Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
1999-08-13
2001-06-19
Picardat, Kevin M. (Department: 2823)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S121000, C438S622000
Reexamination Certificate
active
06248656
ABSTRACT:
BACKGROUND OF THE INVENTION
Many components used in microelectronic assemblies include fine leads which are used for connection to other elements of the device. For example, as taught in commonly assigned U.S. Pat. Nos. 5,148,265; 5,148,266; 5,489,749; 5,536,909; 5,518,964 and 5,619,017 and PCT Publication WO/94/03036 the disclosures of which are hereby incorporated by reference herein, a microelectronic connection component may include a large number of electrically conductive terminals and leads disposed on a suitable support such as a dielectric sheet or a composite element including both metal and dielectric layers. The leads may include connection sections projecting beyond edges of the support or across apertures in the support. The connection sections may be bonded to contacts on a semiconductor chip to thereby connect the chip contacts to the terminals on the component. Most often, the leads are formed principally from a metal such as copper or a copper-based alloy. As disclosed, for example, in commonly assigned U.S. Pat. No. 5,597,470, it is often desirable to provide a layer of a cover metal over some or all of the surfaces of the principal metal portion of the lead. Depending upon the particular application, the cover metal may provide enhanced properties such as easier bonding of the leads to chip contacts or other structures; enhanced fatigue resistance; or enhanced corrosion resistance.
One common procedure for making leads on a support utilizes a thin conductive layer, typically copper, on a dielectric layer such as a dielectric layer of a rigid circuit panel or a flexible circuit panel, commonly referred to as “tape.” A layer of photoresist is applied over the conductive layer and patterned using conventional photographic processes to provide a series of openings in the form of elongated slots at locations where the leads are to be formed. The slots in the photoresist leave portions of the conductive layer at the bottom of each slot exposed. The principal metal such as copper is then deposited in the slots, typically by electroplating the principal metal onto portions of the conductive layer exposed within each slot. The principal metal deposited within each slot fills the bottom portion of the slot. A layer of the cover metal is deposited onto the top surface of the principal metal deposit, facing away from the support, by a further electroplating step. The photoresist is removed and the part is exposed to an etchant which will attack the conductive layer, thereby removing the conductive layer from regions between the leads. In a variant of this process, a layer of the cover metal is deposited on the conductive layer within each slot before deposition of the principal metal, so as to form a cover metal layer on the bottom surface of each principal metal deposit, facing toward the support. After the etching step used to remove the conductive layer, further cover metal may be deposited onto all of the lead surfaces, as by a further electroplating, electroless plating or immersion plating step.
Typically, the etchant which is used to remove the conductive layer will not attack the cover layer appreciably but will attack the principal metal. The cover layer on the top surface of the principal metal will protect the principal metal from the etchant to some degree. A cover layer on the bottom surface can also provide some protection. However, the vertically extending edge surfaces of the principal metal are not covered by the cover metal, and these surfaces are attacked by the etchant. Loss of principal metal results in a lead having an irregular cross-sectional shape and “undercutting” or removal of principal metal from beneath the top cover layer, leaving portions of the top cover layer projecting laterally at edges of the lead. Moreover, the principal metal in the finished lead will have cross-sectional area smaller than the cross-sectional area of the original principal metal deposit. All of these phenomena tend to weaken the lead, and to reduce its electrical performance. These phenomena are subject to some variability depending on variations in the etching process. These phenomena and variations in these phenomena are more significant in the case of fine leads, with small nominal cross-sectional dimensions. Thus, there has been a need for a lead-forming process which will alleviate the problem of edge surface undercutting.
Other metallic elements are also formed by processes similar to the lead-forming process discussed above. For example, metallic terminals are often formed on supports using a process which is the same as the conventional lead-forming process discussed above, except that the openings in the photo resist layer may be in the form. of circular discs, squares, ovals or other desired terminal shapes rather than elongated slots. The openings used to form the terminals may be connected with the elongated slot like openings used to form the leads, so as to form the terminals integral with the leads. Etching of the terminal edge surfaces presents the same problem as discussed above with reference to the leads. Similar problems can occur in formation of still other conductive elements, and hence there has been a similar need for a fabrication procedure which alleviates these problems.
SUMMARY OF THE INVENTION
One aspect of the present invention provides processes for forming metallic features on a base. A method according to this aspect of the invention preferably includes the step of providing a base with an electrically conductive layer on a first surface of the base, and depositing a first resist over the conductive layer. The method also desirably includes applying and patterning a main resist atop the first resist so as to leave openings in said main resist in locations where metallic features are to be formed, and forming openings in said first resist in registration with the openings in the main resist so that the conductive layer is exposed in said openings of said main resist. Once these openings have been provided, principal metal such as a copper alloy is deposited in the openings. After the principal metal has been deposited, the main resist is removed while the first resist is left in place, thereby exposing edges of the principal metal. A jacket of a cover metal is then deposited onto the edges of the principal metal. This jacket protects the edges of the principal metal during subsequent etching steps used to remove the conductive layer. Most preferably, the method includes the step of depositing a bottom cover metal layer in said openings before depositing said principal metal, and the said cover metal jacket deposited onto said principal metal merges with said bottom cover metal layer. The cover metal jacket and the bottom cover layer cooperatively surround the principal metal. For example, where the metallic features formed in the process include elongated leads, the cover metal desirably surrounds each lead around its entire perimeter at at least some points along the length of the lead.
The process in its preferred forms makes it possible to form leads with accurately-controlled dimensions, without substantial erosion of the leads during the etching process used to remove the conductive layer. Moreover, because the cover metals, such as gold, tend to have good bonding properties and good fatigue resistance, the process can provide leads having these properties where they are most needed—on the exterior or surface regions of the leads.
As further discussed below, the first resist may be a conventional photoresist or other dielectric material, or else may be a metal, such as chromium, which resists deposition of the cover metal during the process used to apply the cover metal. Thus, unless otherwise specified herein, term “resist” as used in this disclosure should not be limited to the dielectric materials commonly utilized as resists in the art, but should be understood as encompassing any material which will prevent deposition of the cover metal. Also, the step of depositing the resists may be performed in various ways, including
Baker David R.
Grange John
Light David
Wang Hung-Ming
Collins D. M.
Lerner David Littenberg Krumholz & Mentlik LLP
Picardat Kevin M.
Tessera Inc.
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