# Patent

## Title
Post passivation interconnection process and structures

## Abstract

A system and method for forming post passivation metal structures is described. Metal interconnections and high quality electrical components, such as inductors, transformers, capacitors, or resistors are formed on a layer of passivation, or on a thick layer of polymer over a passivation layer.

## Background
This application is a continuation of application Ser. No. 10/937,543, filed on Sep. 9, 2004, now U.S. Pat. No. 7,423,346.
RELATED PATENT APPLICATIONS
This application is related to MEG00-008CBBC, Ser. No. 10/303,451, filed on Nov. 25, 2002, MEG02-016, Ser. No. 10/445,558, filed on May 27, 2003, MEG02-017, Ser. No. 10/445,559, filed on May 27, 2003, and MEG02-018, Ser. No. 10/445,560, filed on May 27, 2003, all assigned to a common assignee, and all are herein incorporated by reference in their entirety.


BACKGROUND OF THE INVENTION
(1) Field of the Invention
The invention relates to the manufacturing of high performance, high current, low power, and/or low voltage Integrated Circuit (IC's), and, more specifically, to methods of creating high performance, high current, low power, and/or low voltage electrical components on the surface of a semiconductor substrate.
(2) Description of the Related Art
The continued emphasis in the semiconductor technology is to create improved performance semiconductor devices at competitive prices. This emphasis over the years has resulted in extreme miniaturization of semiconductor devices, made possible by continued advances of semiconductor processes and materials in combination with new and sophisticated device designs. Most of the semiconductor devices that are at this time being created are aimed at processing digital data. There are however also numerous semiconductor designs that are aimed at incorporating analog functions into devices that simultaneously process digital and analog data, or devices that can be used for the processing of only analog data. One of the major challenges in the creation of analog processing circuitry (using digital processing procedures and equipment) is that a number of the components that are used for analog circuitry are large in size and are therefore not readily integrated into devices that typically have feature sizes that approach the sub-micron range. The main components that offer a challenge in this respect are capacitors and inductors, since both these components are, for typical analog processing circuits, of considerable size.
When the dimensions of Integrated Circuits are scaled down, the cost per die is decreased while some aspects of performance are improved. The metal connections which connect the Integrated Circuit to other circuit or system components become of relative more importance and have, with the further miniaturization of the IC, an increasingly negative impact on circuit performance. The parasitic capacitance and resistance of the metal interconnections increase, which degrades the chip performance significantly. Of most concern in this respect is the voltage drop along the power and ground buses and the RC delay of the critical signal paths. Attempts to reduce the resistance by using wider metal lines result in higher capacitance of these wires.
Since the 1960's, sputtered aluminum has become a main stream IC interconnection metal material. The aluminum film is sputtered covering the whole wafer, and then the metal is patterned using photolithography methods and dry and/or wet etching. It is technically difficult and economically expensive to create thicker than 2 micrometers aluminum metal lines due to the cost and stress concerns of blanket sputtering. About 1995, damascene copper metal became an alternative for IC metal interconnection. In damascene copper, the insulator is patterned and copper metal lines are formed within the insulator openings by blanket electroplating copper and chemical mechanical polishing (CMP) to remove the unwanted copper. Electroplating the whole wafer with thick metal creates large stress and carries a very high material (metal) cost. Furthermore, the thickness of damascene copper is usually defined by the insulator thickness, typically chemical vapor deposited (CVD) oxides, which does not offer the desired thickness due to stress and cost concerns. Again it is also technically difficult and economically expensive to create thicker than 2 micrometers copper lines.
Current techniques for building an inductor on the surface of a semiconductor substrate use fine-line techniques whereby the inductor is created under a layer of passivation. The current fine-line techniques, either using sputtered aluminum or damascene copper, cannot provide inductors with a high quality factor due to the high resistance of fine-line metals. The resistance of the metal traces used to form the inductor coils will consume electrical energy. In addition, the fine-line techniques further imply close physical proximity between the created inductor and the surface of the substrate over which the inductor has been created (typically less than 10 micrometers), resulting in high electromagnetic losses in the silicon substrate which in turn further results in reducing the Q value of the inductor.
U.S. Pat. No. 5,212,403 (Nakanishi) shows a method of forming wiring connections both inside and outside (in a wiring substrate over the chip) for a logic circuit depending on the length of the wire connections.
U.S. Pat. No. 5,501,006 (Gehman, Jr. et al.) shows a structure with an insulating layer between the integrated circuit (IC) and the wiring substrate. A distribution lead connects the bonding pads of the IC to the bonding pads of the substrate.
U.S. Pat. No. 5,055,907 (Jacobs) discloses an extended integration semiconductor structure that allows manufacturers to integrate circuitry beyond the chip boundaries by forming a thin film multi-layer wiring decal on the support substrate and over the chip.
U.S. Pat. No. 5,106,461 (Volfson et al.) teaches a multi layer interconnect structure of alternating polyimide (dielectric) and metal layers over an IC in a TAB structure.
U.S. Pat. No. 5,635,767 (Wenzel et al.) teaches a method for reducing RC delay by a PBGA that separates multiple metal layers.
U.S. Pat. No. 5,686,764 (Fulcher) shows a flip chip substrate that reduces RC delay by separating the power and I/O traces.
U.S. Pat. No. 6,008,102 (Alford et al.) shows a helix inductor using two metal layers connected by vias.
