Axiocentric scrubbing land grid array contacts and methods for fabrication

Abstract

A contact structure and assembly for a microelectronics device includes first and second electrically conductive contacts being helically shaped. A carrier element is attached to and positioned between the first and second contacts. The first and second contacts are in electrical communication with each other, and the first and second contacts are in a mirror image relationship with each other. A pair of insulating substrates each include electrically conductive members. A contact point on each of the first and second contacts is attached and electrically communicating to respective electrically conductive members such that the first and second electrically conductive contacts between the pair of insulating substrates form an electrically conductive package. A metal layer on the carrier element provides electrical conductivity through a first opening defined by the carrier element between the first and second portions of the helix shaped contact.

Claims

1. A contact assembly for a microelectronics device, comprising: first and second electrically conductive contacts having defined lengths, both the first and second contacts being helically shaped; a carrier element attached to and positioned between the first and second contacts, the first and second contacts being in electrical communication with each other, and the first and second contacts being in a mirror image relationship with each other such that a direction of helicity reverses as the first and second contacts pass through the central carrier plane; a pair of insulating substrates each including electrically conductive members, and at least one contact point on each of the first and second contacts are attached and electrically communicating to respective electrically conductive members such that the first and second electrically conductive contacts between the pair of insulating substrates form an electrically conductive package; and a separate metal layer on the carrier element provides electrical conductivity through a first opening defined by the carrier element between the first and second portions of the helix shaped contact.

2. The assembly of claim 1, wherein a second opening defined by the carrier element provides electrical conductivity through the second opening between the first and second portions of the helix shaped contact.

3. The assembly of claim 1, further comprising: the first and second electrically conductive contacts between the pair of insulating substrates being in a compressed state relative to an at rest state; and a rotational displacement of the first and second contacts on the electrically conductive members between the compressed state and the at rest state.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. The various features of the drawings are not to scale as the illustrations are for clarity in facilitating one skilled in the art in understanding the invention in conjunction with the detailed description. In the drawings:

(2) FIG. 1 is flow chart of a method of the invention according to one embodiment of the invention;

(3) FIGS. 2A-2I are diagrammatic view of process steps depicting the process for manufacturing a hollow LGA array according to an embodiment of the invention;

(4) FIGS. 3A-3B are perspective views of helical contacts in reverse directions;

(5) FIG. 3C is a side elevational view of a contact assembly having helical contact in mirror image relation separated by a carrier element;

(6) FIG. 3D is a perspective view of the contact assembly in FIG. 3C;

(7) FIG. 4 is a perspective view of a plurality of helical contacts used in high frequency electrical simulation;

(8) FIG. 5 is a perspective view of an illustrative metal and photo resist coated molded polymer bump sitting within a form fitting cavity;

(9) FIG. 6 is a plan view of a mask pattern shaped as a four leg helix;

(10) FIG. 7 is a isometric bottom view of the mask pattern shown in FIG. 6;

(11) FIG. 8 is an isometric view of a single side of a molded sacrificial polymer element;

(12) FIG. 9 is an isometric view of the element shown in FIG. 8, having a metal helix spring contact formed over the element;

(13) FIG. 10 is an isometric view of the spring contact of FIG. 9 after the sacrificial polymer element shown in FIGS. 8 and 9 has been ablated;

(14) FIG. 11 is a plan view of a group of four mask pattern openings in a helix pattern;

(15) FIG. 12 is a graph of a return loss in dB (reflected energy) for right hand-left hand (RH-LH) and right hand-right hand (RH-RH) spring contact types; and

(16) FIG. 13 is a graph of effects of spring contact leg rotation on electrical transmissive loss performance.

DETAILED DESCRIPTION

(17) Referring to FIG. 1, a method 10 according to one embodiment of the present invention includes the general steps below including step 14 forming a substrate with a 2d array of vias (for example, conducting vias) as a first step to forming a 2d array of contacts. Two different alternatives can be pursued after step 14, a first path is to manufacture contacts shaped from thermally decomposable polymers in step 40. Contact elements are shaped from thermally decomposable polymers, and the sacrificial polymers are molded into a carrier plane with vias. The plurality of contact may be formed in an array of contacts having a pattern.

