Abstract:
A semiconductor package substrate for assembling an integrated circuit chip operable at fast ramp rate signals and clock rates, comprising an insulating support having a region for attaching said chip; a pattern of electrical interconnections, disposed on said substrate in one metallization level and operable for transmitting waveforms; and a low pass filter for removing unwanted high frequency components from said transmitted waveforms, comprising a network of inductors and capacitors formed within said one metallization level and positioned substantially within said substrate region for chip attachment.

Description:
This application claims benefit provisional application Ser. No. 60/154,313 filed Sep. 16, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related in general to the field of semiconductor devices and processes and more specifically to structure, materials and fabrication of high performance plastic ball grid array packages designed for very high frequency operation. 
     DESCRIPTION OF THE RELATED ART 
     Ball Grid Array (BGA) packages have emerged as an excellent packaging solution for integrated circuit (IC) chips with high input/output (I/O) count. BGA packages use sturdy solder balls for surface mount connection to the “outside world” (typically plastic circuit boards, PCB) rather sensitive package leads, as in Quad Flat Packs (QFP), Small Outline Packages (SOP), or Tape Carrier Packages (TCP). Some BGA advantages include ease of assembly, use of surface mount process, low failure rate in PCB attach, economic use of board area, and robustness under environmental stress. The latter used to be true only for ceramic BGA packages, but has been validated in the last few years even for plastic BGAs. From the standpoint of high quality and reliability in PCB attach, BGA packages lend themselves much more readily to a six-sigma failure rate fabrication strategy than conventional devices with leads to be soldered. 
     A BGA package generally includes an IC chip, a multi-layer substrate, and a heat spreader. The chip is generally mounted on the heat spreader using a thermally conductive adhesive, such as an epoxy. The heat spreader provides a low resistance thermal path to dissipate thermal energy, and is thus essential for improved thermal performance during device operation, necessary for consistently good electrical performance. Further, the heat spreader provides structural and mechanical support by acting as a stiffener, adding rigidity to the BGA package, and may thus be referred to as a heat spreader/stiffener. 
     One of the substrate layers includes a signal “plane” that provides various signal lines, which can be coupled, on one end, to a corresponding chip bond pad using a wire bond (or to a contact pad using flip-chip solder connection). On the other end, the signal lines are coupled with solder “balls” to other circuitry, generally through a PCB. These solder balls form the array referred to in a BGA. Additionally, a ground plane will generally be included on one of the substrate layers to serve as an active ground plane to improve overall device performance by lowering the inductance, providing controlled impedance, and reducing cross talk. These features become the more important the higher the BGA pin count is. 
     In contrast to the advantages of the BGA packages, prevailing solutions in BGA packages have lagged in performance characteristics such as the ability to maintain signal integrity in high speed operation necessary for devices such as high speed digital signal processors (DSP) and mixed signal products (MSP). One major source of signal distortion derives from irregular, unwanted high-frequency radiation originating in the package substrates; another major source is a specific consequence of a BGA manufacturing method aiming at low-cost package fabrication. 
     Normally, packages are designed to minimize inductance and capacitance on the signal and clock traces to allow the transmission of waveforms with minimum distortions. An example for BGA packages can be found in U.S. Patent Application Serial No. 60/147,596 filed Aug. 6, 1999 (Lamson et al., “Structure and Method of High Performance Two Layer Ball Grid Array Substrate”). In some cases, where the signal or clock rise times are fast enough, some radio frequency (rf) radiation will occur in the IC within the package and/or the related circuit board. 
     Within the confined space of a semiconductor package, there is not enough room to accommodate additional external components, or to assemble a multi-level structure as a low pass filter. The latter idea was proposed in U.S. Pat. No. 5,668,511, issued on Sep. 16, 1997 (Furutani et al., “Low Pass Filter”). 
     The layout of high performance ICs is typically so crowded that silicon real estate cannot be freed up to design the components of a low pass filter. This makes the implementation difficult for integrated solutions as described in U.S. Pat. No. 4,176,318, issued on Dec. 29, 1987 (Koike, “Low Pass Filter Formed in an Integrated Circuit”). 
