Detecting Completion of Electroless via Fill

Current multi-chip circuitry design requires the attachment of numerous integrated circuit chips to high density electrical interconnects, also known as multi-chip-modules (MCMs) or as substrates. The electrical interconnects normally include surface pads for bonding to surface mounted chips, a dielectric, and electrical lines buried in the dielectric for connecting selected pads to provide electrical routing between various bond sites on the chips. It is common to use copper for the buried lines and polyimide as the dielectric. The copper lines may form separate layers of orthogonal wiring sets that are interconnected to the surface pads by vertical metal pillars. Thus the construction of metal pillars becomes an essential part of the overall electrical interconnect fabrication process.

The fabrication of such electrical interconnects is a rapidly emerging technology with a number of different material and process options. While many processes have been proposed and put to use by a number of companies, there is typically a serious problem with yields since a substantial percentage of the interconnects are found to be defective. The yield problem is affected by the operator's expertise, choice of equipment, process characterization, and perhaps most importantly the adequacy of the fabrication process self.

For instance, if electroplating is used as one of the fabrication steps then pillar uniformity is likely to be an unavoidable problem. Photosensitive polyimide appears to be promising, but currently suffers from several disadvantages including limited resolution, limited via aspect ration, a high coefficient of thermal expansion (CTE), and a highly limited shelf life. Furthermore, if electroless metallization is used with photosensitive polyimide, the process control (e.g., electroless bath chemistry, wafer preparation, plating conditions) is usually unable to meet the pre-determined, non-flexible plating rate requirement. A disadvantage of conventional electroless via fill plating approaches is the difficulty in detecting when the electroless deposition is completed and the interconnect substrate can be removed from the plating bath.

Consequently, there is a need for better electroless via fill endpoint control.

An object of the present invention is to provide a simple and convenient technique for indicating the completion of electrolessly depositing metal into vias.

Another object of the present invention is a method of forming highly uniform metal pillars in a high density multilayer electrical interconnect (e.g. copper/polyimide substrates exceeding 300 lines per inch) by pattern electroless plating with a relatively simple, inexpensive and well-controlled process without the need for electroplating or a photosensitive dielectric.

A further object of the present invention is the use of a non-autocatalytic metal mask as a plasma etch mask, a plating completion indicator, an electroless via plating uniformity self-controller, and an adhesion layer between underlaying and overlaying layers.

A feature of the present invention is a method for electrolessly fabricating metal pillars in a dielectric and indicating the pillars are fabricated, which includes providing a flat first dielectric base, depositing a first electrically conductive metal over the first dielectric, depositing a second dielectric layer over the first metal, depositing over the second dielectric an electrically conductive detection mask containing an opening above a portion of the first metal, etching the second dielectric through the detection mask opening to form a via which exposes the portion of the first metal, electrolessly depositing a second electrically conductive metal into the via through the opening in the detection mask, wherein the first metal is autocatalytic to the electroless deposition, the detection mask is non-autocatalytic to the electroless deposition, and the second metal deposits on the exposed portion of the first metal and partially fills the via but fails to electrically connect the first metal to the detection mask and fails to deposit on the detection mask, continuing to electrolessly deposit the second metal into the via until the second metal fills the via, thereby forming a metal pillar in the via which electrically connects the first metal to the detection mask, at which time the second metal begins to deposit on the detection mask, continuing to electrolessly deposit the second metal until the second metal covers and changes the visual appearance of the top surface of the detection mask, and then discontinuing the electroless deposition of the second metal.

Another feature of the present invention is that completion of the electroless via fill is indicated by a change in both the visual appearance and electrochemical potential of the detection mask.

Advantages of the present invention include high uniformity between metal pillars particularly in comparison to electroplating, a convenient visual indication that the dielectric vias are filled with metal, fewer process steps than conventional mechanical polishing, lift-off or via-fill approaches, and the use of electroless plating without the need for electroplating or a photosensitive dielectric.

These and other objects, features and advantages of the present invention will be more readily apparent from a review of the detailed description and preferred embodiments which follow.

