Abstract:
An interposer for electrical connection between a CPU chip and a circuit board is provided. The interposer includes a board-shaped base substrate made of glass having a coefficient of thermal expansion ranging from 3.1×10 −6 /K to 3.4×10 −6 /K. The interposer further includes a number of holes having diameters ranging from 20 μm to 200 μm. The number of holes ranging from 10 to 10,000 per square centimeter. Conductive paths running on one surface of the board extend right into respective holes and therethrough to the other surface of the board in order to form connection points for the chip.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to interposers for electrically connecting the terminals of a CPU chip and a circuit board, and further relates to methods used in a critical manufacturing step of interposers. 
       BACKGROUND OF THE INVENTION 
       [0002]    A CPU chip, as a processor core, typically has several hundred contact points on its bottom surface, which are closely spaced to one another and distributed over a relatively small area. Due to this close spacing, these contact points cannot be mounted directly to a circuit board, the so called motherboard. Therefore, an intermediate part is employed which permits to enlarge the connection base. As an intermediate part, often, a glass fiber mat encased in epoxy material is employed, which is provided with a number of holes. Conductive paths running on one surface of the glass fiber mat extend into respective holes to fill them and to lead to the terminals of the processor core at the other surface of the glass fiber mat. For this purpose, an underfill is applied both around the processor core and between the processor core and the glass fiber mat, which protects the wires and mechanically joins the processor core and the glass fiber mat. However, the processor core and the glass fiber mat exhibit different thermal expansions. For example, the glass fiber mat has an expansion coefficient from 15 to 17×10 −6 /K, while the silicon-based core processor has a thermal expansion factor from 3.2 to 3.3×10 −6 /K. Therefore, in case of heating there are differential expansions between the core processor and the glass fiber mat and hence mechanical stresses arise between these two components. This can be detrimental to the contact connections, especially when the two components are not completely joined face to face. In this case the contact points may break easily. 
         [0003]    Another drawback of using a glass fiber mat is related to the mechanical drilling of holes into the glass fiber mat. The hole diameter is limited to 250 to 450 μm. 
         [0004]    Another possibility of designing and manufacturing connecting structures which could be used as a type of interposer is described in WO 02/058135 A2. Wafer technology is employed, including the generation of holes and trenches in dielectric material, such as silicon dioxide, and filling of the holes and trenches with conductive layers. However, this method of producing contact connections is very expensive. 
         [0005]    A similar technology is taught in DE 103 01 291 B3. Recesses are etched into substrates and filled with metal conductive paths, and contacts extending through holes are also provided. This technique is complex and expensive. 
         [0006]    U.S. 2002/0180015 A1 discloses a multi-chip module which includes semiconductor devices and a wiring substrate for mounting the semiconductor devices. The wiring substrate comprises a glass substrate having holes which were formed by a sand blasting treatment. A wiring layer is formed on the surface of the glass substrate. Furthermore, the glass substrate has wirings and an insulation layer. It is aimed at selecting the coefficient of thermal expansion of the glass substrate close to the coefficient of silicon. 
         [0007]    U.S. Pat. No. 5,216,207 discloses ceramic multilayer circuit boards including silver conductors. The layers are fired at low temperatures. The circuit boards have a coefficient of thermal expansion close to that of silicon. 
         [0008]    U.S. 2009/0321114 A1 discloses an electrical testing substrate unit including a multilayer ceramic substrate. Although the materials used have a coefficient of thermal expansion close to the value of the invention, they are not pure glasses. 
         [0009]    U.S. Pat. No. 7,550,321 B1 discloses a substrate having a coefficient of thermal expansion with a gradient in the thickness direction. 
         [0010]    The paper “Femtosecond laser-assisted three-dimensional microfabrication in silica” in Optics Letters, Vol. 26, No. 5, Mar. 1, 2001, pages 277-279 describes direct three-dimensional microfabrication in a silicate glass. The fabrication process is accomplished in two steps. First, the intended patterns are mapped out in the glass using focused femtosecond laser pulses. Then these patterns are etched. 
       GENERAL DESCRIPTION OF THE INVENTION 