     BGA substrate fabrication is under economical pressure to keep manufacturing costs to a minimum. Consequently, batch fabrication techniques are preferred, which deposit interconnection lines to the desired thickness on insulating substrates directly “where wanted”. For metal deposition, a very cost-effective method for batch deposition is plating, provided that it is uniform and controlled. In the BGA substrate fabrication, all lines are electrically coupled together for the period of plating deposition, and separated after completion. When this separation step is executed as a mechanical cut, it is unavoidable that small pieces of metal lines are leftover “dangling” as loose ends. In device operation, these loose ends are available to act as unwanted radiating antennas. 
     Consequently, a serious device signal distortion problem has arisen in high frequency devices with a need to reduce radiation in the IC. An urgent need has therefore appeared to conceive a concept for avoiding the signal distortion without interfering with the low-cost substrate manufacturing practices. Preferably, this concept should be based on a fundamental design solution flexible enough to be applied for different semiconductor product families and a wide spectrum of design and assembly variations. It should not only meet high electrical and thermal performance requirements, but should also achieve improvements towards the goals of enhanced process yields and device reliability. Preferably, these innovations should be accomplished using the installed equipment base so that no investment in new manufacturing machines is needed. 
     SUMMARY OF THE INVENTION 
     According to the present invention for integrated circuit chip packages, a low pass filter, comprising a network of inductors and capacitors, for removing unwanted high frequency components from fast ramp rate signals and clock lines is provided so that it is integral to the package substrate, located substantially in the substrate region intended for chip attachment, and can be created by a single level of metallization. 
     A chip output signal with fast rise time (plotted in the form of voltage as a function of time; voltage and time on linear scale) can be subjected to Fourier analysis. The resultant frequency distribution can be analyzed with regard to its energy content (plotted in the form of power as function of frequency; power on exponential scale, frequency linear scale). This output signal power distribution shows a rapid fall-off towards higher frequency. 
     When chip signals exhibit faster and faster rise times, at constant overall signal power, the output power distribution displays an increase of the power content for higher frequencies. In order to reduce this high frequency power component back to original levels, a low pass filter is added to the IC package design. The invention describes a layout integral to the package substrate without the need for additional individual components or extra metallizations. The design provides passive filtering for fast ramp rate signals and clock lines to remove high frequency components from the transmitted waveform and thus to reduce the power share of the higher frequencies. 
     It is an aspect of the present invention to use a series connection of narrow and wide traces in the layout of the package substrate to create controlled inductances and capacitances to filter out high frequency components of transmitted signal or clock. 
     Another aspect of the invention is to provide design concepts suitable for semiconductor package substrates, especially ball grid array substrates, as well as generally for circuit boards. In all cases, the low pass filter is laid out so that it substantially fits into the region allotted to the chip attachment so that not extra substrate area needs to be consumed. 
     Another aspect of the invention is to provide design concepts for ball grid array substrates applicable to both wire bonded and flip-soldered chip assemblies. 
     Another aspect of the invention is to use only the standard metallization process flow without the need of additional process steps, multi-level structures, or electronic components, such as external capacitors and inductors, to fabricate the device package. The area requirements of the package substrate, and the thickness constraints of the package do not have to be expanded in order to accommodate the filter network according to the invention. 
     Another aspect of the invention is to utilize existing semiconductor fabrication processes and to reach the electrical device goals without the cost of equipment changes and new capital investment, by using the installed fabrication equipment. 
     Another aspect of the invention is to provide design and fabrication solutions such that they are flexible enough to be applied for different semiconductor high-performance device families and a wide spectrum of high speed, high power design and assembly variations. 
     These aspects have been achieved by the electrical design of the capacitance, inductance, and resistance network for bonding wire assembled IC chips in 144-ball grid array devices. As an example, at 500 MHz the output power has been attenuated from −20 dB to −25 dB. A reduction by 3 or 4 dB is sufficient. 
     The technical advances represented by the invention, as well as the aspects thereof, will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the metal connection layout on the surface of a ball grid array substrate before the integrated circuit chip is attached. 
     FIG. 2 is a simplified and schematic cross section of a portion of a ball grid array package substrate after attaching and wire bonding the integrated circuit chip. 
     FIG. 3 illustrates the output waveform from the integrated circuit, plotted as voltage vs. time. 
     FIG. 4 illustrates the output power distribution from the integrated circuit vs. frequency. 
