Compensation structure for a bond wire at high frequency operation

An improved frequency response for a bond wire (1) at high frequency operation is realized by using a matching element (13) including a meander line (9) structure. The frequency response is improved at an operating frequency by design. The matching element (13) compensates the bond wire (1) by tuning it as a length of high impedance transmission line and then completing the combined length of the bond wire (1) and matching element (13) to a length of half of a guided wavelength at the operating frequency

FIELD OF THE INVENTION
 Embodiments of the present invention relate to impedance compensation
 structures, and more particularly to a compensated interconnection
 structure between a semiconductor die or a Monolithic Microwave Integrated
 Circuit ("MMIC") chip, and a substrate that improves the insertion loss of
 the interconnection structure at millimeter wave frequencies.
 BACKGROUND
 Semiconductors for wireless communication applications operate at radio
 frequencies ("RF") and above. As the applications for wireless
 communication increase, so does the desirability of making use of many
 frequency bands in the available spectrum. The millimeter wave spectrum is
 available and desirable for wireless communication purposes. The
 millimeter spectrum, however, presents certain engineering challenges due
 to the increased distortion and loss at millimeter wave frequencies. There
 is also a greater sensitivity to device parasitics at higher frequencies
 than radio frequencies ("RF") which are commonly used in wireless
 communications. Accordingly, the typical parasitics that are tolerated at
 lower frequencies cannot be ignored at millimeter wave frequencies and
 still achieve adequate performance for the applications in which they
 occur.
 One of the typical parasitics of all wireless communication systems that
 include high frequency semiconductors is the complex impedance, primarily
 inductance, of the bond wire. The bond wire is typically a length of gold
 wire or ribbon that is connected using ultrasonic energy to a
 semiconductor device or a MMIC chip contact on one end and an
 interconnecting contact on an opposite end. The interconnecting contact is
 typically a conductive pad on a substrate such as a chip on board printed
 circuit substrate onto which the semiconductor die or MMIC chip is
 directly attached. With this connection style, there is conventionally a
 length of wire that has a significant inductive component at millimeter
 wave frequencies. Conventionally, serial discrete capacitors are used to
 tune the inductance of the bond wire to a resonant condition. Discrete
 capacitors, however, are large and take up too much substrate, which is in
 contravention of the interest in miniaturization. In addition, the normal
 discrete capacitor tolerances and inherent parasitics render accurate
 tuning at millimeter wave frequencies impractical from a manufacturing
 view point. Production errors in certain types of appropriate capacitors
 can also reduce the total yield of the circuit making the manufacturing
 process costly. There remains a need, therefore, for a manufacturable and
 small apparatus for compensating typical bond wire parasitics at
 millimeter wave frequencies.
 SUMMARY
 It is an object of an embodiment according to the teachings of the present
 invention to improve the frequency performance of bond wire interconnects
 at millimeter wave frequencies.
 It is a further object of an embodiment according to the teachings of the
 present invention to provide a compensation device that is small and
 simple.
 It is a further object of an embodiment according to the teachings of the
 present invention to provide a compensation device that is reliably
 manufacturable, repeatable, and low cost.
 A semiconductor die or a MMIC chip for attachment to a substrate has a die
 contact for electrical connection thereto, and an interconnection tuned
 for operation at an operating frequency. The interconnection is a bond
 wire connected to the die contact and extends to a substrate contact. The
 bond wire has a wire length and an associated impedance value. The
 interconnection further includes a matching element connected to the
 substrate contact opposite the bond wire. The matching element has a first
 connecting element, a meander line, and a second connecting element. The
 matching element is tuned to combine with the bond wire to create a high
 impedance transmission line which length is substantially equal to half of
 a guided wavelength of the operating frequency.
 A method for making a compensated bond wire comprises the steps of
 identifying the bond wire to be compensated, and obtaining a predictive
 model for it. The method further comprises identifying an operating
 frequency, and calculating an electrical length of a guided half
 wavelength at the operating frequency. A length of the bond wire is
 identified, and is subtracted from the length of the guided half
 wavelength to arrive at the electrical length of a matching element. The
 method fits a model of the matching element comprising first and second
 connecting elements and a meander line to the calculated electrical length
 of the matching element. The method then calls for simulating the
 electrical behavior of the fitted matching element, and then optimizing a
 frequency response of the matching element by varying a length of the
 meander line to combine with the bond wire to achieve an electrical length
 substantially equal to the guided half wavelength of the operating
 frequency.
 It is a feature of an embodiment according to the teachings of the present
 invention that a meander line is used for compensating the bond wire
 impedance to achieve a total electrical length, when combined with the
 bond wire and the connection elements, equal to a guided half wavelength
 of the operating frequency.
 It is a feature of an embodiment according to the teachings of the present
 invention that the tuning element is easily and repeatably fabricated with
 conventional printed transmission line technology.
 It is an advantage of an embodiment according to the teachings of the
 present invention that a wire bond can be used for interconnection at
 millimeter wave frequencies.
