Method and apparatus to facilitate capacitive communication between chips

<u style="single">Field of the Invention</u>

0001The present invention relates to techniques for communicating between integrated circuits.

<u style="single">Background of the Invention</u>

0002Recent advances in semiconductor technology have not been matched by corresponding advances in inter-chip communication technology. Semiconductor chips are typically integrated onto a printed circuit board that contains multiple layers of signal lines for inter-chip communication. However, signal lines on a semiconductor chip are about 100 times more densely packed than signal lines on a printed circuit board. Consequently, only a tiny fraction of the signal lines on a semiconductor chip can be routed across the printed circuit board to other chips. This mismatch creates a bottleneck that continues to grow as semiconductor integration densities continue to increase.

0003Researchers have begun to investigate alternative techniques for communicating between semiconductor chips. One promising technique involves integrating arrays of capacitive transmitters and receivers onto semiconductor chips to facilitate inter-chip communication. If a first chip is situated face-to-face with a second chip so that transmitter pads on the first chip are capacitively coupled with receiver pads on the second chip, it becomes possible to transmit signals directly from the first chip to the second chip without having to route the signal through intervening signal lines within a printed circuit board.

0004Face-to-face communication requires that transmitters and corresponding receivers are in close proximity to each other. This can be difficult to accomplish for a number of reasons. When a group of chips are brought together, the chips may have different functions and correspondingly different thicknesses. For example, a group of chips may include processors, memory, field programmable gate arrays, optical drivers, receivers, etc. Even chips of the same function, but manufactured on different wafers, may have different thicknesses. Wafers can be thinned to some accuracy and that accuracy can be improved through closed loop (measure/adjust) mechanical machining operations. However, achieving a uniform thickness may still be a problem. Note that non-uniform chip spacing can cause significant problems in achieving uniform signal propagation during capacitive face-to-face communication between chips.

0005For example, consider face-to-face chips arranged in a checkerboard pattern as shown in FIG. 1. In FIG. 1, chips 101-113 communicate with each other through overlapping regions on their four corners. In this arrangement, each chip communicates with four neighboring chips. Note that other arrangements of the chips will be apparent to a practitioner with ordinary skill in the art.

0006As shown in FIG. 1, chip 107 overlaps chips 104, 105, 109, and 110. If the surfaces of chips 104, 105, 109, and 110 are co-planar, then chip 107 can make equal contact with each of chips 104, 105, 109, and 110. However, if chips 105 and 109 are thicker, or are situated higher by a supporting structure, then they will tend to separate chip 107 from chips 104 and 110. In this case, chip 107 has an average spacing to chips 104 and 110, which is equal to the difference in chip surface height between chips 104 and 110 on one hand, and chips 105 and 109 on the other. If chip 107 tilts along the diagonal axis formed by the contact regions to chips 105 and 109, the gap to chip 104 or 110 will become larger and the gap to the other chip will become smaller. In other words, one of chips 104 and 110 can contact chip 107 and the other chip will have a gap of twice the difference in chip thickness.

0007Chips with a triangular shape rather than a rectangular shape contact on three corners. This ensures that the chip will not wobble along a diagonal axis as it can when in contact with four neighboring chips. However, if the three contacted chips are not co-planar, the triangular chip will still have some gaps relative to the three neighbors because of the tilt caused by the different thicknesses.

0008Proximity communication can tolerate some amount of spacing variation between the face-to-face chips. Larger spacing causes less coupling between the transmitter and receiver and hence smaller input signals. Hence, more sensitive receivers with lower offsets can deal with larger spacing. For example, in 180nm CMOS technology, one implementation of capacitive coupling accommodates air gaps of 5 to 10 microns. Chips are presently delivered with thicknesses of 12 to 15 mils, or roughly 300 to 375 microns. Variations in this thickness are not well controlled because normal chip packaging techniques can tolerate wide variations in the chip thickness (for example, wire bonds and ball bonds can accommodate some amount of mechanical variability). Note that a chip thickness variation of 0.25 mils (6 microns) can lead to a 12 micron gap, and a variation of 0.5 mils can lead to a 25 micron gap.

<u style="single">Summary of the Invention</u>

0009One embodiment of the present invention provides a system that improves communications between capacitively coupled integrated circuit chips. The system operates by situating an interposer over capacitive communication pads on a first integrated circuit chip, wherein the interposer is made up of material that is anisotropic with respect to the transmitting capacitive signals. A second integrated circuit chip is situated so that communication pads on the second integrated circuit chip are aligned to capacitively couple signals between the integrated circuit chips through the interposer. The increased dielectric permittivity caused by the interposer can improve capacitive coupling between opposing communication pads on the integrated circuit chips. The interposer can also reduce cross talk between communication pads on the first integrated circuit chip and pads adjacent to the opposing communication pads on the second integrated circuit chip.

0010In one embodiment, the anisotropic material is made up of a plurality of columns, wherein each column has a higher permittivity than the intervening material between the columns.

