106  is adjusted and the spherical shell is basically regarded as the former status of the bubbles  108 , a volume ratio of the spherical shell to the mixture is correspondingly adjusted. In some embodiments, the volume ratio of the spherical shell to the mixture is adjusted between about 5 vol.% and 50 vol.%, such as 5, 10, 15, 20, 25, 30, 35, 40, 45 or 50 vol.%. In some embodiments, the volume ratio of the spherical shell to the mixture is adjusted between about 5 vol.% and about 10 vol. %, such as 5, 6, 7, 8, 9 or 10 vol. %. 
     Next, in operation  206 , a solder mask layer is formed on a substrate with the mixture the spherical shell and the liquid solder mask material. In some embodiments, a screen print process and a curing process are performed to form an unpatterned solder mask layer on the substrate. An operation number of the screen print process and the curing process is based on process parameters. In some embodiments, a patterning process used to form a patterned solder mask layer (e.g., the solder mask layer  106  in  FIG. 1A  and FIG.  1 B) can include a photolithography process, an etching process, a curing process, any suitable process, or combinations thereof. 
     Referring to  FIG. 3 , a simplified schematic diagram of the method  300  for forming a spherical shell (e.g., operation  202  in the method  200 ) according to some embodiments of the present disclosure. The method  300  may not produce a complete spherical shell. Accordingly, it is understood that additional processes can be provided before, during, and/or after the method  300 , and that some other processes may be briefly described herein. For clarity of discussion, some instruments or systems may be omitted in a simplified schematic diagram (that is,  FIG. 3 ) of the method  300 . 
     In  FIG. 3 , a first chamber  302  contains a liquid solder mask material  304 . The first chamber  302  has a nozzle  306  protruding from the first chamber  302  and inserting into a second chamber  310 . An orifice  308  of the nozzle  306  is directed toward an interior space of the second chamber  310 . In some embodiments, the nozzle is connected to the first chamber  302  via a pipe (not shown) rather than directly configured on the first chamber  302 . The first chamber  302  can have a fluid system (not shown), such as a pump, a piston, any other suitable instrument, or combinations thereof, to drive a flow of the liquid solder mask material  304  toward the nozzle  306 . In some embodiments, a piston is used in the first chamber  302  to drive the flow of the liquid solder mask material  304  toward the nozzle  306  within the first chamber  302 . After flowing through the orifice  308  of the nozzle  306 , the liquid solder mask material  304  enters into the interior space of the second chamber  310 . 
     When the liquid solder mask material  304  passes through the nozzle  306 , a pressure drop/flow velocity change of the liquid solder mask material  304  occurs through the nozzle  306 . In some embodiments, the pressure drop/flow velocity change of the liquid solder mask material  304  is large enough to from a cavitation phenomenon in the liquid solder mask material  304 . In some embodiments, the nozzle  306  has a conical shape, shown as in  FIG. 3 . That is, a cross-sectional area at an inlet of the nozzle  306  (e.g., an end of the nozzle  306  configured on the first chamber  302 ) is larger than that at an outlet of the nozzle  306  (e.g., the orifice  308  configured on the second chamber  310 ). 
     Due to a sudden shrinking of cross-sectional areas during a course of the liquid solder mask material  304  through the nozzle  306 , a flow velocity of the liquid solder mask material  304  is sharply increased and a pressure of the liquid solder mask material  304  is sharply deceased. If the pressure drop/flow velocity change of the liquid solder mask material  304  is large enough to from the cavitation phenomenon, a cavitation bubble  312  is formed in the liquid solder mask material  304 . Then, the cavitation bubble  312  becomes a liquid spherical shell  314  after the cavitation bubble  312  along with the liquid solder mask material  304  flows out of the orifice  308  of the nozzle  306  and enters into the interior space of the second chamber  310 . The liquid spherical shell  314  is made from the liquid solder mask material  304 . 
     In some embodiments, the second chamber  310  is kept at a temperature to thermally cure the liquid spherical shell  314  as soon as the liquid spherical shell  314  enters into the interior space of the second chamber  310 . In some embodiments, the liquid spherical shell  314  is cured and turned into a solid spherical shell. In another words, the liquid spherical shell  314  is cured to form a spherical shell  316  with a hollow structure. In some embodiments, the temperature used in the second chamber  310  is capable of curing the liquid spherical shell  314 , such as about 50° C., about 100° C., about 150° C., about 200° C., about 250° C., about 300° C., about 350° C., or about 400° C. In some embodiments, the temperature used to cure the liquid spherical shell  314  in the second chamber  310  is between about 180° C. and about 220° C. In some embodiments, the second chamber  310  is kept at 200° C. to cure the liquid spherical shell  314 . 
