Excimer induced topography of flexible interconnect structures

Abstract: A raised metal connection feature (29) is created by forming a conductive pad (17) on a flexible substrate (11) and applying a flexible dielectric insulator (19) and photoresist layer (21) over the conductive pad (17). An excimer laser (25) is used to ablate a via (27) through the dielectric insulator (19) and photoresist (21) to the conductive pad (17). Gold or other metal is then plated up in the via (17), and the photoresist (21) is removed, leaving a metal feature (29) extending above the dielectric insulator (19).

Description: The subject invention relates to a method of forming an interconnection feature and, generally, to flexible printed circuit fabrication and, more particularly, to a method for defining and building raised metal features for use in connection applications. From DE-A-37 02 354 there is known a method to form a semiconductor device having an electrode part formed on a wafer, a passivating film formed on the wafer exposing the electrode part, an insulating film formed on the passivating film, the insulating film comprising a predetermined thickness for forming a concave portion on the electrode part, and conducting material which is filled in the concave portion at least to the height of the isolating film. From EP-A-0 164 564 there is known a method wherein an excimer laser is used to burn a blind hole through an insulating layer and to bring a conductive material into the hole for contacting a conductor path. Various applications exist where it is desirable to form raised metal connection features above a surface. One such application is in the formation of connector assemblies wherein such raised features form interconnection contact points on flexible structures, for example, as illustrated in U.S Patent No. 4,453,795 assigned to Hughes Aircraft Company. According to the disclosure of that patent, a first wafer has copper contact pads on which are formed raised metal connector features in the form of copper buttons. A second wafer has copper pads located to mate with the copper buttons. The first and second wafers are then aligned and compressed together to establish connection between the copper buttons of the first wafer and the pads of the second wafer. The conventional method of creating raised metal connection features, such as the copper buttons of the ′795 patent, requires multiple photolithographic steps. First, photoresist is laminated, exposed and developed three times to create a circuit with an interconnection feature which extends above an acrylic adhesive dielectric film substrate surface .002 to .003 inches. Then photoresist is "tented" around the raised feature and another photoetch step is performed to define circuitry on a copper pad located on the substrate. Finally, a predrilled coverlay is applied over the circuitry, and gold plating is applied to the areas exposed by the coverlay to define the feature. The foregoing process has several problems. Numerous steps are required, resulting in a laborintensive procedure. Second, trying to tent photoresist around the raised feature is difficult and frequently results in voids. Additionally, the raised feature holds the coverlay off the copper pad, resulting in poor line definition. Finally, alignment of a predrilled coverlay is difficult and still leaves exposed copper on the pad area which must be gold plated. Gold plating degrades exposed areas of the acrylic adhesive substrate. SUMMARY OF THE INVENTION It is therefore an object of the invention to improve procedures for defining and building raised metal connection features; It is another object of the invention to reduce the number of steps in such a procedure over those conventionally employed; and It is another object to provide a simplified procedure which eliminates sources of inaccuracies and degradation in the prior art. The object is solved by the method of forming an interconnection feature in accordance with claim 1. According to the process of the invention, metal connection features are established after completion of processes which tend to induce dimensional changes such as lamination and etching. According to the process of the invention, an etched circuit is covered with a dielectric film insulator. The circuit is then laminated with a masking layer such as a plating resist. The feature location is then established by ablating the masking layer and the underlying dielectric film using a laser. Subsequently, the feature is plated up in the ablated location and the masking layer is stripped away, leaving the feature extending above the dielectric surface. The method of the invention thus achieves the formation of a feature possessing enough height above the dielectric to achieve an interconnect. Additionally, the method allows for more precise placement of connecting features, since the features are placed after the product has completed processes which tend to induce dimensional changes. Finally, the laser will ablate features smaller than those obtainable by conventional photolithography, thereby allowing increased interconnection density. BRIEF DESCRIPTION OF THE DRAWINGS The preferred embodiment of the just-summarized invention will now be described in detail in conjunction with the drawings, of which: FIGS. 1-6 are cross-sectional drawings illustrating various stages in the construction of a raised metal feature according to the preferred embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a flexible dielectric film substrate 11, typically formed of Pyralux™ available from DuPont, Wilmington, Delaware. Many other known materials may be used for the dielectric film, such as polyimide, polyimide with an acrylic adhesive, polyester, and teflon. A copper conductor layer 13, approximately .0014 inches thick, is formed on the substrate 11 by conventional printed circuit fabrication procedures such as lamination. An exposed photoresist layer 15 is formed on the copper conductor layer 13. The exposed photoresist layer 15 is of a width such as .020 inches. The width is selected to define a pad in the layer 13 on which a connection feature will ultimately be formed. To form the resist portion 15, the conductive layer 13 is coated with a photosensitive resist. The resist may be laminated onto the conductive layer 13 using a photoresist laminator, a device well-known in the art. With a photoresist laminator, photoresist is rolled on using heat and pressure. The approximate temperature used in the heating process is 225-250°F. The applied photosensitive resist is then exposed through a mask, thereby defining the portion 15, in an "exposure unit," a commercial well-known exposure device. The exposure unit (not shown) polymerizes the unmasked resist 15 by subjecting it to ultraviolet (UV) radiation. The resist is then developed. The developer solution removes the unpolymerized resist (the resist which was unexposed because it was masked), leaving the exposed or polymerized portion 15. An alternate method of defining the portion 15 would be to print the pattern using a silk screen and an alkali soluble etch resist. This alternative would eliminate the resist lamination, exposure and developing processes. Vinyl, polyvinyl or latex may also be used in lieu of photoresist, as known in the art. After the photoresist 15 is formed, a conventional etching step is performed and the photoresist 15 is stripped away, resulting in formation of a pad 17, as shown in FIG. 2. As further shown in FIG. 2, a flexible dielectric film insulator 19 is formed over the pad 17 at a thickness of, e.g., .001-.002 in. Pyralux™ again may be used as the dielectric film insulator 19. Pyralux™ is a staged cast resin which is laminated under pressure and temperature. Many other known dielectric film materials again could be used. After the Pyralux™ insulator layer 19 is applied, a layer 21 of photoresist is laminated onto the layer 19, as shown in FIG. 3. At this stage, the part comprising layers 11, 19, 21 and the copper pad 17 are ready for laser ablation. As shown in FIG. 4, a laser beam 25 is interrupted by an off-contact mask 23 and focused on the workpiece by suitable optics, shown schematically at 24. The mask 23 contains openings which, by interrupting and guiding the laser beam 25, define the desired features on the workpiece. As shown in FIG. 4, the focused laser light 25 ablates a feature opening or "via" 27 in the structure. The opening or via 27 extends through the photoresist and the Pyralux™ dielectric film layers 19, 21 to the surface of the copper pad 17. The laser employed is preferably an excimer laser emitting laser pulses of a 248-nanometer (nm) wavelength. The laser is used to ablate a via 27 having a depth of, for example, .005 to .008 inches. With the via 27 thus defined, copper 29 is electrolytically deposited in the opening 27 by a conventional electrolytic deposition procedure, as illustrated in FIG. 5. The copper 29 is built up level with the top surface 22 of the photoresist layer 21. Finally, as shown in FIG. 6, the photoresist layer 21 is stripped away, leaving the desired feature 29 formed upon the pad 17 and extending above the Pyralux™ insulator layer 19 a sufficient distance to achieve interconnection with a connection pad located on, e.g., a mother board. As will be appreciated, the just-described method eliminates numerous steps attendant to the conventional method of forming features such as feature 29. The feature 29 is also deposited after the etching process which forms the pad 17, thereby contributing to more precise feature placement. Finally, laser ablating as illustrated in FIG. 4 provides for creation of smaller features and, hence, increased interconnection density. For simplicity of discussion, the creation of one metal feature 29 has been described. Those skilled in the art will appreciate that the method of the preferred embodiment is applicable to creating various patterns of a large number of such features, using various appropriately constructed masks such as mask 23.