U.S. Pat. No. 5,372,967 (Sundaram et al.) discloses a helix inductor.
U.S. Pat. No. 5,576,680 (Ling) and U.S. Pat. No. 5,884,990 (Burghartz et al.) show other helix inductor designs.
U.S. Pat. No. 6,495,442 to M. S. Lin et al and U.S. Pat. No. 6,383,916 to M. S. Lin, add, in a post passivation processing sequence, a thick layer of dielectric over a layer of passivation and layers of wide and thick metal lines on top of the thick layer of dielectric.
Co-pending U.S. patent application Ser. Nos. 10/445,558, 10/445,559, and 10/445,560 apply the post-passivation process of U.S. Pat. No. 6,383,916 in addition to creating high quality electrical components, such as an inductor, a capacitor or a resistor, on a layer of passivation or on the surface of a thick layer of dielectric.
SUMMARY OF THE INVENTION
It is the primary objective of the invention to improve the RF performance of High Performance Integrated Circuits.
Another objective of the invention is to provide a method for the creation of a high-Q inductor.
Another objective of the invention is to provide a method for the creation of high quality inductors, capacitors, or resistors in integrated circuits.
Yet another objective of the invention is to provide a method for mounting discrete electrical components on integrated circuits in a post-passivation process.
It is yet another objective of the invention to provide a method for fabricating post-passivation metal interconnections and devices having a much smaller RC product than that of the fine line metal interconnections under the passivation layer.
A further objective of the invention is to provide a method for fabricating post-passivation metal interconnections and devices having a structure different from the structure of the fine line metal interconnections underlying the passivation layer.
A still further objective of the invention is to provide a method for fabricating post-passivation metal interconnections and devices by a selective deposition process.
A still further objective is to provide post-passivation metal interconnection and device structures having a structure different from the structure of the fine line metal interconnections underlying the passivation layer.
Another objective is to provide post-passivation metal interconnection and device structures having a much smaller RC product than that of the fine line metal interconnections under the passivation layer.
In accordance with the objectives of the invention, a method of forming post-passivation interconnections and devices is achieved. A semiconductor substrate is provided. Fine line metal interconnection comprising one or more layers of metals overlying the semiconductor substrate is provided formed by a blanket metal deposition process and overlaid with a passivation layer, wherein the passivation layer comprises at least one passivation opening through which is exposed at least one top level metal contact point on the fine line metal interconnection. A post-passivation metal structure comprising one or more layers of metals formed over the passivation layer is formed by a selective metal deposition process and connected to at least one top level metal contact point wherein the at least one passivation opening is formed to a width larger than about 0.1 um.
Also accordance with the objectives of the invention, a post-passivation metal interconnection and device structure is achieved. The post passivation system of the invention comprises a semiconductor substrate, fine line metal interconnection comprising one or more layers of metals overlying the semiconductor substrate, a passivation layer overlying the fine line metal interconnection, wherein the passivation layer comprises at least one passivation opening through which is exposed at least one top level metal contact point on the fine line metal interconnection, and a post-passivation metal structure comprising one or more layers of metals formed over the passivation layer and connected to at least one top level metal contact point wherein the passivation opening's width is larger than about 0.1 um.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional representation of the interconnection scheme shown in U.S. Pat. No. 6,383,916.
FIG. 2 is a cross sectional representation of an inductor of the invention, created on a thick layer of polyimide.
FIGS. 3-9 depict, in cross-sectional form, the creation of gold metal structures of the invention, through a layer of polymer.
FIGS. 10-14 depict the creation of copper metal structures of the invention, through a layer of polymer.
FIG. 15 shows an inductor of the invention above a layer of passivation.
FIG. 16a is a cross sectional representation of a transformer according to the invention, formed over a polymer layer, over a layer of passivation.
FIG. 16b is a cross sectional representation of a transformer according to the invention, with the bottom coil formed on a layer of passivation.
FIGS. 17a-17c are cross sectional representations of a capacitor of the invention, formed over a polymer layer over passivation.
FIG. 18 is a cross sectional representation of a resistor of the invention, formed over a passivation layer.
FIGS. 19a-19b are cross sectional representations of a resistor of the invention, formed over a thick polymer layer, over a passivation layer.
FIG. 20 is a cross sectional representation of a silicon substrate over which a discrete electrical component has been mounted, on the top of a thick polymer layer, using surface mount technology.
FIG. 21 is a cross sectional representation of a silicon substrate, having a passivation layer on the surface of which a discrete electrical component has been mounted, using surface mount technology.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
The post-passivation process, described in U.S. Pat. Nos. 6,383,916 and 6,495,442, to the same inventor as the current invention, teaches an Integrated Circuit structure where re-distribution and interconnect metal layers are created in layers of dielectric over the passivation layer of a conventional Integrated Circuit (IC). A layer of passivation is deposited over the IC, a thick layer of polymer is alternately deposited over the surface of the layer of passivation, and thick, wide metal lines are formed over the passivation.