(18) A second path is to mold elastomeric contact elements into a carrier plane in step 20. After either steps 20 or 40, the contact elements are metalized using known methods in step 44. Both paths continue in step 48 wherein the protruding contact elements are coated with metal layers and photo resist. A conformally coating resist such as electrophoretically deposited photo resist is used to coat the protruding contacts.

(19) A metal mask for positioning over both sides of the LGA protrusions includes cavities of complementary shape to the protrusions and is placed over an array of protrusions in step 52. The cavities are of similar shape to a desired contacts, but slightly larger to account for the photo resist thickness and any tolerance. The masks have patterns in the top formed as slots by wire electric discharge machining (EDM), laser, and other techniques. The patterns transmit light into a pattern onto the photo resist covered contacts or LGA protrusions, without allowing for significant reflections from one contact to another because they are nestled into individual cavities. Then the metal mask is clamped together, e.g., top and bottom portions of the metal mask can be clamped together.

(20) Referring to FIG. 2, a process 100 for manufacturing hollow LGA array using a mold includes a perforated insulator 104 as shown in FIG. 2A. The perforated insulator 104 includes a large hole 106 and a small hole 108. A polymer element 112 shown in FIG. 2B is oval shaped, however, other shapes may be used. A desired radius lip 620 shown in FIGS. 5 and 8 transitions from the polymer element to the substrate surface and is formed where the polymer contacts the substrate 408 in FIG. 5. The polymer element 112 is molded into the perforation or hole 106. A metal layer 120 is formed over the insulator 104 as shown in FIG. 2C, metalizing, for example, with copper (for example, 12.5 micrometers (um) thick). Another layer 124 is added over the metal layer 120. The layer 124, for example, can be beryllium about 2.5 microns thick. Another metal layer 128, for instance a copper layer of about 12.5 um, is formed over the Be layer 124 in FIG. 2E. Onto this metal stack is deposited a photo resist 132 shown in FIG. 2F. The photoresist may be a conformal coating obtainable with electrophoretic resist. The metal stack with photo resist is referred to as a sample 136, and is inserted between a form fitting top layer mask 140 and a form fitting bottom layer mask 142 as shown in FIG. 2G. Machined fine slots shown as curved and straight thin lines 148 in the body of masks 140 and 142 of FIG. 2G are cut into the masks 140, 142 to define the pattern of light that will be able to penetrate to the metal stack with photo resist. A helical pattern is projected onto the metal stack with photo resist, and a linear strip is patterned leading to a via to the right of the polymer bump 112 which will act as an electrical conduit through the hole 108 to connect the final product of contact structure embodied as top and bottom helical springs 160 shown in FIG. 2I. After the sample 136 has been removed from the mask 140, 142, the resist 132 developed, and the exposed metal etched away the pre-finished part 150 includes metal remaining only where it is desired for manufacturing the final part. The pre-finished part 150 includes the polymer bump 112, and the metal coating remaining in discrete layers 120, 124, 128. Fabricating the 3D masks 140, 142 utilized in process step 2G may use fine wire EDM, laser cutting, plasma cutting, water jet cutting, all in combination with any standard machining that may also be required.

(21) Referring to FIG. 2I, a high temperature bake out and anneal is performed in inert or chemically reducing gas environment during which the polymer 112 is decomposed and the metal layers 120, 124, 128 diffuse into one another forming a single metal alloy layer 164. Upon cooling or quenching at a speed chosen to give the desired metallurgical microstructure (which varies from a sudden liquid nitrogen quench 10 deg. C./second, to a slow cool of 1 deg. C./day) the final contact structure 170 is formed as shown in FIG. 31. The contact structure 170 includes the original insulator 104 and the double sided spring contact 160 which is electrically connected through the central plane by the single metal allow layer 164 via the small hole 108.