     In some other embodiments, the second chamber  310  is provided with radiation to cure the liquid spherical shell  314  as soon as the liquid spherical shell  314  enters into the side of the second chamber  310 . In some embodiments, the liquid spherical shell  314  is cured with radiation and turned into a solid spherical shell. In another words, the liquid spherical shell  314  is cured with radiation to form a spherical shell  316  with the hollow structure. In some embodiments, radiation can be UV. 
     Since the spherical shell  316  has the hollow structure, an interior space of the spherical shell  316  includes gas, varying with an environment of processes. In some embodiments, the interior space of the spherical shell  316  includes air. 
     Finally, the spherical shell  316  is collected. In some embodiments, the collected spherical shell  316  is further screened for particular sizes. For example, a sieve is used to screen the spherical shell  316 . In some embodiments, the spherical shell  316  with a diameter less than about 10 μm is collected by screening. The following operation can be operation  204  in the method  200  after the spherical shell  316  is collected. Based on the disclosure herein, other operations, instruments, or systems used in a similar concept of the method  300  are within the scope and spirit of this disclosure. 
     Referring to  FIG. 4 , a flow diagram of method  400  for forming a spherical shell (e.g., operation  202  in the method  200 ) according to some embodiments of the present disclosure. Operations in the method  400  can be performed in a different order or not performed depending on specific application. The method  400  may not produce a complete spherical shell. Accordingly, it is understood that additional processes can be provided before, during, and/or after the method  400 , and that some other processes may be briefly described herein. 
     The method  400  for forming the spherical shell will be described in more detail below with reference to  FIG. 5 ,  FIG. 6A ,  FIG. 6B ,  FIG. 7A ,  FIG. 7B ,  FIG. 8A ,  FIG. 8B ,  FIG. 8C ,  FIG. 9A  and  FIG. 9B .  FIG. 5 ,  FIG. 6A ,  FIG. 7A ,  FIG. 8A , and  FIG. 9A  are schematic diagrams of the method  400  at one of various process stages according to some embodiments of the present disclosure.  FIG. 6B ,  FIG. 7B ,  FIG. 8B , and  FIG. 9B  are cross-sectional views of the spherical shell at one of various process stages according to some embodiments of the present disclosure.  FIG. 8C  is an enlarged view of a portion of the spherical shell shown in  FIG. 8B  according to some embodiments of the present disclosure. 
     Referring to  FIG. 4 , the method  400  begins with operation  402  and the process of mixing a liquid solder mask material and a solvent to form two separated layers. Take  FIG. 5  as an example, a liquid solder mask material  500  and a solvent  502  are mixed together to form two separated layers because of immiscibility of the liquid solder mask material  500  with the solvent  502 . In some embodiments, a formation of two separated layers is due to a difference between a polarity of substances, such as oil and water. Therefore, properties of the liquid solder mask material  500  can determine a selection of the solvent  502 . In some embodiments, the solvent  502  is chosen to be polar or with stronger polarity, such as water. In some embodiments, the solvent  502  is chosen to be nonpolar or with weaker polarity, such as hexane. 
     Referring to  FIG. 4 , the method  400  continues with operation  404  and the process of stirring at an interface between two separated layers to form a sphere including a liquid shell and a liquid core. For example, as shown in  FIG. 6A , a stirring instrument  602  stirs at an interface between the liquid solder mask material  500  and the solvent  502  to distribute a portion of the liquid solder mask material  500  into the solvent  502 , and then a sphere  600  can be formed in the solvent  502 . The sphere  600  includes a liquid shell  604  and a liquid core  606 , as shown in  FIG. 6B . In some embodiments, the liquid shell  604  is made of the liquid solder mask material  500 , and the liquid core  606  is made of the solvent  502 . In some embodiments of the sphere  600 , the liquid shell  604  entirely covers the liquid core  606 ; that is, the liquid solder mask material  500  entirely covers the solvent  502 . 
     Referring to  FIG. 4 , the method  400  continues with operation  406  and the process of curing the liquid shell of the sphere. For example, as shown in  FIG. 7A , the sphere  600  formed in the solvent  502  is cured and turned into a sphere  700  during a curing process. The curing process of the sphere  600  includes a thermal curing process, a radiation curing process, a curing process with additives, any other suitable process, or combination thereof. Referring to  FIG. 7B , in some embodiments, the liquid shell  604  is cured and turned into a solid shell  702 , and the liquid core  606  is remained its original status (e.g., in liquid state). 