Claims: A method of forming an interconnection feature (29) comprising the steps of: forming a conductive layer (13) on a flexible substrate (11); forming a conductor pad (17) from said conductive layer (13); forming a dielectric insulator film (19) over said pad (17); forming a masking layer (21) on said dielectric insulator film (19); ablating said masking layer (21) and dielectric insulator film (19) to form a via (27), said via (27) leading to said pad (17); plating up conductive material in said via (27), said conductive material filling at least a portion of the via (27) above the dielectric insulator film (19); and removing said masking layer (21) to form said interconnection feature (29). The method of Claim 1 wherein said step of ablating comprises ablating said masking layer (21) and dielectric insulator film (19) with a laser (25). The method of Claim 2 wherein said laser comprises an excimer laser. The method of forming an interconnection feature of one of Claims 1-3 wherein said step of ablating comprises: positioning a mask (23) with respect to said masking layer (21), said mask including means for defining said feature; and passing a laser light beam (25) through said mask (23) to ablate said masking layer (21) and said dielectric insulator film (19). The method of one of the previous Claims wherein said step of forming a dielectric insulator film (19) comprises applying a staged cast resin over said pad (17). The method of one of the previous Claims wherein said masking layer (21) comprises a photoresist layer and said step of removing comprises stripping away said photoresist. The method of Claim 4 wherein said step of passing a laser light beam (25) includes the step of generating said laser light beam with an excimer laser. The method of Claim 7 wherein said excimer laser generates a wavelength of 248 nanometers.

Inventors: SCHREIBER CHRISTOPHER M, SCHREIBER, CHRISTOPHER M.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1990
Main classification: H05K3
IPC codes: H05K3, H05K1, H01R12