0011In one embodiment, the cross-sectional area of the column is small in comparison with the cross-sectional area of a capacitive communication pad on the integrated circuit chips.

0012In one embodiment, the interposer comprises a non-conductive material with metal particles imbedded in the plurality of anisotropic columns.

0013In one embodiment, the interposer comprises two layers of metal pads and micro electro-mechanical (MEM) springs that couple metal pads on the first layer of metal pads to corresponding metal pads on the second layer of metal pads.

0014In one embodiment, the interposer is attached to one of the integrated circuit chips with an adhesive.

0015In one embodiment, the system provides a plurality of alignment spurs on the integrated circuit chips to align the integrated circuit chips.

0016In one embodiment, the system uses a plurality of alignment springs to align the integrated circuit chips.

<u style="single">Brief Description of the Drawings</u>

0017Various embodiments of the invention will now be described in detail by way of example only with reference to the following drawings: <ul id="ul0001" list-style="none" compact="compact"><li>FIG. 1 illustrates a checkerboard pattern of integrated circuit chips that use face-to-face communications.</li><li>FIG. 2 illustrates a side view of an interposer in accordance with an embodiment of the present invention.</li><li>FIG. 3 illustrates a top view of an interposer in accordance with an embodiment of the present invention.</li><li>FIG. 4 illustrates an interposer with MEM springs in accordance with an embodiment of the present invention.</li><li>FIG. 5 illustrates a circuit model for the interposer of FIG. 4 in accordance with an embodiment of the present invention.</li><li>FIG. 6 illustrates a vertical interposer column with embedded metal particles in accordance with an embodiment of the present invention.</li><li>FIG. 7A illustrates a sheet of interposer material in accordance with an embodiment of the present invention.</li><li>FIG. 7B illustrates an interposer cut from a sheet of interposer material in accordance with an embodiment of the present invention.</li><li>FIG. 7C illustrates the process of attaching an interposer to an integrated circuit chip in accordance with an embodiment of the present invention.</li><li>FIG. 8 illustrates alignment spurs on an integrated circuit chip in accordance with an embodiment of the present invention.</li><li>FIG. 9 illustrates the process of aligning integrated circuit chips without an interposer in accordance with an embodiment of the present invention.</li><li>FIG. 10 illustrates the process of aligning integrated circuit chips with an interposer in accordance with an embodiment of the present invention.</li><li>FIG. 11 illustrates a side-view of a system that uses springs to align integrated circuit chips in accordance with an embodiment of the present invention.</li><li>FIG. 12 illustrates a top-view of a system that uses springs to align integrated circuit chips in accordance with an embodiment of the present invention.</li></ul>

<u style="single">Detailed Description</u>

<u style="single">Interposer</u>

0018FIG. 2 illustrates a side view of an interposer 210 in accordance with an embodiment of the present invention. The interposer may be used to facilitate capacitive communication between face-to-face chips. Interposer 210 includes multiple vertical columns 212 of higher permittivity, which cause the interposer to have anisotropic properties. Note that the spacing and size of the columns may vary from one embodiment to another; the 50-50 ratio illustrated in Figure 2 yields satisfactory results.

0019The size of an individual column may be small in comparison with communication pads 202, 204, 206, and 208. Note that communication pads 202 and 206 form a communicating pair as do communication pads 204 and 208. One pad in each pair is a transmitting pad, while the other pad is a receiving pad.

0020The anisotropic characteristic of the columns enhance communication between a communicating pair while simultaneously reducing crosstalk from adjacent transmitting pads. For example, if pads 202 and 204 are transmitting pads, communications between pads 202 and 206 are enhanced, while crosstalk between pads 202 and 208 is reduced.

0021FIG. 3 illustrates a top view of an interposer 210 in accordance with an embodiment of the present invention. In FIG. 3, vertical columns within interposer 210 are illustrated as squares. Note that other shapes and arrangements are possible. Communication pads 302 are shown by the dashed lines. Note that each communication pad covers multiple vertical columns. The number of communication pads covered is a design selection based upon the size of each column, the shape of the columns, and the layout of the columns.

<u style="single">Interposer with MEM Springs</u>

0022FIG. 4 illustrates an interposer with Micro-Electrical-Mechanical (MEM) springs in accordance with an embodiment of the present invention. The interposer illustrated in FIG. 4 includes two layers 402 with embedded metal pads 404. High dielectric material 408 is interposed between the metal pads 404. Corresponding metal pads 404 on the two layers 402 are coupled together through conductive micro electro-mechanical (MEM) springs 406. This structure gives the interposer some mechanical compliance. Note that the top ends of the MEM springs 406 can be sealed on the structure to prevent shorting of adjacent metal pads 404. Note that the springs do not have to be of the standard "spiral" variety. Other spring types, such as cantilever torsion springs can be used.