     FIG. 5 is a block diagram of a low pass filter used by the invention. 
     FIG. 6 illustrates the filtered output waveform on the basis of the invention, plotted as voltage vs. time. 
     FIG. 7 illustrates the filtered output power distribution from the integrated circuit vs. frequency on the basis of the invention. 
     FIG. 8 is a simplified and schematic cross section of a portion of a ball grid array package substrate integral with low pass filter located in the substrate area for chip attachment according to the invention. 
     FIG. 9 shows the metal connection and low pass filter layout on the surface of a ball grid array substrate according to the invention, before the integrated circuit is attached. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is the top view of the substrate generally designated  100  of a ball grid array (BGA) package for a semiconductor device requiring  144  connections to the “outside world”. The BGA device belongs the large family of high performance high input/output (I/O) integrated circuit (IC) devices. The insulating layer  101  of the substrate is typically made of organic material selected from a group consisting of polyimide, polymer strengthened by glass fibers, FR- 4  (an epoxy resin), FR- 5 , and BT resin. Alternative materials are cyanate ester resins reinforced with a woven glass cloth. The insulating layer preferably has a thickness in the range from about 70 to 150 μm and a dielectric constant between 4 and 5. Suitable materials are commercially available from Sheldahl Micro Products. Longmont, Colo., 80503 USA; Motorola Inc., USA; Shinko Corp., Japan; or Ibiden Corp., Japan. 
     The dotted line  102  in FIG. 1 encloses the area reserved for attaching the IC chip. The small rectangular areas  103  are the metallization pads for attaching the stitch bonds when the chip terminals are electrically connected to the substrate  101  by bonding wires. Commonly, these stitch pads comprise copper layers, or copper with gold flash surface, or refractory metal layers followed by copper layers with gold flash. Alternatively, they may comprise copper layers followed by nickel layers of a silver flash. The metal pads  103  preferably have a thickness in the range of about 7 to 15 μm. 
     The contact areas  104  serve as terminals for the metal-filled vias through the insulating layer. On the opposite side of the vias (shown in FIG.  2 ), counter-terminals serve as attachment sites for the solder balls of the BGA package. Connecting lines  105  connect contact areas  104  to bond stitch sites  103 . Typically, these metal areas and connections are made of copper with gold flash; thickness commonly ranges from about 7 to 15 μm. A convenient and low-cost manufacturing process flow consists of depositing a thin “seed” metallization, defining interconnections and pads by photolithography, and depositing the desired metal thickness in the subsequent plating step. 
     The popular preference for a plating process has an important consequence for a problem solved by the present invention. Successful plating control requires uniform electrical potential applied to all areas intended to receive equal amounts of plating material. The best way to insure uniform deposition is to couple all areas-to-be-plated electrically together. After completing the plating process, the connecting lines can be severed. Unfortunately, numerous line segments will be left over as useless “dangling dead ends”. Connecting lines  106  in FIG. 1 are examples of such leftovers from this plating interconnection need. In IC operation, these leftover lines  106  can act as antennas radiating unwanted high frequency radiation. 
     FIG. 2 is a cross section along line  110  through a portion of the substrate in FIG. 1 after the IC chip has been assembled. However, the cross section is simplified and schematic. The IC chip  201  is attached by adhesive polymer  202  to insulating support layer  203 . The semiconductor chip  201  is usually made of silicon, but could be silicon germanium, gallium arsenide, or any other semiconductor material used in electronic device production. In the case of silicon ICs, the thickness of the chip is typically 225 to 475 μm, and the chips may belong to product families such as digital signal processors, mixed signal devices, analog and logic circuits, application-specific ICs, and many other IC components. Attach material  202  is commonly an epoxy, sometimes silver-filled, in the thickness range from about 20 to 30 μm; it may also be a polyimide material. 
     Bonding wires  204  connect the chip terminals to the substrate. In standard bonding operation, ball bonds  205  are formed on the circuit bonding pads, and stitch bonds  206  are formed on the substrate metallization. Typically, standard round wire of diameter between about 18 to 33 μm is used; the ball has a typical diameter from about 1.2 to 1.6 wire diameters. The metallurgical stitch bond is formed by an imprint of the capillary used in wire bonding (about 1.5 to 3 times the wire diameter), and contact pad  207  can have small area (corresponds to  103  in FIG. 1, metal composition and thickness as mentioned above). 