U.S. Pat. No. 6,303,423 and the co-pending related patent applications, also assigned to a common assignee as the current invention, address, among other objectives, the creation of an inductor whereby the emphasis is on creating an inductor of high Q value above the passivation layer of a semiconductor substrate. The high quality of the inductor of the invention allows for the use of this inductor in high frequency applications while incurring minimum loss of power. The invention further addresses the creation of a capacitor and a resistor on the surface of a silicon substrate whereby the main objective (of the process of creating a capacitor and resistor) is to reduce parasitics that are typically incurred by these components in the underlying silicon substrate.
Referring now more specifically to FIG. 1, there is shown a cross section of one implementation of U.S. Pat. No. 6,383,916. The surface of silicon substrate 10 has been provided with transistors 11 and other devices (not shown in FIG. 1). The surface of substrate 10 is covered by an interlevel dielectric (ILD) layer 12, formed over the devices.
Layers 14 represent metal and dielectric layers that are typically created over ILD 12. Layers 14 contain one or more layers of dielectric, interspersed with one or more metal interconnect lines 13 that make up a network of electrical connections. At a top metal layer are points 16 of electrical contact. These points 16 of electrical contact can establish electrical interconnects to the transistors and other devices 11 that have been provided in and on the surface of the substrate 10. These metal layers are referred to as fine line metal interconnections. Typically, the intermetal dielectric (IMD) layers comprise silicon-based oxides, such as silicon dioxide formed by a chemical vapor deposition (CVD) process, CVD TEOS oxide, spin-on-glass (SOG), fluorosilicate glass (FSG), high density plasma CVD oxides, or a composite layer formed by a portion of this group of materials. The IMD layers typically have a thickness of between about 1000 and 10,000 Angstroms. The fine line metal interconnections are typically formed by sputtering aluminum or an aluminum alloy and patterning the aluminum to form the fine metal lines. Alternatively, the fine lines may be formed by a copper damascene process. In the copper damascene process, the copper is protected by an adhesion/barrier layer not only underlying the copper, but also surrounding the copper at the sidewalls of the line through the IMD. These fine lines typically have a thickness of between about 1000 and 10,000 Angstroms. In the fabrication process of the fine line metal interconnections, a typical clean room environment of class 10 or less is required; that is, having no more than 10 particles larger than 0.5 microns in any given cubic foot of air. The fine line IC metal is fabricated using 5.times. steppers or scanners or better and using a photoresist layer having thickness of less than 5 microns.
A passivation layer 18, formed of, for example, a composite layer of silicon oxide and silicon nitride, is deposited over the surface of layers 14, and functions to prevent the penetration of mobile ions (such as sodium ions), moisture, transition metal (such as gold, copper, silver), and other contamination. The passivation layer is used to protect the underlying devices (such as transistors, polysilicon resistors, poly-to-poly capacitors, etc.) and the fine-line metal interconnection.
The key steps of U.S. Pat. No. 6,383,916, begin with the deposition of an optional thick layer 20 of polymer that is deposited over the surface of passivation layer 18. Access must be provided to points of electrical contact 16; for this reason, a pattern of openings 22, 36 and 38 is formed through the polymer layer 20 and the passivation layer 18. The pattern of openings 22, 36 and 38 aligns with the pattern of electrical contact points 16. Contact points 16 are, by means of the openings 22/36/38 that are created in the layer 20 of polymer, electrically extended to the surface of layer 20.
Layer 20 is a polymer, and is preferably polyimide. Polymer 20 may optionally be photosensitive. Examples of other polymers that can be used include benzocyclobutene (BCB), parylene or epoxy-based material such as photoepoxy SU-8 (available from Sotec Microsystems, Renens, Switzerland).
After formation of openings 22/36/38, metallization is performed to create patterned thick, wide metal layers 26 and 28, and to connect to contact points 16. Lines 26 and 28 can be of any design in width and thickness to accommodate specific circuit design requirements, which can be used for power distribution, or as a ground or signal bus. Furthermore, metal 26 may be connected off-chip through wire bonds or solder bumps.
Contact points 16 are located on top of a thin dielectric (layers 14, FIG. 1), and the pad size must be kept small to minimize capacitance with underlying metal layers. In addition, a large pad size will interfere with the routing capability of the layer of metal.
Layer 20 is a thick polymer dielectric layer (for example, polyimide) having a thickness in excess of 2 micrometers (after curing). The range of the polymer thickness can vary from 2 micrometers to 150 micrometers, dependent on electrical design requirements. For a thicker layer of polyimide, the polyimide film can be multiple coated and cured. The polymer is formed by spin-on, printing, or laminating.
U.S. Pat. No. 6,383,916 B1 allows for the interconnection of circuit elements at various distances, over the path 30/32/34 shown in FIG. 1, using the thick, wide (as compared to the underlying “fine line” metallization in layers 14) metal of 28. Thick, wide metal 28 has smaller resistance and thicker dielectric 20 has smaller capacitance than the fine line metallization 14 and is also easier and more cost effective to manufacture.
In more detail, the clean room environment of the post-passivation metal process can be of a class 100 or more; that is, having 100 or more particles larger than 0.5 microns in any given cubic foot of air. During photolithography in the post-passivation metal process, aligners or 1.times. steppers are used with a photoresist having a thickness of greater than 5 microns. The thick, wide metal lines have a thickness of between about 2 and 100 microns and a width larger than about 2 microns. The lines can be very wide, as would be used for power and ground planes.