(22) Referring to FIGS. 3A and 3B, two LGA helical spring contacts are shown. A right handed helical contact 204 extending in the clockwise direction is shown in FIG. 3A. A left handed helical contact 208 extending in the counter clockwise direction is shown in FIG. 3B. Referring to FIGS. 3C and 3D, a contact assembly 212 includes a right handed helical contact 204 in mirror image relation with a left handed helical contact 208 having a carrier element 216 therebetween. The mirror image relation of the right handed and left handed helical contacts 204, 208, respectively, or helix reversal, imparts to a signal performance enhancements of the contacts at high frequencies.

(23) Referring to FIG. 4, a plurality of helical contact assemblies 212 (as shown in FIG. 3C,) including right hand and left hand wind directions as described regarding FIG. 3C, are used in a package 300. The package 300 includes the contact assemblies 212 between upper and lower substrates 304, 308, respectively. The package 300 provides helical contact assemblies 212 for a high frequency electrical applications.

(24) Referring to FIG. 5, an illustrative mask and polymer bump combination 400 includes a metal and photo resist coated molded polymer bump having a top portion 404 and a bottom portion 406 separated by a substrate 408. The bump sits within a form fitting cavity 412 of a top portion of a mask solid 410. The mask solid 410 includes a pattern 416 open to the outside. This example shows a slot pattern 416, however, other patterns including multileg helix patterns can be used. A bottom portion of the mask (not shown) typically is clamped together with the top portion 410. The closed mask solid is rotated and illuminated to provide light access through the slot pattern 416, producing the desired image on the bump 404, 406.

(25) Referring to FIGS. 6 and 7, a mask 500, is cut out of metal, and the pattern is cut out from the mask 500 as a four legged 504 helix pattern 502. The helix pattern 502 allows light to pass through the cut away portions as shown in FIG. 6. The helix pattern 502 includes the four legs 502 having circular pads 508.

(26) Referring to FIG. 8, a single side of a molded sacrificial polymer element 600 is frustoconically shaped. The element, as used in the embodiments of the invention depicted herein, are two sided, i.e., having two elements 600 in opposing relation such that the element 600 is joined through a perforation in a center carrier.

(27) Referring to FIG. 9, an intermediate fabrication step includes molded polymer element 600 under a lithographically formed metal helix spring contact 610. The helix spring contact 610 comprises different layers of metal (not shown) that will ultimately form an alloy after heat treatment and quenching in later processing.

(28) Referring to FIG. 10, the spring contact 610 shown in FIG. 9 is shown after a fabrication heating step wherein the sacrificial polymer element has been ablated (burned out or vaporized or other wise decomposed) and metal layers have been annealed to form alloy, and then the contact 610 is quenched to achieve the final metallurgy. Also shown is a conducting metal baseplate 620 which may be used in some instances to form mechanical anchoring of the helix legs and to electrically connect top and bottom sides of the contact 610. The baseplate 620 may be use instead of, or in addition to, a conductive element connecting the contact through a separate via as shown in FIG. 2.

(29) Referring to FIG. 11, a metal mask 650 includes a plurality of metal helix spring contacts 502 in a pattern on the metal mask 650. In an alternative embodiment, a large plurality of contact 502 may be patterned on a metal mask, for example, hundreds or thousands of contacts to form a desired array of contacts

(30) Referring to FIG. 12, a graph 700 of return loss in dB (reflected energy) for right handed-right handed (RH-RH) 704 and right handed-left handed (RH-LH) 708 spring contacts is shown. The graph 700 shows much lower reflective losses for RH-LH spring contacts even at relatively low frequencies. Referring to FIG. 13, a graph 750 of effects of spring contact leg rotation on electrical transmissive insertion loss performance is shown. RH-RH 758 is shown to have a more severe insertion loss than RH-LH 754. This becomes especially pronounced above frequency of 5 GHz.