     Contact pads  207  are connected by metal lines  208  to contact lands  209 . These metallizations are usually created in a single photolithographic process step from a single metal deposition layer. Contact lands  109 , in turn, serve as terminals for the metal-filled vias  210  through the insulating layer  203 . On the opposite side of vias  210  are the counter-terminals  211 . These counter-terminals are made of solderable metal, usually copper and/or nickel with flash of gold or palladium; sometimes a flash of platinum is used, when metal interdiffusion in the solder process is to be minimized. 
     Attached to counter-terminals  211  are solder balls  212 . As defined herein, the term solder “balls” does not imply that the solder contacts are necessarily spherical. They may have various forms, such as semispherical, half-dome, truncated cone, or generally bump. The exact shape is a function of the deposition technique (such as evaporation, plating, or pre-fabricated units) and reflow technique (such as infrared or radiant heat), and the material composition. The solder balls comprise attach materials selected from a group consisting of tin/lead, tin/indium, tin/silver, tin/bismuth, solder paste, and conductive adhesive compounds. 
     As a comparison of FIG. 2 with FIG. 1 shows, some connecting metal lines  208  from stitch pads to contact lands are short (for instance, to contact land  209 ), others have to be designed considerably longer. For example, the connecting lines to contact lands  219  and  229  have to be laid out longer than  208 . As pointed out above, these metallizations are created together by the process steps of metal seeding, metal patterning, and metal plating. The connecting lines needed for applying the common electrical potential during the plating step are severed, line segments such as  213  in FIG. 2 are left over as “dangling dead ends”. Since these line segments  213  can act as antennas radiating unwanted high frequency radiation during IC operation, the low pass filter of the present invention must attenuate this unwanted radiation in order to preserve fast ramp rate signal and clock lines of the IC. 
     FIG. 3 illustrates an example of an ideal signal output from the IC, plotted as voltage ramp (in V) versus time (in s×10E-8). In signal portion  301 , the signal increases linearly and uniformly; after reaching its maximum value at  303 , it remains constant for the length of signal portion  302 . It is common practice to obtain signal plots like FIG. 1 from calculations using a “Simulation Program with Integrated Circuit Emphasis” (SPICE) as provided by the University of California at Berkeley, USA, for use with IC designs. 
     The IC output signal of FIG. 3 can be subjected to Fourier analysis. The resultant frequency distribution can be plotted in the frequency domain as a function of its energy, or power, content. In FIG. 4, the output power distribution (expressed in dB) is plotted as a function of frequency (in Hz). Curve  401  shows a rapid fall-off with increasing frequency; most power is concentrated in the frequency regime less than 0.5×10E8 Hz (curve  402  in FIG.  4 ). It should be noted that the frequency regime around 9×10E8 Hz is still represented by about −23 dB (curve  403  in FIG.  2 ), a power generally too high for electronic operation and therefore in need of filtering. 
     Instead of individual calculations, it is convenient to transform plots of time-domain into plots of frequency-domain using computer programs such as MATLAB®, Simulink®, and Stateflow™, commercially available from MathWorks, Inc., 24 Prime Park Way, Natick, Mass., 01760-1500 USA. 
     FIG. 5 shows the block diagram of a low pass filter. According to the invention, the filter is created by a single metallization level, integral to the semiconductor package substrate. The filter does not require external components. Furthermore, it is confined substantially into the substrate area reserved for attaching the IC chip. The low pass filter, generally designated  500 , receives the signal  501  from the driver  502  in the IC. Generally the low pass filter consists of at least one resistance  503 , at least one inductance  504 , at least one capacitance  505 . The electrical circuit is closed by the buffer  506  (transmission line) on the assembly board. 
     In U.S. Patent Application Serial No. 60/116,274, filed Jan. 19, 1999, (Lamson et al., “System for Electrically Modeling an Electronic Structure, and Method of Operation”), a method is described to calculate electrical parameters from geometrical data and vise versa. The electrical parameters especially include inductance, capacitance, and resistance values. By way of example, a low pass filter for the high frequency IC attached to the substrate of FIG. 1 is to be calculated. The first step is to determine the high frequency attenuation of a low filter constructed of a set of electrical components. The next step is to construct the geometries of the electrical components within the constraints of the allotted area of the package substrate in FIG.  1 . The modeling steps have to be repeated until both goals of high frequency attenuation and geometrical fit are achieved. 