       [0011]    An object of the invention is to provide an interposer for electrical connection between a CPU chip and a circuit board, which is economical to produce and enables to produce microholes with a hole diameter in the order of 20 μm and 200 μm, and wherein the interposer body exhibits a thermal expansion similar to that of the CPU chip material. The novel interposer should be able to meet the following requirements: 
         [0012]    Multiple small holes (10 to 10,000) are to be accommodated in each interposer, with close tolerances of the holes to each other. It has to be possible to ensure a hole spacing down to 30 μm. Hole diameters down to a size of 20 μm should be possible. A ratio of the thickness of the interposer to the hole diameter, the so-called aspect ratio, from 1 to 10 should be possible. A center-to-center distance of the holes in a range from 120 μm and 400 μm should be possible. 
         [0013]    The hole should have a conical or crater-shaped inlet and outlet to the hole, but the inner walls of the hole in the center should be of cylindrical shape. The hole should have smooth walls (fire-polished). Optionally, a bead may be produced around the edge of the hole, having a height of not more than 5 mm. 
         [0014]    The interposer according to the invention is characterized in that its board-shaped base substrate is made of glass having a coefficient of thermal expansion ranging from 3.1×10 −6 /K to 3.4×10 −6 /K. Silicon-based chip boards exhibit an expansion coefficient between 3.2×10 −6 /K and 3.3×10 −6 /K. Therefore, large mechanical stresses between the interposer and the CPU chip due to different thermal expansion behavior are not to be expected. 
         [0015]    The number of holes in the interposer is selected according to the particular requirements and may amount up to 10,000 holes per cm 2 . A usual number of holes ranges from 1000 to 3000. The center-to-center spacing of the holes ranges from 50 μm to 700 μm. To meet the requirements of miniaturization of components, holes are provided which have a diameter ranging from 20  682  m to 200 μm. To establish an electrical connection between the CPU chip and its circuit board, conductive paths extend on one of the surfaces of the interposer board to and into the holes and therethrough to form connection points for the CPU chip. 
         [0016]    The glass of the base substrate should have an alkali content of less than 700 ppm. Such a glass has a low coefficient of thermal expansion, as required, and exhibits very good signal-insulating properties, due to the high dielectric value. Furthermore, the risk of contamination of silicon processors with alkalis is largely avoided. 
         [0017]    For reasons of environmental protection, an arsenic or antimony content of the glass composition is less than 50 ppm. 
         [0018]    Interposer boards have a thickness of less than 1 mm, but not below 30 μm. The number of holes of an interposer is chosen according to the needs, and is in the order from 1000 to 3000 holes per cm 2 . The invention targets to offer interposers on the market having microholes smaller than 100 p.m. Hence, the holes are closely packed, with a center-to-center distance of the holes that may range from 150 μm to 400 μm. However, the edge-to-edge distance of the holes should not be less than 30 μm. The holes need not all have the same diameter, it is possible that holes of different diameters are provided in the board-shaped base substrate. The ratio of the thickness of the glass board to the hole diameter, the so-called aspect ratio, may be selected in a wide range from 0.1 to 25, an aspect ratio from 1 to 10 being preferred. The holes generally have a thin cylindrical shape, but may have rounded-broken edges at the inlet and outlet of the hole. 
         [0019]    In order to accurately position the holes, which may have a diameter ranging from 20 μm to 200 μm, focused laser pulses are used in a wavelength range of transparency of the glass, so that the laser beams penetrate into the glass and are not already absorbed in the surface layers of the glass. The laser radiation used has a very high radiation intensity, so as to result in local non-thermal destruction of the glass along filamentary channels. These filamentary channels are subsequently widened to the desired diameter of the holes, for which purpose dielectric breakdowns may be employed which cause electro-thermal heating and evaporation of the material of the hole edges, and/or the filamentary channels are widened by supplying reactive gases. 
         [0020]    It is also possible to precisely mark the intended perforation points using RF coupling material which is printed onto the base substrate in form of dots. Such marked points are heated by RF energy to lower the breakdown strength to electrical high voltage in the region of the intended holes, and to finally cause dielectric breakdowns at these points. The perforations may be widened by supplied etching gas. 