<u style="single">Interposer Circuit Model</u>

0023FIG. 5 illustrates a circuit model for the interposer of FIG. 4 in accordance with an embodiment of the present invention. The path for normal communications from transmitting pad 502 to receiving pad 504 is from transmitting pad 502 through a capacitance C1, resistance R, and capacitance C3 to receiving pad 504. Note that capacitance C1 and C3 relate to the high dielectric material 408 described above in conjunction with FIG. 4. The crosstalk path from transmitting pad 506 to receiving pad 504 is a parallel path from transmitting pad 506 through capacitors C 1 and C2, resistance R and capacitor C3 to receiving pad 504. If the capacitance of C2 is small compared to the capacitance of C 1 and C3, the crosstalk signal will be smaller than the normal signal at receiving pad 504. Since the capacitance of C2 includes the lower dielectric constant of the material between the columns, the capacitance for C2 will be smaller than that of C 1 and C3.

<u style="single">Embedded Metal Particles</u>

0024FIG. 6 illustrates a vertical interposer column with embedded metal (or other conductive material) particles in accordance with an embodiment of the present invention. Vertical interposer column 602 includes multiple particles 604 embedded within the material. These metal particles 604 can be implanted in the interposer material using standard implantation techniques. These metal particles 604 increase the dielectric constant within the interposer column so that the interposer column has a higher, and thus more favorable, dielectric constant for capacitive communication. Note that the metal particles do not need to be placed uniformly throughout the column.

<u style="single">Interposer Material</u>

0025FIG. 7A illustrates a sheet of interposer material 702 in accordance with an embodiment of the present invention. Note that the sheet of interposer material 702 can include an adhesive on one surface. Note also that the columns of high dielectric material are shown as circles 710 in FIGs. 7A, B, and C. The density of these columns is a design consideration and may have a much higher density than shown. Cut lines 704 indicate where the interposer material 702 is cut to form an interposer for a specific chip.

0026FIG. 7B illustrates an interposer cut from a sheet of interposer material in accordance with an embodiment of the present invention. By cutting the interposer material 702 along cut lines 704, interposer 706 is formed. Note that the shape and size of interposer 706 is determined from the shape and size of the chip to which interposer 706 is attached.

0027FIG. 7C illustrates the process of attaching an interposer to an integrated circuit chip in accordance with an embodiment of the present invention. Interposer 706 is attached to integrated circuit chip 708 through adhesive on a surface of interposer 706. Note that any type of known adhesive material can be used. After interposer 706 is attached to integrated circuit chip 708, integrated circuit chip is aligned relative to another integrated circuit chip as described in conjunction with FIGs. 8-12 below to allow capacitive communication between the integrated circuit chips. Note that the interposer illustrated in FIG. 7 can be of the simple column type as is illustrated in FIG. 2, or the MEM type illustrated in FIG. 4, or any other appropriate type.

<u style="single">Alignment Spurs</u>

0028FIG. 8 illustrates alignment spurs on an integrated circuit chip in accordance with an embodiment of the present invention. Integrated circuit chip 802 includes alignment spur 804, exposed pads 806, and covered pads 808. Exposed pads 806 are used for ohmic contacts, for example power and ground connections. Covered pads 808 are used for capacitive communications with other integrated circuit chips. Spur 804 is used to align the integrated circuit chips to enable communications.

<u style="single">Alignment Without an Interposer</u>

0029FIG. 9 illustrates the process of aligning integrated circuit chips without an interposer in accordance with an embodiment of the present invention. Integrated circuit chips 902 and 904 are situated so that their communicating pads are facing each other. Alignment spurs 804 provide positive alignment of the communication pads when alignment spurs 804 are positioned against the chip ends as shown in FIG. 9.

<u style="single">Alignment with an Interposer</u>

0030FIG. 10 illustrates the process of aligning integrated circuit chips with an interposer in accordance with an embodiment of the present invention. Integrated circuit chips 1002 and 1004 are situated so that their communicating pads are facing each other, and interposer 1006 is positioned between the communication pads. Alignment spurs 804 provide positive alignment of the communication pads when alignment spurs 804 are positioned against the chip ends as shown in FIG. 9. Note that interposer 1006 is held securely in position between the faces of integrated circuit chips 1002 and 1004.

<u style="single">Alignment Springs</u>

0031FIG. 11 illustrates a side view of a system that uses springs to align integrated circuit chips in accordance with an embodiment of the present invention. Integrated circuit chips 1102 and 1104 are positioned in wells within integrated chip fixtures 1106 and 1108, respectively. Springs 1110 apply pressure to the sides and tops of integrated circuit chips 1102 and 1104 to maintain their positions within integrated circuit chip fixtures 1106 and 1108.

0032FIG. 12 illustrates a top view of a system that uses springs to align integrated circuit chips in accordance with an embodiment of the present invention. Integrated chip 1204 is positioned at the top left of well 1202 in the integrated circuit fixture and is held in place by pressure from springs 1206.

0033The foregoing descriptions of embodiments of the present invention have been presented for purposes of illustration in order to enable a person skilled in the art to appreciate and implement the invention. They are provided in the context of particular applications and their requirements, but are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art, and the scope of the present invention is defined by the appended claims and their equivalents.