Abstract:
A device ( 10 ) is provided for matching the CTE between substrates ( 12, 14 ), e.g., a semiconductor substrate and packaging material. The first substrate ( 12 ) has a first coefficient of thermal expansion and the second substrate ( 14 ) has a second coefficient of thermal expansion. At least two layers ( 16 ) of liquid crystal polymer are formed between the first substrate ( 12 ) and the second substrate ( 14 ), each layer having a unique coefficient of thermal expansion progressively higher in magnitude from the first substrate ( 12 ) to the second substrate ( 14 ).

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to integrated circuit packaging, and more particularly to attaching an integrated circuit to a substrate. 
   BACKGROUND OF THE INVENTION 
   With the growth of the use of personal communication devices, e.g., cell phones and two way radios, high performance and high frequency packaging materials have increased in importance. Desired characteristics for electronic packaging include high electric and thermal performance, thinness, low weight, small size, high component density, and low cost. 
   When attaching an integrated circuit to a packaging material, e.g., a printed circuit board or a polymer material, it is known that the coefficient of thermal expansion (CTE) of the integrated circuit and the packaging material must be matched. When the CTE of the two materials are matched, the two materials will expand and contract simultaneously over temperature so as to avoid deformities, cracking, detachment, and loss of functionality, especially after a number of temperature cycles. The importance of this matched CTE becomes apparent in many applications having large temperature swings, e.g., automotive electronics. 
   Conventional packages are fabricated from materials such as plastic, Teflon or ceramics. The type of material that is used depends on a number of factors which include frequency of operation, environment and cost. Plastic packages are typically the lowest in cost but may not be suitable for high frequencies of operation or very high temperatures. Applications that require exposure to extreme temperatures will commonly use ceramics. The metallization that can be used will typically differ depending on the packaging material. As the frequency of operation increases, factors such as surface roughness and metal thickness become important. In addition to these factors, as the frequency of operation increases it becomes advantageous to utilize materials that have lower dielectric constants to allow for the implementation so that the final package, with interconnects, will avoid noise or signal loss associated with high speed circuits 
   One known solution is to place the integrated circuit on the substrate and within a hole formed in a liquid crystal polymer material; however, this adds complexity to the manufacturing process. 
   Another known solution involves the formation of a single layer of liquid crystal polymer between two non-liquid crystal polymer substrates; however, this results in layers that will not have as good a performance at a high frequency as liquid crystal polymer. 
   Accordingly, it is desirable to provide a liquid crystal polymer package that matches the CTE of an integrated circuit to that of the packaging material. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention. 
   BRIEF SUMMARY OF THE INVENTION 
   A device is provided for matching the CTE of two substrates. The device comprises a first substrate having a first coefficient of thermal expansion and a second substrate having a second coefficient of thermal expansion. At least two layers of liquid crystal polymer are formed between the first substrate and the second substrate, each layer having a unique coefficient of thermal expansion progressively higher in magnitude from the first substrate to the second substrate. The first and second substrates may comprise a semiconductor substrate and packaging material. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
       FIG. 1  is a graph demonstrating the moisture barrier properties of the material used in an exemplary embodiment of the present invention. 
       FIG. 2  is a cross-sectional view of an exemplary embodiment of the present invention taken along the line  3 - 3  of  FIG. 3 ; and 
       FIG. 3  is a top cross-sectional view taken along the line  2 - 2  of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. 
   Liquid crystal polymer (LCP) combines the properties of polymers with those of liquids and provide superior thermal and electrical properties including low loss, low dielectric constant, and low coefficient of thermal expansion (CTE) characteristics. LCP packages are especially advantageous for RF devices due to their low signal toss and low dielectric constant (3.01 at 1 MHz) over a wide frequency range and superior moisture barrier properties (0.04% water absorption).  FIG. 1  shows that LCP  4  demonstrates superior moisture barrier properties as compared with two other conventional substrate materials, an organic material  6  and polyimide  8 , relative to loss tangent. The dielectric constant is important for RF packaging because it determines the characteristic impedance of the circuitry, which relates to size and to the signal loss of the circuitry. Loss tangent is important and directly related to circuit signal losses and Q factor. Q factor is a figure merit in filters, resonators, and low noise circuits. 
   LCP is an ordered thermoplastic polymer with long stiff molecules that offer an excellent combination of electronic, thermal, mechanical and chemical properties that make them an excellent material choice for electronic packaging. LCPs are highly crystalline materials based on aromatic ring-structured compounds that are very stable after polymerizing. Characteristics of a particular LCP depend on the manufacturer, but exist in a variety of unfilled, glass-filled, mineral-filled, carbon fiber reinforced, and glass fiber-reinforced grades that allow for numerous options in key properties such as the CTE. LCP laminates have a CTE that can be readily matched to that of silicon and other materials. Also, the high moisture and chemical resistance improve LCP performance in unfriendly operating environments, and the low CTE, low dielectric constant, and high dielectric strength make it desirable as circuit board laminates for electronics packaging. Furthermore, LCP has a high moisture barrier which may be used to seal and protect electronic components from high humidity. 
   In an exemplary embodiment and referring to the device  10  of  FIG. 2 , a cross sectional view is shown as viewed along the line  3 - 3  of  FIG. 3 . The CTE of the first substrate  12  is lower than the CTE of the second substrate  14  so that attaching them directly together would cause deformities or cracks, for example, in one or both of the first substrate  12  or second substrate  14 . In general, graded layers  16  of LCP provide a transition in CTE between the CTE of first substrate  12  and the CTE of second substrate  14 . The material for each of the graded layers  16  can be selected, as desired, for a particular first substrate  12  material and a particular second substrate  14 . More particularly, each of the graded layers  16  are selected such that the CTE is “stair-stepped” from the first substrate  12  to the second substrate  14 . 
   More particularly, the graded layers  16  comprise a LCP wherein the CTE of layer  22  adjacent to the first substrate  12  is closely matched to the CTE of the first substrate  12 , but slightly higher. The layer  24  adjacent layer  22  is closely matched thereto; however the CTE of layer  24  is slightly higher than the CTE of layer  22 . Each successive layer of the graded layers has a CTE closely matching that of the adjacent layer, but the CTE of each of the graded layers increases as it gets closer to the second substrate  14 . The CTE of the layer  28  adjacent to the second substrate  14  is closely matched to the CTE of the second substrate  14 . Effectively, this approach reduces the stress build up at any one layer-to-layer interface by distributing it across multiple layer-to-layer interfaces. 
   In the exemplary embodiment shown in  FIG. 2 , one of the first or second substrates  12 ,  14  may comprise, for example, an integrated circuit substrate comprising silicon, but may alternative comprise any type of material used for integrated circuits, such as germanium, silicon/germanium, or a III-V compound. The other of the first or second substrates  12 ,  14  may comprise, for example, packaging material comprising any type of material used for electronic packaging, such as that used for printed circuit boards, a polymer, glass, etc. The thickness of the graded layers  16  may range from 1 mil to 30 mils; however the thickness of the graded layers  16  is dependent upon the frequency needed for a particular application. 
   An example of the thicknesses and CTEs for the first substrate  12 , second substrate  14  and graded layers  16  is illustrated in the chart as follows: 
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
                 