FIGS. 1-9 are fragmentary elevational views in cross-section illustrating successive stages for selectively depositing a metal on a dielectric,

FIGS. 10-16 are fragmentary elevational views in cross-section illustrating successive stages for depositing a second layer of dielectric on the metal, providing a patterned detection mask on the second dielectric, and forming vias in the second dielectric,

FIGS. 17-18 are fragmentary elevational views in cross-section illustrating successive stages for electrolessly filling the vias with metal to form uniform metal pillars indicating the vias are filled,

FIGS. 19-23 are fragmentary elevational views in cross-section illustrating successive stages for fabricating additional layers in accordance with one aspect of the present invention, and

FIGS. 24-27 are fragmentary elevational views in cross-section illustrating successive stages for fabricating additional layers in accordance with another aspect of the present invention.

The present invention will now be described in conjunction with a particularly preferred embodiment in which various layers of a high density multichip interconnect module are fabricated. During the initial steps, metal is selectively deposited on the dielectric, as shown in FIGS. 1-9.

As seen in FIG. 1, the reference numeral 10 generally indicates any suitable substrate. Substrate 10 includes a ceramic base 12 covered with a dielectric layer shown as 10 micron thick polyimide 14 having a smooth top surface 16. Other suitable dielectrics include polymers such as polyquinoline and benzocylobutene. In FIG. 2, trilayer 18 is deposited on polyimide 14. Trilayer 18 consists of 500 angstroms chromium 20 deposited on polyimide 14, 2500 angstroms copper 22 deposited on chromium 20, and 500 angstroms titanium 24 deposited on copper 22. Layers 20, 22 and 24 are preferably sputter deposited. Chromium 20 provides adhesion between copper 22 and polyimide 14, copper 22 provides an electroless plating activator, and titanium 24 provides adhesion for overlaying photoresist as well as protection of copper 22 from oxidation and corrosion. In FIG. 3 first photoresist mask 28 is deposited on titanium 24, and in FIG. 4 mask 28 is patterned by standard lithographic techniques to form openings 30. In FIG. 5, titanium 24 exposed by openings 30 is etched in a mild solution such as 0.5% hydrofluoric acid. Thereafter, in FIG. 6 a first electrically conductive metal shown as lines of copper 32 deposited through openings 30 onto copper 22. Electroless deposition of copper 32 is preferred but other approaches such as electroplating are suitable. In FIG. 7 photoresist 28 is stripped, and in FIG. 8 titanium 24, copper 22, and chromium 20 outside copper 32 are etched and removed. In FIG. 9 an optional protective overcoat shown as electroless nickel 34 is deposited over copper 32. Nickel alloys are also preferred overcoats. Together, copper 32 and nickel overcoat 34 form copper/nickel 35.

Numerous other approaches for selectively depositing metal lines on a dielectric are known in the art. Conventional electrolytic plating will do. An electroless approach using a mask is taught in U.S. Pat. No. 4,701,351 by Jackson, and maskless laser direct-write patterning for electroless plating is described in U.S. Pat. No. 4,981,715 by Hirsch et al.

Additional steps can now performed to deposit a second layer of dielectric on the the first metal, provide a detection mask on the second dielectric, and form vias in the dielectric over the first metal. This is depicted in FIGS. 10-16.