FIG. 2 shows how the interconnect aspect of U.S. Pat. No. 6,383,916, can be modified to form an inductor on the surface of the thick layer 20 of polyimide. The inductor is created in a plane that is parallel with the surface of the substrate 10 whereby this plane however is separated from the surface of the substrate 10 by the combined heights of layers 12, 14, 18, and 20. FIG. 2 shows a cross section 40 of the inductor taken in a plane that is perpendicular to the surface of substrate 10. The wide and thick metal will also contribute to a reduction of the resistive energy losses. Furthermore, the low resistivity metal, such as gold, silver and copper, can be applied using electroplating; the thickness can be about 20 micrometers.
By increasing the distance between the inductor and the semiconductor surface, as compared to prior art approaches in which the inductor is formed under the passivation, the electromagnetic field in the silicon substrate will be reduced as the distance is increased, and the Q value of the inductor can be increased. The inductor overlies the layer of passivation and by, in addition, creating the inductor on the surface of a thick layer of dielectric (such as a polymer) formed over the passivation layer. In addition, by using wide and thick metal for the creation of the inductor, the parasitic resistance is reduced.
In an important feature of the invention, the openings 19 in passivation layer 18 may be as small as 0.1 um wide. Thus, contact pads 16 may also be nearly as small, which allows for greater routing capability in the top fine-line metallization layer, and lower capacitance.
In another important feature of the invention, the openings 22/36/38 in polymer 20 are larger than the passivation openings 19. The polymer openings 22/36/38 are aligned with passivation openings 19. The larger polymer openings allow for relaxed design rules, simpler opening formation, and the use of a thick metal layer for the post-passivation metallization of the invention.
FIG. 2 illustrates interconnect structure 26 as well as inductor 40, wherein the inductor includes two contacts 41 and 43, through polymer layer 20 to contact pads 16.
In another feature of the invention, the FIG. 2 structure may be covered by an additional layer of polymer (not shown).
Referring now to FIGS. 3-8, further details are provided for forming the post passivation inductor (and other passive devices) of the invention. In FIG. 3, a substrate 80 is shown, which could be an underlying dielectric layer, and a metal contact point 81, preferably comprising aluminum. A layer 84 of passivation has been patterned creating an opening 82 through layer 84 that exposes the contact pad 81. Layer 86 is a layer of polymer, preferably polyimide, as earlier described, deposited over the layer 84 of passivation, including the exposed surface of the contact pad. Polymer layer 86, such as polyimide, is typically spun on. For some thick layers of polymer, the polymer can be screen printed. Alternately, a laminated dry film polymer may be used.
FIG. 4 illustrates forming an opening 87 in polymer 86, wherein the polymer opening 87 is larger than passivation opening 82. Opening 87 is depicted having sloped sides 85. Polymer layer 86 is exposed and developed to form opening 87, which initially has vertical sidewalls. However, the subsequent curing process causes the sidewalls to have a slope 85, and a opening 87 to have a resultant partially conical shape. The sidewall slope 85 may have an angle of 45 degrees or more, and is typically between about 50 and 60 degrees. It may be possible to form the sidewalls with an angle as small as 20 degrees.
By creating relatively large vias through the layer of polyimide or polymer, aligned with smaller vias created through the underlying layer of passivation, aligned with underlying sub-micron metal layer, it is clear that the sub-micron metal vias can effectively be enlarged when progressing from the sub-micron metal layer to the level of the thick, wide metal.
Continuing to refer to FIG. 4, one metallization system and selective deposition process for forming the post passivation interconnect and inductor of the invention is depicted. First, a glue/barrier layer 88, preferably comprising TiW, TiN, TaN, Ti, or Cr is deposited, preferably by sputtering to a thickness of between about 500 and 5,000 Angstroms. A gold seed layer 90, is next sputter deposited over the glue/barrier 88, to a thickness of between about 300 and 3,000 Angstroms.
Referring now to FIG. 5, a bulk layer 92 of gold (Au) is next formed by electroplating, to a thickness of between about 1 and 20 micrometers. Electroplating is preceded by deposition of a thick photoresist 94 (to a thickness greater than the desired bulk metal thickness), and conventional lithography to expose the gold seed layer 90 in those areas where electroplating thick metallization is desired.
Thus, a selective deposition process forms the post-passivation metal structure. An advantage of the selective deposition process of the invention is a minimization of wasted material. In the selective deposition process, the metal is electroplated only where it is needed. In contrast, in the standard copper damascene process used for fine line metallization, copper is electroplated everywhere and then etched or polished away where it is not needed. Covering the whole wafer with thick copper creates stress which causes the process problem. This is a waste of copper. The removed copper is often contaminated and may not be able to be reused.
Furthermore, in the selective deposition process of the invention, the thickness of selective electroplated metal is defined by the thickness of photoresist, which can be formed as thick as 100 microns. In other words, it is feasible and cost-effective to form thick metal by selective electroplating. By contrast, it is technically difficult to form thick metal by a damascene copper process. A primary limitation to forming thick copper damascene lines is the thickness of the chemical vapor deposited (CVD) oxides which define the damascene copper thickness. CVD oxides cannot be thickly deposited due to stress concerns. It is also very expensive to deposit thick CVD oxides.
After electroplating, photoresist 94 is removed, as shown in FIG. 6. Glue/barrier Layer 88 and gold seed layer 90 are now removed, as shown in FIG. 7, by etching, using bulk Au layer 92 as a mask. During the self-aligned wet etching of the adhesion/barrier layer, an undercut 89 is formed in the adhesion/barrier layer, as shown in FIG. 16. The undercut is usually between 0.1 to 1.0 micron per side, depending on etching recipe and over-etch time.
One coil of inductor 40 is shown, but it would be understood that the complete inductor would be formed at the same time.