 It is an advantage of an embodiment according to the teachings of the
 present invention that a tuned bond wire is reliably manufacturable.
 It is an advantage of an embodiment according to the teachings of the
 present invention that a tuned bond wire is small and performs according
 to predictive modeling.
 It is a further advantage of an embodiment according to the teachings of
 the present invention that a compensated bond wire can be manufactured at
 a low cost.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 With specific reference to FIG. 1 of the drawings, there is shown a side
 view of a typical bond wire or bond ribbon 1 as it is attached between a
 die 3 and a substrate 5. For purposes of the present disclosure, the term
 "bond wire" is used to refer to any uncompensated interconnection between
 two electrical points. Ends of the bond wires 1 are typically attached to
 a die or MMIC contact 6 and a substrate contact 7 respectively using
 conventional bond wire attachment technology. Design of a compensation
 structure for the bond wire 1 begins by establishing a lumped element
 equivalent model of the bond wire. This establishment can occur in many
 ways including using a model already established or using a software
 program such as ANSOFT's "Maxwell" software program based upon the
 materials and geometry of the bond wire of interest. FIG. 2 of the
 drawings shows a lumped element equivalent model of the bond wire at high
 frequencies in which the bond wire is modeled as an inductor (L) in series
 with a resistor (R.sub.S). Other typical parasitics included in the model
 are two parallel resistor-capacitor circuits (R.sub.p1 &C.sub.p1, R.sub.p2
 &C.sub.p2) between each end of the bond wire and reference potential. As
 an example, a typical wire bond length is approximately 470 microns. For
 the example bond wire and with reference to FIG. 2 of the drawings, lumped
 element values with f as the operating frequency are:

L 0.24 nH
 R.sub.s 0.04 f .OMEGA.
 C approx. 0
 C.sub.p1 15 fF
 C.sub.p2 13 fF
 R.sub.p1 86 k.OMEGA.
 R.sub.p2 65 K.OMEGA.
 Note that while the value of the capacitor, C, in parallel with the
 inductive, L, impedance of the bond wire was determined to be zero, it is
 included in the initial model for completeness. FIGS. 3A and 3B are graph
 respectively showing the verification of the lumped element equivalent
 circuit model results as compared to the full wave simulation results as a
 function of frequency showing that the lumped element equivalent model
 provides a good indication of bond wire behavior at the frequencies of
 interest. In the figure captions, M and S stand for the extracted lumped
 model response and the full-wave simulation results, respectively. It
 should be noted that it is unimportant to the teachings of the present
 invention that a particular kind of interconnect or bond wire is used. It
 is only important that a reliable and accurate lumped element equivalent
 circuit is known for the bond wire, bond ribbon, or other similar
 structure. An operating frequency is identified which is based upon the
 frequency or frequency range of the desired application. The operating
 frequency is the frequency at which the electrical behavior of the bond
 wire will be tuned to be most ideal. If a frequency range is of interest,
 a frequency value that is centered in the range is used as the operating
 frequency for the remainder of the compensated interconnect design process
 In order to find the lumped-model component values, the scattering
 parameters of the bond wire over a broad frequency range that includes the
 operating frequency are obtained Scattering parameters may be generated
 using ANSOFT's "Maxwell" computer program and are imported as a data set
 into a simulation program such as HEWLETT KARD Company's "Libra"
 software. Alternatively, the scattering parameters might also be obtained
 from the direct measurement of the bond wire itself. After obtaining the
 scattering parameters, the simple lumped model of the bond wire is drawn
 in the "Libra" software and the component values are extracted through an
 optimization algorithm built in "Libra" software by comparing the model
 response with scattering parameters obtained from bond wire measurement or
 the "Maxwell" software.
 A matching element 13 is chosen to tune the lumped element equivalent
 circuit of the modeled bond wire at the operating frequency. The design
 process for the matching element strives to create a structure in such a
 way that when the total length of the matching element is combined with
 the length of the bond wire, the overall circuitry exhibits a transmission
 line response whose length is half of the guided wavelength of the
 operating frequency. From the classical transmission line theory, it is
 known that a half wavelength transmission line translates its input
 impedance to its output (i.e. 50 Ohm.). Accordingly, a distributed
 equivalent model of the matching circuit is drawn in the "Libra" software
 as being placed in series with the bond wire lumped element equivalent
 model. The matching element 13 according to the teachings of the present
 invention comprises a first connecting element 8a comprising a length of
 transmission line connected at a right angle to a meander line 9 at an
 open end 16 of the meander line 9. The meander line 9 comprises a first
 parallel length 10 of transmission line connected to a spacing element 12
 by a first right triangle element 14a. The first parallel length 10 is
 contiguous with the first right triangle element 14a, which is contiguous
 with the spacing element 12. A second parallel length 11 of transmission
 line is connected to an opposite end of the spacing element 12 by a second
 right triangle element 14. As FIGS. 4 and 6 of the drawings show, the
 spacing element 12 is contiguous with the second right triangle element 14
 which is contiguous with the second parallel length 11. A second
 connecting element 8b is connected to the second parallel length 11 at an
 open end 16 of the meander line 9 to complete the matching element 13. The
 "Libra" model for the matching element 13 as described comprises a series
 circuit of a transmission line modeling the first connecting element 8a, a
 transmission T connection modeling a right angle interconnect, a
 transmission line modeling the first parallel length 10, a right bend
 modeling the first right triangle element 14a, a transmission line
 modeling the spacing element 12, a left bend modeling the second right
 triangle element 14, a transmission line modeling the second parallel
 length 11, a transmission T modeling a right angle interconnect, and a
 transmission line modeling the second connecting element 8b. The length
 and width of the transmission lines that make up the constituent parts of
 the matching element 6 are variable in order to optimize the tuning of the
 matching element 13. For completeness, the first and second parallel
 lengths 10, 11 of the meander line 9 are modeled as coupled transmission
 lines and a gap is modeled at the open end 16 of the meander line 9.