In FIG. 10, a second dielectric layer of polyimide 36 is coated over the first metal shown as copper/nickel 35 and the entire top of substrate 10, such as by spin coating or extrusion coating. Top surface 38 of polyimide 36 is then planarized by polishing. In FIG. 11, a detection mask 40 is deposited on polyimide surface 38. Detection mask 40 must be electrically conductive as well as non-autocatalytic (incapable of providing a seed or plating activator) for an electroless bath to follow. Suitable materials for the detection mask include chromium, aluminum, nickel, titanium, copper, tungsten, and combinations thereof, as well as conductive polymers. In some embodiments a particular detection mask material may be autocatalytic to a particular electroless bath, in which case the detection mask can be rendered non-autocatalytic by being coated with an inert oxide film. As shown, detection mask 40 is a trilayer 42 of sputtered titanium/copper/chromium similar to trilayer 18. Sputtered chromium 44 can be quite porous and when subjected to a plasma etch may allow etch pits to form in underlaying polyimide 36. Copper 46 minimizes this problem since the likelyhood of recesses in copper 46 aligning with recesses in chromium 44 is small. In addition, copper 46 is highly resistant to oxygen plasma etching which is commonly used for etching polymers. Titanium 48 further minimizes the formation of etch pits 44 and protects copper 46 from oxidation and corrosion. As seen in FIG. 12 a second photoresist mask 50 is deposited on detection mask 40 and selectively patterned to form openings 52 which expose regions 54 of detection mask 40. Openings 52 lay directly above a portion of copper/nickel 35 and define where the via holes (and metal pillars) will be positioned. In FIG. 13 the exposed detection mask regions 54 are etched to form detection mask openings 56. In addition, regions 58 of polyimide 36 become exposed. In FIG. 14 photoresist mask 50 is stripped. In FIG. 15 exposed polyimide regions 58 are oxygen plasma etched through detection mask openings 56 to form vertical cylindrical via holes or vias 60 which expose portions 62 of copper/nickel 35. Polyimide regions 58 can be etched by other suitable methods including laser drilling. In FIG. 16 titanium layer 48 and copper layer 46 are etched and removed, leaving only chromium layer 44 of detection mask 40. Chromium 44 alone can provide a better adhesion layer than the chromium/copper/titanium trilayer 42.

The purpose of the steps in FIGS. 17-18 is to 1) electrolessly deposit a second metal into the vias to form uniform metal pillars, and 2) provide an indication that the vias are filled.

With reference now to FIG. 17, a second electrically conductive metal is now electrolessly deposited into the vias. Suitable metals can be electrolessly deposited, such as chromium, gold, palladium, silver, copper and preferably nickel. Nickel is preferred over copper since nickel plates well to nickel without requiring a catalyst, whereas plating well-adhering copper to nickel or copper normally requires a catalyst, such as palladium, which might inadvertently end up on detection mask 40 as well. In all cases there must be an autocatalytic material (for which a metal in an electroless bath will spontaneously deposit thereon) provided in the via, illustrated herein as first metal 35 below vias 60. Conversly, dielectric 36 and detection mask 40 must each be non-autocatalytic. Initially, substrate 10 is immersed in an electroless nickel bath and nickel 66 is electrolessly deposited on exposed copper/nickel portions 62. It is critical to note that as nickel 66 begins to deposit into and partially fills (from bottom to top) vias 60, but before the deposited nickel 66 contacts detection mask 40 and electrically connects copper/nickel 35 to detection mask 40, nickel 66 does not plate to or cover top surface 70 of mask 40. Until at least one nickel pillar 68 reachs the top of it's surrounding via the nickel pillars shall fail to contact detection mask 40 (which lays above vias 60). Thus, prior to filling at least one via 60 with nickel 66, detection mask 40 is electrically isolated from copper/nickel 35 (which lays below vias 60) and nickel 66 will not plate or deposit on detection mask 40.

Referring now to FIG. 18, the electroless nickel plating is continued. Via fill occurs when the electrolessly deposited nickel 66 fills vias 60 to form nickel pillars 68. At this point at least one of nickel pillars 68 contacts detection mask 40 at a lower detection mask opening edge 71. Now nickel pillars 68 electrically connect copper/nickel 35 to mask 40, and two important changes occur. First, the electrochemical potential of mask 40 changes to the electroless plating electrochemical potential (of copper/nickel 35 and nickel pillars 68). For instance, the electrochemical potential of detection mask 40 may change from 0 volts to 0.2 volts. Furthermore, an electronic device such as a voltmeter can be provided to detect the change in electrochemical potential (voltage) of mask 40. The device can then communicate the completion of the via fill to an operator or to additional electrical equipment in order to discontinue the electroless bath. Second, as illustrated herein, as electroless nickel deposition continues nickel 66 begins to plate on and cover mask 40. In other words, the deposition of nickel 66 on detection mask 40 is not localized to the areas around nickel pillars 68. Instead, a nickel layer 72 is uniformly deposited over the entire top surface 70 of detection mask 40. As a result, the appearance of the entire top of the detection mask changes from the sputtered chromium's shiney fine grain appearance to electroless nickel's dull large grain appearance. If the mask is a different color than the second metal then obviously the completion of via fill will also be readily apparent. The visual contrast allows for visual detection that the vias are filled. Furthermore, detection mask 40 functions as a via electroless plating uniformity controller since nickel pillars 68 have uniform heights above detection mask top surface 70 equal to the thickness of nickel layer 72.