         [0021]    The manufacturing of the conductive paths on the board-shaped glass substrate and through the holes is accomplished by known method patterns and need not be further described here. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Exemplary embodiments of the invention will be described with reference to the drawings, wherein: 
           [0023]      FIG. 1  schematically illustrates, in a longitudinal sectional view, one way of producing an interposer; and 
           [0024]      FIG. 2  illustrates a second way of producing an interposer. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    In a first method step, perforation points  10  on a board-shaped glass substrate  1  are marked by focused laser pulses  41  emanating from an array  4  of lasers  40 . The radiation intensity of these lasers is so strong that it causes local non-thermal destruction in the glass along a filamentary channel  11 . 
         [0026]    In a second method step, filamentary channels  11  are widened into holes  12 . For this purpose, opposing electrodes  6  and  7  may be employed, to which high voltage energy is applied, resulting in dielectric breakdowns across the glass substrate along filamentary channels  11 . These breakthroughs are widened by electro-thermal heating and evaporation of the perforation material until the process is stopped by switching off the power supply when the desired hole diameter is achieved. 
         [0027]    Alternatively or additionally, the filamentary channels  11  may be widened using reactive gases, as illustrated by nozzles  20 ,  30 , which direct the gas to the perforation points  10 . 
         [0028]    In the next method step, conductive paths  13  to the perforation points  10  are applied on the upper surface of glass board  1 , and the holes  12  are filled with conductive material  14  to complete the connections to the contact points of a CPU chip or the like at the bottom surface of the board. (For mounting on the motherboard, glass board  1  is turned around.) 
         [0029]      FIG. 2  shows another way of producing microholes. Perforation points  10  are marked by precisely imprinted RF coupling material. At these points  10  high frequency energy is applied by means of electrodes  2 ,  3 , so that the coupling points themselves and the glass material between the upper surface coupling points and the lower surface coupling points is heated, causing the dielectric strength of the material to be lowered. When a high voltage is applied, dielectric breakdowns will occur along narrow channels  11 . By continuing the supply of high-voltage energy, these narrow channels  11  may be widened to the size of holes  12 . 
         [0030]    However, it is also possible, for widening the narrow channels  11  resulting from dielectric breakdowns, to use reactive gas which is supplied through nozzles  20 ,  30 . 
         [0031]    Finally, conductive paths  13  to the holes  12  are applied on the upper surface of the glass substrate, and the holes are filled with conductive material  14  in order to establish the connections for the CPU chip, with the glass board  1  turned around. 
         [0032]    It should be noted that interposers need not to be produced separately, rather glass substrate boards for a plurality of interposers may be processed, and the large-sized glass substrate boards may be cut to obtain the individual interposers. Glass substrate boards of a size with edge lengths of 0.2 m by 3 m (or less) can be processed. Round board formats may have dimensions of up to 1 m. 
       EXEMPLARY EMBODIMENTS 
       [0033]    Glasses were melted at 1620° C. in Pt/Ir crucibles from conventional, essentially alkali-free raw materials, apart from unavoidable impurities. The melt was refined for one and a half hour at this temperature, then poured into inductively heated platinum crucibles and stirred for 30 minutes at 1550° C. for homogenization. 
         [0034]    The table shows fifteen examples of suitable glasses and their compositions (in wt. % based on oxide) and their main features. The refining agents SnO 2  (Examples 1-8, 11, 12, 14, 15) and As 2 O 3  (Examples 9, 10, 13) with a proportion of 0.3 wt. % is not listed. The following properties are specified:
       the coefficient of thermal expansion α 20/300  (10 −6  1K)   the density ρ 60  (g/cm 3 )   the dilatometric glass transition temperature T g  (°C.) according to DIN 52324   the temperature at a viscosity of 10 4  dPa.s (referred to as T4 [°C.])   the temperature at a viscosity of 10 2  dPa.s (referred to as T2 [°C.]), calculated from the Vogel-Fulcher-Tammann equation   an “HCl” acid resistance as weight loss (material removal value) of glass boards measuring 50 mm×50 mm×2 mm and polished on all sides after treatment with 5% hydrochloric acid for 24 hours at 95° C. (mg/cm 2 ).   a “BHF” resistance to buffered hydrofluoric acid as a weight loss (material removal value) of glass boards measuring 50 mm×50 mm×2 mm and polished on all sides after treatment with 10% NH 4 F NH4F.HF for 20 min at 23° C. (mg/cm 2 )   the refractive index n d .       
 