(31) More specifically, the electrical performance of the helical contacts are illustrated in terms of their scattering parameters up to 20 GHz with ports defined at the top and the bottom of the contact. The reflection (referred as return) shown in FIG. 12 of the RH-LH type of contact is smaller than the RH-RH type, meaning less energy gets reflected when the signal propagates vertically through the contact. On the other hand, the transmission (referred as insertion), shown in FIG. 13, of the RH-LH type of contact is larger than the RH-RH type, indicating more energy gets transmitted through the contact. Both graphs/plots 700, 750 consistently demonstrate that the RH-LH type of contact has superior electrical performance to the RH-RH type in terms of signal integrity for high-speed signals.

(32) Thereby the present invention provides helical spring positioned in mirror image relation to each other. For example, a contact structure comprised of two helical springs that are mirror images of each other across the plane defined by a central carrier. Further, referring to FIGS. 2A-2I, the present invention provides a process including the steps of: molding polymer template of LGA contacts into the top and bottom of an insulating carrier plane; metalizing over the polymer; coating with photo resist, preferably electrophoretic or other type of conformal coating method; sandwiching the metalized molded polymer LGA within a carrier between top and bottom 3D masks; photoexposing; removing LGA from mask, and completing the lithography; and heat treating the sample to burn away (cleanly vaporize) polymer template leaving hollow contact and complete the alloying of constituent metals.

(33) In another embodiment of the invention similar to the process shown in FIGS. 2A-2I, however, the process uses elastomeric molded contacts instead of polymers meant to decompose, and eliminates the high temperature burn out step.

(34) The present invention achieves a rotational or twisting effect of the helical contacts. This rotation upon compression is desirable to achieve scrubbing through oxide and other thin contaminant layers normally present on electronic contacts. A feature of these helical electrical contact structures is that the direction of helicity reverses as it passes through the central carrier plane, i.e. that the top and bottom helix structures are mirror images with respect to the carrier plane. This preserves the signal integrity of a computer signal at high frequencies by causing significant cancellation of electromagnetic induction.

(35) The advantage of having a rotational scrubbing over typical lateral scrubbing is that as xy dimensions of LGA arrays are decreased, traditional lateral scrubbing increases the chance of the contact moving off the mating contact pad and resulting in an open circuit. Rotational scrubbing of an axiosymmetric contact does not move the contact relative to the position of the mating surface pad, and thus reduces the chances of a contact moving off a mating contact pad.

(36) Additionally, metalization over a polymer, may include the methods for metalizing including electroplating, electro less plating, physical vapor deposition such as meal evaporation or sputtering, chemical vapor deposition, plasma spray, powder coating, etc. The metalizing could be a single layer or multiple layers of different metals.

(37) In addition, coating with a photo resist, may include electrophoretic or other type of conformal coating method. The extreme z-dimension of LGA contacts complicates photolithography processes in several ways. In order to apply photo resist uniformly, one alternative is to use electrophoretic type photo resists. Electrophoretic resist may be used to provide uniform coverage of photo resist. Other methods of photo resist coating include spraying, spinning and liquid dipping.

(38) 3D Masks are used to expose all coated surface with a uniform dose of light using the photo resist coated LGA pre-contacts inserted into form fitting cavities on the underside of a mask. The desired contact pattern is then cut into the top of the mask using a very fine resolution machining technique called wire EDM (electro discharge machining). The LGA precontact protrusions on both sides of a carrier plane are accordingly inserted into masks in a sandwich form. Thus, the part can be illuminated and photo lithographically defined from both sides. 3D masks include a plate of metal where the bottom has machined cavities that are form fitting to the metalized and photo resist coated LGA contact arrays. In practical application the cavities need to be a little bigger in dimension than the coated LGA contacts to account for any fabrication tolerances etc.

(39) The final desired contact pattern is imparted to the mask by cutting or etching light pathways, or slots by wire EDM (wire electrodeposition machining) using very fine cutting wires. Alternatively, slots can be made by laser cutting, chemical etching, plasma etching etc. This slotting is expected to be most practically exercised cutting at right angle to the plane of the mask, i.e., through the z-direction of the mask metal. However, it can be advantageous to cut in a direction at right angles (normal) to the surface of the contact at any given location along the contact. Such normal-to-surface (NTS) slotting allow superior lithographic resolution and superior illumination uniformity. This process may be combined with chemical etching to achieve slot cuts of desired geometry.