The structure of the post-passivation metal interconnect and device, such as the inductor coil 40 shown, is different from the structure of the fine line metallization. In addition to the undercut 89 in the adhesion/barrier layer, there is a clear boundary between the sputtered thin gold layer 90 and the electroplated thick gold 92. This can be seen, for example, in a transmission electron microscope (TEM) image. The boundary is due to different grain sizes and/or grain orientation in the two gold layers 90 and 92. For example, in a 1,000 Angstroms thick sputtered gold layer 90 under a 4 microns thick electroplated gold layer 92, the grain size of the sputtered gold layer 90 is about 1,000 Angstroms, and the grain boundary is perpendicular to the surface of substrate. The grain size of the electroplated gold 92 is greater than 2 microns with the grain boundary not perpendicular, and typically, at an angle of about 45 degrees from the substrate surface. In the fine line metal interconnections, there is no undercutting or clear boundary of grain size difference inside the aluminum layer.
In another feature of the invention, polymer opening 87 may be only partially filled, as shown in FIGS. 8-9, which provides tight design rules for fine-pitch inductors. The design rule of polymer opening 87 is typically about 15 um, while the metal traces of inductor are as tight as a 4 um pitch. Therefore, patterning metal inside the polyimide opening is a very important feature of this technology.
Glue/barrier layer 88 and Au seed layer 90 are sputtered as previously described, and photoresist 95 formed as shown in FIG. 8, followed by electroplating gold bulk layer 92. Photoresist 95 is then stripped, and the seed layer and glue/barrier etched as previously described, and as shown in FIG. 9.
In another embodiment of the invention, copper may be used as the bulk metal in the post-passivation metallization scheme. The FIG. 4 structure is a starting point. Next, as shown in FIG. 10, a glue/barrier layer 100 of TiW, TiN, TaN, Cr, or Ti is sputter deposited to a thickness of between about 200 and 2000 Angstroms. Next, a Cu seed layer 102 is sputter deposited to a thickness of between about 2,000 and 10,000 Angstroms. Bulk layer 104 of Cu is next electroplated to a thickness of between about 3 and 20 micrometers, also using a photoresist 94′ and conventional lithography to define the areas to be electroplated in the selective deposition process of the invention. Finally, an optional cap layer 106 comprising Ni may also be formed, also by electroplating, to a thickness of between about 0.1 and 3 um.
Referring to FIG. 11, photoresist 94′ is stripped, exposing Cu seed layer 102. Glue/barrier layer 100 and Cu seed layer 102 are now removed, as shown in FIG. 21, by etching. The bulk Cu layer 104 is used as a mask for this etch. In this case, there is an undercut at the edge of the glue/barrier layer 100 (Cr or Ti layer).
If the optional Ni cap layer 106 is used, it acts as an etch stop during the etching of glue/barrier 100 and seed layer 102. With the Ni cap, a faster Cu etch recipe can be used for removing the seed layer since there is no loss of Cu bulk in this configuration. In this case, there is an undercut in the bulk copper layer 104 and seed copper layer 102, in addition to the undercut in the glue/barrier layer 100. In other words, there is an overhang of the Ni cap at the edge of the bulk copper layer 104 and seed copper layer 102.
One coil of inductor 40 is shown, but it would be understood that the complete inductor would be formed at the same time.
As described above, an undercut 101 in the adhesion/barrier layer is formed during etching of that layer in the post-passivation metal process. Additionally, the adhesion/barrier layer 100 in the post-passivation metal structure is formed only under the copper line, as shown in FIG. 12. In the copper damascene process of the fine line metallization, an adhesion/barrier layer is needed not only at the bottom, but also at the sidewalls of the copper line. This is needed primarily to protect the underlying active devices from copper ions. However, in the post-passivation scheme of the invention, the passivation layer 84 provides the barrier to copper ions.
In another feature of the invention and as earlier described, polymer opening 87 may be only partially filled, as shown in FIGS. 13-14. Glue/barrier layer 100 and Cu seed layer 102 are sputtered as previously described, and photoresist 95′ formed as shown in FIG. 13, followed by electroplating Cu bulk layer 104 and Ni 106. Photoresist 95′ is then stripped, and the seed layer and glue/barrier etched as previously described, and as shown in FIG. 14.
Referring now to FIG. 15, layers similar to earlier descriptions are shown whereby in this case no layer of polyimide has been deposited over the layer of passivation. An inductor 19 has been created on the surface of layer 18 of passivation. The ohmic resistivity of the metal that is used for inductor 19 must be as low as possible. For this reason, the use of a thick layer of, for instance, gold is preferred for the formation of inductor 19. It has been shown that a thick layer of gold increased the Q value of inductor 19 from about 5 to about 20 for 2.4 GHz applications.
The FIG. 15 inductor may be connected to other elements in various configurations, as earlier described and as shown in the related patent applications.
An additional layer of polymer 21 may optionally be formed over inductor 19.
FIGS. 16a-16b depict a transformer made according to the invention. The transformer consists of bottom coil 60, and top coil 62, isolated by a dielectric layer 47. Polymer layers 20, 47 and 64 are formed, and comprise materials, previously described. Openings 66 are provided in top polymer layer 64 for connections to the top coil 62.
FIG. 16b is a cross-sectional representation of a transformer of the invention, in which the bottom coil 60 is formed directly on passivation layer 18. Solenoid and toroidal inductors may be fabricated in the same way, as described in related patent applications MEG02-016, Ser. No. 10/445,558, filed on May 27, 2003, MEG02-017, Ser. No. 10/445,559, filed on May 27, 2003, and MEG02-018, Ser. No. 10/445,560, filed on May 27, 2003, herein incorporated by reference in their entirety.
Besides inductors, it is very useful to form other passive devices, such as capacitors and resistors, using the method and structure of the invention.