       EXAMPLES 
       [0043]    Compositions (in wt. % based on oxide), and essential properties of glasses according to the invention 
         [0000]    
       
         
               
               
               
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
             
             
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
               
               
                   
               
               
                 SiO 2   
                 60.0 
                 60.0 
                 59.9 
                 58.9 
                 59.9 
                 61.0 
               
               
                 B 2 O 3   
                 7.5 
                 7.5 
                 7.5 
                 8.5 
                 7.5 
                 9.5 
               
               
                 Al 2 O 3   
                 21.5 
                 21.5 
                 21.5 
                 21.5 
                 21.5 
                 18.4 
               
               
                 MgO 
                 2.9 
                 2.9 
                 2.0 
                 2.0 
                 2.9 
                 2.2 
               
               
                 CaO 
                 3.8 
                 2.8 
                 3.8 
                 3.8 
                 4.8 
                 4.1 
               
               
                 BaO 
                 4.0 
                 5.0 
                 5.0 
                 5.0 
                 3.1 
                 4.5 
               
               
                 ZnO 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 α 20/300  (10 −6  /K) 
                 3.07 
                 3.00 
                 3.01 
                 3.08 
                 3.13 
                 3.11 
               
               
                 ρ (g/cm 3 ) 
                 2.48 
                 2.48 
                 2.48 
                 2.48 
                 2.47 
                 2.45 
               
               
                 Tg (° C.) 
                 747 
                 748 
                 752 
                 741 
                 743 
                 729 
               
               
                 T 4(° C.) 
                 1312 
                 1318 
                 1315 
                 1308 
                 1292 
                 1313 
               
               
                 T 2 (° C.) 
                 1672 
                 1678 
                 1691 
                 1668 
                 1662 
                 1700 
               
               
                 n d   
                 1.520 
                 1.518 
                 1.519 
                 1.519 
                 1.521 
                 1.515 
               
               
                 HCl (mg/cm 2 ) 
                 1.05 
                 n.m. 
                 0.85 
                 n.m. 
                 1.1 
                 n.m. 
               
               
                 BHF (mg/cm 2 ) 
                 0.57 
                 0.58 
                 0.55 
                 0.55 
                 0.56 
                 0.49 
               
               
                   
               
               
                   
                 7 
                 8 
                 9 
                 10 
                 11 
                 12 
               
               
                   
               
               
                 SiO 2   
                 58.5 
                 62.8 
                 63.5 
                 63.5 
                 59.7 
                 59.0 
               
               
                 B 2 O 3   
                 7.7 
                 8.2 
                 10.0 
                 10.0 
                 10.0 
                 9.0 
               
               
                 Al 2 O 3   
                 22.7 
                 16.5 
                 15.4 
                 15.4 
                 18.5 
                 17.2 
               
               
                 MgO 
                 2.8 
                 0.5 
                 2.0 
                 1.0 
                   
                 2.0 
               
               
                 CaO 
                 2.0 
                 4.2 
                 5.6 
                 6.6 
                 8.3 
                 9.0 
               
               
                 BaO 
                 5.0 
                 7.5 
                 3.2 
                 3.2 
                 3.2 
                 3.5 
               
               
                 ZnO 
                 1.0 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 α 20/300  (10 −6 /K)  
                 2.89 
                 3.19 
                 3.24 
                 3.34 
                 3.44 
                 3.76 
               
               
                 ρ (g/cm 3 ) 
                 2.50 
                 2.49 
                 2.42 
                 2.43 
                 2.46 
                 2.50 
               
               
                 Tg (2° C.) 
                 748 
                 725 
                 711 
                 719 
                 714 
                 711 
               
               
                 T 4 (2° C.) 
                 1314 
                 1325 
                 1320 
                 1327 
                 1281 
                 1257 
               
               
                 T 2 (2° C.) 
                 1674 
                 1699 
                 n.m. 
                 n.m. 
                 1650 
                 1615 
               
               
                 n d   
                 1.520 
                 1.513 
                 1.511 
                 1.512 
                 1.520 
                 1.526 
               