 Relative constraints of the variable lengths include the assumptions that
 the first and second parallel lengths 10, 11 are equal in length and the
 first and second connecting elements 8a, 8b are equal in length For
 purposes of the design of the meander line, the first and second parallel
 lengths should be spaced a distance greater or equal than one substrate
 height from each other so as to prevent coupling between the elements that
 may deteriorate the response. However, the best way of verifying this is
 to use a circuit simulator because the coupling depends on the electrical
 width of the lines as well as the spacing. In the case presented here, the
 parallel sections of the meander line are separated by 100 microns, and
 for the sake of completeness, the meander line is simulated by using
 coupled lines in the circuit simulator. Note that a meander line is used
 instead of a straight line to reduce the area required by the matching
 circuit.
 The length of the bond wire is then subtracted from the guided half
 wavelength of the operating frequency in order to establish an approximate
 length for the matching element 13. The bond wire 1 acts as a high 25
 impedance transmission line because its structure is narrow and suspended
 in air. Accordingly, the meander line 9 is formed out of a high impedance
 transmission line whose characteristic impedance is selected near to 100
 Ohms at the operating frequency. In this way the meander line is a natural
 continuation, from a transmission line point of view, of the wire bond.
 From another point of view, the bond wire with the parameters L, R.sub.S,
 C.sub.p1, C.sub.p2, R.sub.p1, and R.sub.p2 (See FIGS. 2 and 6 of the
 drawings) is treated as a small high-impedance transmission line segment.
 The parameters of the matching element 13 are adjusted so that the total
 length of the bond wire and matching element 13 is substantially half of
 the guided wavelength at the operating frequency. Therefore, the overall
 compensated bond wire system acts as a half-wavelength impedance
 transformer. Intuitively, it can be said that the principle of operation
 is to shift the resonance point as seen in FIG. 5 of the drawings, to the
 operating frequency by adjusting certain parameters of the matching
 element 13. Accordingly, the matching element 13 has the following
 variable parameters: length of the first and second connecting elements
 8a, 8b, width of the first and second connecting elements 8a, 8b, length
 of the first and second parallel lengths 10,11 of the meander line 9,
 width of the first and second parallel lengths 10,11 of the meander line,
 and length of the spacing element 12. Using the relative constraints as
 well as the constraint that the total length of the matching element 13
 plus the bond wire equals the guided half wavelength of the operating
 frequency, the individual parameters are varied to arrive at an impedance
 value, which is primarily capacitive, that compensates for the primarily
 inductive impedance of the bond wire as predicted by the bond wire model.
 The parameters are varied by using an optimization algorithm or by hand at
 the operating frequency until the insertion loss is minimized at the
 operating frequency. Since there are relatively few variable parameters,
 one of ordinary skill in the art can find the solution by tuning the
 elements manually in a circuit simulator. Strictly speaking, there can be
 more than one set of possible parameters.
 With specific reference to FIG. 6 of the drawings, there is shown a model
 of a compensated bond wire according to the teachings of the present
 invention. FIGS. 7 and 8 show the comparison of the reflection coefficient
 and insertion loss, respectively, of the uncompensated bond wire as
 compared to the compensated bond wire as a function of frequency. The
 simulated data show that at the operating frequency, there is a
 significantly improved impedance match in the compensated bond wire
 according to the teachings of the present invention For purposes of
 completeness, FIGS. 9 and 10 of the drawings show the comparison of the
 reflection coefficient and insertion loss as measured for three different
 bond wires showing the strong correlation to the simulated data and the
 strong repeatability of the design approach. Note that the 0.24 nH
 inductance corresponds to approximately 18.5 mil bond wire length for the
 configuration previously discussed. The slight mismatch between the
 measured and the simulated data could be due to over or under estimated
 bond wire lengths, and imperfect positioning of the bond wire connection
 point on the matching element side.
 FIG. 11 of the drawings shows a measured bond wire and compensation circuit
 Note that the bond wire shown in the picture is a bond wire from one pad
 on a microstrip circuit to another pad on the same plane. As stated
 previously, the method is applicable to any kind of bond wire as far as
 the lumped model is available or found.