(40) During photoexposure the entire part and mask sandwich assembly is tilted and rotated to affect as uniform a photoexposure as possible. This is most easily accomplished by moving the assembly with rotation and tilt stages programmed to move through a path optimal for a given contact pattern. Alternatively, the light source can be made to move around the part. Alternatively, the light source can be shaped by holographic and other types of lenses to provide a uniform distribution of light from many directions at once. Once the lithography is completed, the LGA can be removed from the mask sandwich, and lithography steps of developing resist and etching metal into desired pattern are completed.

(41) To obtain hollow contacts by cleanly burning or vaporizing away the polymer protrusions, e.g., the sacrificial polymer, heat is used, and also causes diffusion of multi layers of deposited metal into one another so they form alloys. Other methods of removing the temporary polymer bump, such as by chemical dissolving may be use.

(42) In an alternative embodiment of the present invention, metal on elastomer contacts are formed using steps of 2A-2E shown in FIG. 2 on molded elastomeric protrusions (bumps) rather than those made with sacrificial polymer. In this case, the metal will be left on the elastomer bump and used as part of the final contact.

(43) In an alternative embodiment, a process includes depositing constituent metal layers over the sacrificial polymer protrusions. For instance, to make a thin film copper beryllium contact the contact may consist of variable Cu to Be ratios depending on the final properties desired. Alternatively, for example, electroplating 12.50 m copper, then sputter deposit 2.5 m of Beryllium, followed by the deposition of a second layer of 12.5 m copper. This metal deposition would be followed by coating with photo resist (e.g. electrophoretic photo resist) and would be sandwiched into 3D egg carton like mask and exposed to light (e.g. Ultraviolet). The part would then be removed from the mask, the photo resist developed to remove protection from any metal desired to be removed. The unprotected areas of the metal would then be etched away. The part is then heated in an oven at sufficiently high temperatures and long enough period of time for the metals to diffuse together to form the alloy of interest after cooling at appropriate rates to obtain the temper of interest by controlled quenching. In this example, Cu and Be would need temperatures of 850 degrees C. for about an hour to diffuse (see Table 1 below). In the heating process the sacrificial polymer protrusions would have burned away and the remaining photo resist will have burned away (or could be removed chemically after the etching).

(44) Table 1 shows the calculated time for a three-layer thin film (12.5 micron Cu/2.5 micron Be/12.5 micron Cu) to completely diffuse into one another. This informs us that if these three materials were sequentially deposited onto the sacrificial polymer, that heating to 850 deg. C. would be required to achieve diffusion within one hour in a furnace, and that a temperature of 1000 deg. C. would be required to shorten the time to 6 minutes. Once homogeneously dispersed, the attainment of desired mechanical properties requires controlled cooling to facilitate desired precipitation and achieve desired metallurgical microstructure.

(45) TABLE-US-00001 TABLE 1 Temp C. time (h)* 400 1040042.8 425 296942.0 450 92456.9 475 31122.9 500 11241.0 525 4327.6 550 1765.5 575 759.4 600 342.8 625 161.7 650 79.5 675 40.6 700 21.4 750 6.6 800 2.2 850 0.8 900 0.3 950 0.2 1000 0.1

(46) Further, alloys other than BeCu can be made using the above technique, such as Ni into Copper, which would take 3.3 hours at 1000 C. Other varieties of metal combinations of two or more constituent metals could be achieved in this way in the desired contoured shape.

(47) The methods according to the present invention are applicable to any combination of metals including bi-metal alloys, ternary alloys, as well as any number of metals together to create an alloy. Other metals may be present, incidentally, for example, from adding adhesions layers, such as titanium and chromium.

(48) While the present invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that changes in forms and details may be made without departing from the spirit and scope of the present application. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated herein, but falls within the scope of the appended claims.