     For the low pass filter of FIG. 5, the goal is the attenuation of high frequency components in the vicinity of 9×10E8 Hz by about 3 to 4 dB. This goal is achieved by electrical components having a resistance in the range from about 0.16 to 0.2 Ω, an inductance in the range from about 10 to 30 nH, and a capacitance in the range from about 6 to 9 pF. Using these values, other frequencies are attenuated as well. 
     The result of the filtering on the signal output from the IC is shown in FIG.  6 . The signal ramp  601  is approximately linear. The constant portion  602  is reached with only minimal overshoot  603 ; the oscillations  604  are rapidly damped. 
     The result of the filtering on high frequency attenuation is shown in FIG.  7 . The IC output power distribution  701  exhibits a significant shift towards reduced power, as a comparison with the curve  401  in FIG. 4 indicates. In the vicinity of 9×10E8 Hz ( 702  in FIG.  7 ), the high frequency components are attenuated by about 3 to 4 dB, in the vicinity of 5×20E8 Hz ( 703  in FIG.  7 ), by about 4 to 5 dB. Similar attenuations are accomplished at other frequencies. In summary, the low pass filter is effective in lowering the high frequency radiation with only minimal distortions of the signal waveforms. 
     According to the invention, the constituents of the low pass filter are integrated into the substrate (especially the substrates of semiconductor packages) within the process scope of the standard metallization step, however without requiring additional process steps and without requiring additional substrate real estate. FIG. 8 indicates the solution according to the invention. The advantages of the invention become clear when FIG. 8 is compared with FIG.  2 . FIG. 8 is a schematic and simplified cross section of a portion of an IC chip  801  mounted on substrate  803 . In contrast to FIG. 2, the chip attach material  802  is only applied after the patterned metal structure  820  of the low pass filter has been created substantially within the area required for the chip attachment. The attach material is an epoxy, however not silver-filled, or sometimes a polyimide, of approximately 20 to 50 μm thickness overall. 
     The metal consumed by the low pass filter structures  820  is deposited and patterned in the same process steps needed for forming the remainder of metal structures  807 ,  808 ,  809  and  810  on the surface of the substrate  803 . Consequently, no extra process step is required to create the low pass filter structures of the invention. 
     The advantages of the invention become clear, when FIG. 9 is compared with FIG.  1 . FIG. 9 is the top view of the substrate generally designated  900  of a ball grid array package for a semiconductor device requiring  144  ball connections (on the bottom surface of the substrate, not shown in FIG.  9 ). The insulating layer  901  of the substrate is made of organic material in the thickness range from about 70 to 150 μm, as mentioned in conjunction with FIG.  1 . 
     The dotted line  902  encloses the area reserved for attaching the IC chip. Substantially within this area are accommodated one inductance  910  and one capacitor  920  of the low pass filter of the invention. The one lead  911  of the inductance  910  originates with the metallization  903 , cooperatively intended for a bonding wire stitch terminal. The other lead  912  of the inductance  903  is placed on terminal pad  904   a , which connects to a metal-filled via and a solder ball (not shown in FIG.  9 ). During the plating step of the metal deposition process, metal line  913  is connected to the applied plating potential. 
     The inductance lead  912  further doubles as the “inner” comb terminal of capacitor  920 . The “outer” comb  922  is connected to terminal pad  904   b , which connects to a metal-filled via and a solder ball at ground potential (not shown in FIG.  9 ). During the plating step of the metal deposition process, metal line  923  is connected to the allied plating potential. 
     While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. As an example, the material of the semiconductor chip may comprise silicon, silicon germanium, gallium arsenide, or any other semiconductor material used in manufacturing. As another example, the package may be a BGA package or any other semiconductor device package. As another example, the electrical characteristics of inductance and/or capacitance of the low pass filter can be modified in order to attenuate or filter different regimes of the frequency spectrum. As another example, the geometrical layout of capacitance and/or inductance of the low pass filter can be patterned differently, if the IC chip is attached to the substrate by a flip-solder process instead by adhesive and wire bonding. It is therefore intended that the appended claims encompass any such modifications or embodiments.