     A suitable curing temperature is applied in the thermal curing process, such as about 50° C., about 100° C., about 150° C., about 200° C., about 250° C., about 300° C., about 350° C., or about 400° C. In some embodiments of water used in the solvent  502 , the curing temperature can be below or equal to about 100° C., such as about 50° C., about 60° C., about 70° C., about 80° C., about 90° C., or about 100° C. In some embodiments of hexane used in the solvent  502 , the curing temperature can be below or equal to about 70° C., such as about 50° C., about 60° C., or about 70° C. When a curing temperature of the liquid solder mask material  500  is higher than a boiling temperature of the solvent  502 , curing the liquid shell  604  to form the solid shell  702  substantially occurs along with removing the liquid core  606  from the sphere  700 . In some embodiments, UV can be used in the radiation curing process to cure the liquid shell  604  turning into the solid shell  702 . In some embodiments, curing agents can be used in the curing process with additives to cure the liquid shell  604  turning into the solid shell  702 . 
     Referring to  FIG. 4 , the method  400  continues with operation  408  and the process of removing the liquid core of the sphere to form the spherical shell. For example, as shown in  FIG. 8A , the sphere  700  is left after most of the solvent  502  (with reference of  FIG. 7A ) is removed. Then, the liquid core  606  of the sphere  700  illustrated in  FIGS. 8B and 8C  is removed. Approaches of removing the liquid core  606  of the sphere  700  include an evaporation process, a dry process, or combinations thereof. In some embodiments, the evaporation process is performed by increasing a process temperature to evaporate the liquid core  606  of the sphere  700 . During the evaporation process, the liquid core  606  vaporized in gas state moves from inside the solid shell  702  to outside the solid shell  702  through pores  800  of the solid shell  702 , as shown in  FIG. 8C . In some embodiment, the liquid core  606  vaporized in a gas state effuses through the pores  800  of the solid shell  702 . In some embodiments, the dry process is performed by using flowing gas, such as flowing air, through the pores  800  to dry the liquid core  606  of the sphere  700 . In addition, in some embodiments of the evaporation process, after most of the liquid core  606  is removed due to evaporation, the rest of the liquid core  606  left within the solid shell  702  can further be removed by the dry process. 
     Referring to  FIG. 9A , after the liquid core  606  of the sphere  700  is removed, the remaining solid shell  702  forms a spherical shell  900  with a hollow structure. Specifically speaking, the spherical shell  900  includes the solid shell  702  and gas inside the solid shell  702  as shown in  FIG. 9B . The gas varies with an environment of processes. In some embodiments, the spherical shell  900  includes the solid shell  702  and air inside the solid shell  702 . 
     Referring to  FIG. 4 , the method  400  continues with operation  410  and the process of collecting the spherical shell. For example, the spherical shell  900  (shown in  FIG. 9B ) is collected. In some embodiments, the collected spherical shell  900  is further screened for particular sizes. For example, a sieve is used to screen the spherical shell  900 . In some embodiments, the spherical shell  900  with a diameter less than about 10 μm is collected by screening. The following operation can be operation  204  in the method  200  after the spherical shell  900  is collected. In some embodiments, the method  400  can be an application of microencapsulation technique. Based on the disclosure herein, other operations, instruments, or systems used in a similar concept of the method  400  are within the scope and spirit of this disclosure. 
     Based on embodiments of the present disclosure, a spherical shell with a hollow structure formed by the method  300  or the method  400  can define a size of bubbles disposed in a solder mask layer in the later process. In another words, a diameter of the spherical shell substantially equals to a diameter of the bubbles. As a result, the size of bubbles can be adjusted by controlling process parameters of forming the spherical shell. Furthermore, the spherical shell and the solder mask layer are formed with the same solder mask material so that the solder mask layer excluding bubbles can be homogenous. That is, except for a boundary between the bubbles and the solder mask material, no other boundaries are observed within the solder mask layer. 
     The foregoing outlines mixing the spherical shell into the solder mask material and further forming the package structure having the solder mask layer with bubbles. With bubbles (e.g., air) disposed within the solder mask layer, the Dk and the Df of the solder mask layer can be reduced. 
     Although the present disclosure has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present disclosure without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the present disclosure cover modifications and variations of this disclosure provided they fall within the scope of the following claims.