FIG. 17a is a cross section of a capacitor that has been created over a substrate 10. A layer (or layers) 14 of fine line conductive interconnect lines and contact points 16 have been created over substrate 10. A layer 18 of passivation has been deposited over layer 14, with openings created in layer 18 of passivation through which contact pads 16 can be accessed.
A capacitor contains, as is well known, a lower plate, an upper plate and a layer of dielectric that separates the upper plate from the lower plate. FIG. 17a includes lower plate 42, upper plate 45, and dielectric layer 46. The upper and lower plates are formed as earlier described, using electroplated Au or Cu for the bulk metals. An optional protective polymer, preferably polyimide, may be formed over the capacitor. Contacts to the capacitor may be made as described in the related patent applications for inductor terminals (both down, one up and one down, or both up).
Lower plate 42 is formed to a thickness of between about 0.5 and 20 micrometers. Layer 46 of dielectric is between about 500 and 50,000 Angstroms. Upper plate 45 is between about 0.5 and 20 micrometers thick.
The post-passivation capacitor shown in cross section in FIG. 17a has:
reduced parasitic capacitance between the capacitor and the underlying silicon substrate
allowed for the use of a thick layer of conductive material for the capacitor plates, reducing the resistance of the capacitor; this is particularly important for wireless applications
can use high-dielectric-constant material in addition to a polymer, such as TiO.sub.2 or Ta.sub.2O.sub.5, Si.sub.3N.sub.4 or SiO.sub.2, for the dielectric between the upper and the lower plate of the capacitor, resulting in a higher capacitive value of the capacitor.
The capacitor of FIG. 17a may alternately be formed above a polymer layer (deposited over passivation 18), similar to the transformer of FIG. 16a. 
Dielectric layer 46 is formed of a high-K dielectric material such as Si.sub.3N.sub.4, TEOS, Ta.sub.2O.sub.5, TiO.sub.2, SrTiO.sub.3, or SiON, which are typically deposited by CVD (Chemical Vapor Deposition).
Alternately, the dielectric layer 46 can be a polymer film, including polyimide, benzocyclobutene (BCB), parylene or an epoxy-based material such as photoepoxy SU-8.
FIGS. 17b-17c show a cross section where, as in FIG. 17a, a capacitor is created. In the cross section that is shown in FIG. 17b a thick layer 20 of polymer has been deposited over the surface of the passivation layer 18 and has been patterned in order to make the contact pads 16 accessible though the thick layer 20 of polymer. FIG. 17b shows the polymer vias having a smaller via diameter than the vias created through the layer of passivation. It is however preferred, as shown in FIG. 17c, that larger vias be used in conjunction with smaller passivation vias. The thick layer 20 of polymer moves most of the capacitor, that is the lower plate 42, the upper plate 45 and the dielectric 46, further from the surface of substrate 10 by a distance equal to the thickness of layer 20. It has previously been stated that the range of polyimide thickness can vary from 2 micrometers to 150 micrometers, depending on electrical design requirements. This leads to a significant increase in distance between the capacitor and underlying structures, including metal lines and/or the silicon substrate, so that parasitic capacitance is significantly reduced.
FIGS. 17a-17c depict both capacitor terminals being connected down to a lower layer. The capacitor may also be contacted in one-up-one-down configuration or a two-up technique.
The capacitor of FIGS. 17a-17c may optionally be covered with a protective layer of polymer, as previously described.
FIG. 18 shows a cross section of a substrate 10 over which has been deposited a layer 18 of passivation, with a resistor 48 formed over passivation layer 18. A resistor, as is well known, is created by connecting two points with a material that offers electrical resistance to the passage of current through the material. For the creation of layer 48 a resistive material is used, such as TaN, NiCr, NiSn, tungsten (W), TiW, TiN, Cr, Ti, TaSi or Ni. Among these resistive materials, NiCr provides the best TCR (Temperature Coefficient of Resistance), which can be as small as 5 ppm/.degree. C. Resistor dimensions such as thickness, length and width of deposition of high resistive material 48 are application dependent. The resistor that is shown in cross section in FIG. 18 is, as are the capacitors of FIGS. 17a-17c, created in a post-passivation process on the surface of layer 18 of passivation.
FIGS. 19a-19b shows the resistor of the invention formed over a thick layer of polymer 20, connected to contact pads 16. By increasing the distance between the body of the resistor and the substrate (by the thickness of the polymer layer 20 and other intervening layers) the parasitic capacitance between the body of the resistor and the substrate is reduced, resulting in an improved resistive component (reduced parasitic capacitive loss, improved high frequency performance).
FIGS. 18, 19a and 19b show a “two-down” system for contacting the terminals of the resistor 48. The resistor may also be contacted in one-up-one-down configuration.
An additional layer of polymer, to protect the resistor, may optionally be formed over the resistor of FIGS. 18, 19a and 19b. 
Further applications of the post-passivation processing of the invention are shown in FIGS. 20 and 21, which concentrate on making contact points between contact pads 16 and an overlying electric component, such as a discrete inductor, capacitor, resistor or other passive device. Interconnect metal 50 of the invention is formed in polymer openings, as previously described, which are aligned with smaller passivation openings, to connect to pads 16, and serves as an under-bump metal (UBM). Solder contact bumps are formed over UBM 50 using conventional methods of solder deposition (selective plating, ball mounting, or screen printing on the surface of contacts 50), the application of a flux on the deposited solder and flowing the solder. A discrete device 54 is connected to solder balls 52 and has solder 53 to facilitate the connection. This is similar to the surface mount technology used in the assembly of printed circuit boards. The discrete electrical component may be, but is not limited to, devices such as inductors, capacitors or resistors.
FIG. 21 illustrates mounting of discrete device 54, using solder bumps 56, and UBM 50, directly over passivation layer 18.