The present invention contrasts the prior art approach of electrolessly plating nickel pillars into vias in polyimide. In the prior art, once the vias are filled the nickel fails to plate the entire top surface of the polyimide, and instead forms nickel "mushrooms" over the vias. See T. G. Tessier et al, "Process Considerations in Fabricating Thin Film Multichip Modules," International Electronics Packaging Society Conference Proceedings 1989, pp. 294-313. Such miniature mushrooms are much harder for an operator to detect than a change in appearance of the entire top surface of the interconnect. In the present invention, nickel layer 72 provides for readily detectable visual contrast to mask 40 and thus allows for timely visual detection that nickel pillars 68 are completed and the electroless deposition can be discontinued. This prevents unnecessary time delays and provides better endpoint process control.

Referring now to FIGS. 19-23, if desired substrate 10 can undergo additional fabrication steps to form additional layers of a multilayer interconnect in accordance with one aspect of the present invention.

In FIG. 19 the nickel layer 72 above mask surface 70 is removed, such as by wet chemical etching or mechanical polishing. The current substrate top surface 80 consists of chromium top surface 78 and tops 82 of nickel pillars 68 in chromium openings 56. Nickel pillar tops 82 are in and aligned with chromium top surface 78 to form a smooth planar upper surface 80. In FIG. 20 a third photoresist mask 84 is deposited on top surface 80. Mask 84 is patterned with openings 86 which expose pillar tops 82 and adjacent regions 88 on chromium surface 78. In FIG. 21 a third electrically conductive metal such as chromium, gold, silver, nickel, or preferably copper 90 is electrolytically deposited through openings 86 on pillar tops 82 and adjacent detection mask regions 88. Copper 90 thus protrudes above surface 80 and can be patterned into a layer of circuitry or lines wider than pillars 66 and orthogonally Oriented to copper/nickel lines 35. Alternatively, if additional buried lines are not necessary, copper 90 can be configured as bonding pads or bumps. In either case, nickel pillars 66 serve as wiring connections between copper/nickel 35 and copper 90. In FIG. 22 photoresist mask 84 is stripped, and chromium 44 outside copper 90 is etched and removed. Chromium portions 88, however, remain to provide adhesion between the underlaying polyimide 36 and overlaying copper 90. As seen in FIG. 23, the steps shown in FIGS. 9-18 can be repeated to fabricate a multilayer structure comprising second protective overcoat 92, second pillars 94, third dielectric 96, second detection mask 98, and second uniform layer 100 over second detection mask 98.

Referring now to FIGS. 24-27, if desired substrate 10 can undergo additional fabrication steps to form more layers in accordance with another aspect of the present invention.

In FIG. 24 fourth mask 102 exposes portions 104 of second layer 100 but covers second pillars 94 and adjacent portions 106 of second detection mask 98. In FIG. 25, the exposed second layer portions 104 are wet etched and the second detection mask 98 beneath portions 104 is also removed by wet etching. This forms bumps 108 from unetched layer 100, and in FIG. 26 fourth mask 102 is stripped. Finally, in FIG. 27 the steps shown in FIGS. 9-18 are repeated to fabricate third protective overcoat 112, third pillars 114, fourth dielectric 116, third detection mask 118, and third uniform layer 120 over third detection mask 118.

The process for forming vertical pillar protrusions in FIGS. 24-27 may be simpler than that of FIGS. 19-23, however, the electrolessly deposited copper 100 (FIGS. 24-14 27) may not adhere to as well to the chromium 98 as the electrolytically plated copper 90 (FIGS. 19-23) adheres to chromium 44. While chromium can be treated to improve the adherence of electrolessly plated metals, the treatment is difficult.

The present invention is therefore well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While presently preferred embodiments of the invention have been described for the purpose of disclosure, numerous other changes in the details of construction and arrangement of parts may be made without departing from the spirit of the present invention and the scope of the appended claims.