                 
             
             
                 
               MATERIAL 
               THICKNESS (mils) 
               CTE (ppm/C.) 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               first substrate 12 
               24 
               2.6 
             
             
                 
               layer 22 
               2 
               3.5 
             
             
                 
               layer 24 
               2 
               4.4 
             
             
                 
               layer 26 
               2 
               5.3 
             
             
                 
               layer 28 
               2 
               6.2 
             
             
                 
               second substrate 14 
               125 
               7 
             
             
                 
                 
             
           
        
       
     
   
   A via  32  may be formed through the graded layers  16  in a manner known to those in the industry for providing electrical connection between circuitry on the first substrate  12  and circuitry on the second substrate  14 . A via  34  also formed in the graded layers  16  may make electrical contact with circuitry on the integrated substrate by a wire bond  36 . A via  38  may be formed through layers  22  and  24  to terminate at the junction  40  between layers  24  and  26 . Another via  42  may be formed through layers  26  and  28  to also terminate at the junction  40 . 
   Referring to  FIG. 3 , the device  10  is shown as viewed along line  2 - 2  of  FIG. 2 . Functional circuitry  44  is formed on the surface  40  of layer  26  in a manner known to those in the industry. Functional circuitry  44  may comprise any type of electronic circuitry, for example, a microstrip transmission line as shown, a coplanar waveguide, a resistor, an inductor, or filtering structures. A first end  46  of the functional circuitry  44  is connected to the via  42  and a second end  48  would be connected to the via  38 . 
   LCP layers would be manufactured in sheet form using standard processes known to the industry. A selection of off-the-shelf and/or customized CTE LCP layers would be made for a particular application. The layers would be laminated using interleaved adhesive layers or alternating single sided metalized LCP layers, or other standard method in conjunction with the proper heat and pressure to achieve proper bonding. 
   While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.