The discrete components of FIGS. 20 and 21 have the advantages of performance and cost savings since the discrete component does not have to be mounted on a Printed Circuit Board as is the common practice in the art.
UBM 50 is formed using the metallization scheme of the invention (as shown and described with respect to FIGS. 10-12), except that a thick layer of solder 52 (about 5 to 100 microns) is electroplated on top of Ni 106 (refer to FIG. 10), before the photoresist 94 stripping. More specifically, the UBM 50 in this case comprises Ti or Cr as the adhesion layer, sputtered Cu as seed layer, and electroplated Cu and Ni as the solder diffusion barrier layer. If the solder layer 52 is formed by screen printing or solder ball mounting, the UBM layer 50 is prepared as follows: again following the process steps in FIGS. 10-12, except that a thin layer of gold is electroplated on top of Ni 106 (refer to FIG. 10), before the photoresist 94 stripping. The thinner range of gold is preferable to avoid a high gold concentration in the solder near the UBM/solder interface, after solder screen printing or solder ball mounting, and reflow processing. More specifically, the UBM 50 in this case comprises Ti or Cr as the adhesion layer, sputtered Cu as seed layer, electroplated Cu and Ni as the solder diffusion barrier layer, and a thin layer of gold as the solder wettable layer.
The invention and its various features provide the advantages of:
the discrete components provide optimized parameters and can be mounted close to the circuits, which offer true system-on-chip performance
the discrete components mounting close to the circuits also minimizes parasitics
the post-passivation process of the invention allows for the selection of discrete component design parameters that result in reduced resistance of the discrete capacitor and the discrete inductor, this is further clear from the following comparison between prior art processes and the processes of the invention.
Prior approaches in the art use thinner metal for inductors, requiring wider coils (to minimize resistance), resulting in increased surface area, increasing the parasitic capacitance of the inductor and causing eddy current losses in the surface of the substrate.
The present invention by contrast, can use easily formed thick metal layers, the thickness reducing resistance. Use of polymer 20 further separates the components formed from underlying structures, reducing capacitance. With the reduced capacitance, a higher frequency of operation results due to a higher resonant frequency.
Resistance of metal interconnections in an integrated circuit is determined by the material to be used and metal thickness and width, while capacitance is related to dielectric types, thickness, and metal line width and spacing. In a first example, assuming an extreme case of the topmost layer of the fine line metals, fine line metal thickness is about 2 micrometers, fine line IMD thickness is about 2 micrometers, and the line spacing is about 10 micrometers. Post-passivation metal thickness is about 5 micrometers, dielectric thickness is about 5 micrometers, and lines spacing is also about 10 micrometers. The metal thickness difference results in about 2.5 times reduction in resistance in the post-passivation metal structure over the fine line metal structure. The dielectric thickness results in about 2.5 times difference in capacitance in the post-passivation metal structure over the fine line metal structure. Then, the reduction in resistance times capacitance (RC product) is 6.25 times, or about 5 times. Increasing the line spacing of the post-passivation metal structure so that the capacitance decreases further by about 2.5 times will result in an RC product that is about 10 times smaller than the RC product of the fine line metal structure.
In a second example, fine line metal thickness is about 0.4 micrometer, fine line IMD thickness is about 0.4 micrometer, and the line width is about 0.2 micrometer. Post-passivation metal thickness is about 5 micrometers, dielectric thickness is about 5 micrometers, and line width is about 10 micrometers. The metal thickness difference results in about 12.5 times reduction in resistance in the post-passivation metal structure over the fine line metal structure, the dielectric thickness results in a 4 times 12.5 difference in capacitance in the post-passivation metal structure over the fine line metal structure, and the line width difference results in about 50 times difference in capacitance in the post-passivation metal structure over the fine line metal structure. Then, the reduction in RC product is 7,800 times. However, typically the line spacing in the post passivation metallization would be greater than that in the fine line metallization, so the difference in RC product will be greater than 10,000 times.
In another example, if the sheet resistance of the post-passivation metal of the invention is about 10 times smaller than that of the fine line metal (because of the metal thickness difference) and the total capacitance of the post-passivation metal line is also about 10 times smaller than the capacitance of the fine line metal (because of the thickness difference of the dielectric), then the RC product is about 100 times smaller than that of the fine line metal. If the width of the post-passivation metal is additionally about 10 times wider than the fine line metal, the resistance of the post-passivation about 100 times smaller than the fine line metal. This makes the RC product of the post-passivation metal about 1000 times smaller than the RC product of the fine line metal. Thus, the RC product of the post-passivation metal structure can be between about 5 to 10,000 times smaller than the RC product of the fine line metal structure.
It is difficult to achieve 100 times smaller RC product for the top layer metal of a fine line metallization system, when compared to the bottom layer metal in the fine line metal interconnection process. For example, the metal line resistance at the top layer metal can be reduced by designing a wide piece of metal, however, this will increase the capacitance of that metal line accordingly (because the IMD is thin). Essentially, it is hard for fine line IC metals to achieve even 10 times smaller RC product for its top metal layer versus its bottom metal layer.
Although the preferred embodiment of the present invention has been illustrated, and that form has been described in detail, it will be readily understood by those skilled in the art that various modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