               
                 HCl (mg/cm 2 ) 
                 n.m. 
                 0.30 
                 0.89 
                 n.m. 
                 n.m. 
                 0.72 
               
               
                 BHF (mg/cm 2 ) 
                 0.62 
                 0.45 
                 0.43 
                 0.40 
                 0.44 
                 0.49 
               
               
                   
               
             
          
           
               
                   
                 13 
                 14 
                 15 
               
               
                   
               
               
                 SiO 2   
                 61.4 
                 59.5 
                 63.9 
               
               
                 B 2 O 3   
                 8.2 
                 10.0 
                 10.4 
               
               
                 Al 2 O 3   
                 16.0 
                 16.7 
                 14.6 
               
               
                 MgO 
                 2.8 
                 0.7 
                 2.9 
               
               
                 CaO 
                 7.9 
                 8.5 
                 4.8 
               
               
                 BaO 
                 3.4 
                 3.8 
                 3.1 
               
               
                 ZnO 
                 — 
                 0.5 
                 — 
               
               
                   α20/300  (10 −6 /K) 
                 3.75 
                 3.60 
                 3.21 
               
               
                 ρ (g/cm 3 ) 
                 2.48 
                 2.48 
                 2.41 
               
               
                 Tg (2° C.) 
                 709 
                 702 
                 701 
               
               
                 T 4 (2° C.) 
                 1273 
                 1260 
                 1311 
               
               
                 T 2 (2° C.) 
                 1629 
                 1629 
                 n.m. 
               
               
                 n d   
                 1.523 
                 1.522 
                 n.m. 
               
               
                 HCl (mg/cm 2 ) 
                 0.41 
                 0.97 
                 n.m. 
               
               
                 BHF (mg/cm 2 ) 
                 0.74 
                 0.47 
                 n.m. 
               
               
                   
               
               
                 n.m. = not measured 
               
             
          
         
       
     
         [0044]    As the exemplary embodiments illustrate, the glasses have the following advantageous properties:
       a thermal expansion α 20/300  of between 2.8×10 −6 /K and 3.8×10 −6 /K, in preferred embodiments 3.6×10 −6 /K, in particularly preferred embodiments &lt;3.2×10 −6 /K, and thus matched to the expansion behavior of both amorphous silicon and also increasingly polycrystalline silicon.   with T g &gt;700° C., a high glass transition temperature, i.e. a high temperature resistance. This is essential for a lowest possible production-related shrinkage (“compaction”) and for use of the glasses as substrates for coatings of amorphous Si layers and subsequent annealing thereof.   with ρ&lt;2.600 g/cm 3 , a low density   a temperature at a viscosity of 10 4  dPa.s (working point V A ) of not more than 1350° C., and a temperature at a viscosity of 10 2  dPa.s of not more than 1720° C., which is a suitable viscosity characteristic in terms of hot-shaping and meltability.   with n d ≦1.526 a low refractive index.   a high chemical resistance, as is evident inter alia from good resistance to buffered hydrofluoric acid solution.       
 
         [0051]    The glasses exhibit high thermal shock resistance and good devitrification stability. The glasses can be produced as flat glasses by various drawing methods, e.g. microsheet down-draw, up-draw, or overflow fusion methods, and, in a preferred embodiment, if they are free of As 2 O 3  and Sb 2 O 3 , also by the float process. 
         [0052]    With these properties, the glasses are highly suitable for use as substrate glass for producing interposers. 
         [0053]    By using the base substrate of low-alkali glass and with a coefficient of thermal expansion very close to that of the chip of silicon material, difficulties resulting from different thermal expansions of the interposer and the CPU chip are largely avoided. If adjacent joint material layers or boards have an only slightly different heating behavior and a slightly different coefficient of thermal expansion, there will be fewer mechanical stresses between these joint layers or boards, and there will be no warpage or cracking between the layers or boards. 
         [0054]    Interposers which are occupied more densely with holes as compared to previous interposers, take smaller substrate sizes, thereby still further reducing the amount of different expansions and contractions of the involved layers or boards and thus the risk of warpage and hence cracking between the involved layers or boards. 
         [0055]    Finally, cost savings can also be expected because (with reduced interposer size and hole size) less glass material and less conductive material for filling the holes has to be used.