## Claims

1. An integrated circuit comprising:
a silicon substrate;
a transistor in or on said silicon substrate;
a first metallization structure over said silicon substrate, wherein said first metallization structure comprises a first metal layer and a second metal layer over said first metal layer;
a dielectric layer between said first and second metal layers;
a separating layer over said first metallization structure and said dielectric layer, wherein said separating layer comprises a nitride, wherein a first opening in said separating layer is over a first contact point of said first metallization structure, and said first contact point is at a bottom of said first opening;
a first polymer layer over said separating layer, wherein said first polymer layer has a thickness between 2 and 150 micrometers;
a second metallization structure on said first polymer layer, wherein said second metallization structure is connected to said first contact point through said first opening and a second opening in said first polymer layer, wherein said second metallization structure comprises a glue layer and an electroplated copper layer over said glue layer, wherein an undercut has an edge of said glue layer recessed from an edge of said electroplated copper layer; and
a second polymer layer on said second metallization structure.

2. The integrated circuit of claim 1, wherein said first polymer layer comprises polyimide.

3. The integrated circuit of claim 1, wherein said second metallization structure further comprises a copper seed layer between said glue layer and said electroplated copper layer.

4. The integrated circuit of claim 1, wherein said glue layer comprises titanium.

5. The integrated circuit of claim 1, wherein said glue layer comprises titanium nitride.

6. The integrated circuit of claim 1, wherein said glue layer comprises a titanium-tungsten alloy.

7. The integrated circuit of claim 1, wherein a third opening in said separating layer is over a second contact point of said first metallization structure, and said second contact point is at a bottom of said third opening, wherein said second metallization structure is connected to said second contact point through said third opening and a fourth opening in said first polymer layer, wherein said first contact point is connected to said second contact point through said second metallization structure.

8. The integrated circuit of claim 1, wherein said second metallization structure further comprises a nickel layer on said electroplated copper layer.

9. The integrated circuit of claim 8, wherein said nickel layer has a thickness between 0.1 and 3 micrometers.

10. The integrated circuit of claim 1, wherein said electroplated copper layer has a thickness between 3 and 20 micrometers.