# Patent

## Title
Passivation layer surface topography modifications for improved integrity in packaged assemblies

## Abstract

A structure and method for producing the same is disclosed. The structure includes an organic passivation layer with solids suspended therein. Preferential etch to remove a portion of the organic material and expose portions of such solids creates enhanced surface roughness, which provides a significant advantage with respect to adhesion of that passivation layer to the packaging underfill material.

## Background
FIELD OF THE INVENTION
The present invention relates to packaging integrated circuit chips and more specifically to improved adhesion of a chip to a packaging assembly at the interface between the chip passivation layer and the packaging underfill or encapsulation material.
BACKGROUND OF THE INVENTION
The integrity of a Flip-Chip Package (FCPBGA) assembly depends directly on the adhesive quality of the interface formed between chip-level passivation polyimide coating and the underfill material that is used to fill interstitial gaps between solder bumps in the cavity between the packaging plastic laminate and the chip. The integrity of a wirebond assembly similarly depends on the adhesion between the passivation and encapsulant materials. Historically, chips having die sizes in excess of about 150 mm2 experience sufficiently high chip edge coefficient of thermal expansion (CTE) forces during thermal cycling that this interface is prone to failure if it is not of adequate integrity. Failure initiates at high DNP (distance to neutral point) locations, such as a corner. Once the interface begins to come apart, this delamination can spread over large areas of the chip corner, and can even propagate as a crack down into the BEOL levels of the chip. Deep thermal cycle and accelerated temperature cycling stress testing of parts is used to force failure of any interface with substandard reliability, during the normal technology and product evaluation phase. Chip parts with die sizes approaching 20 mm per edge and above (400 mm2+) are particularly sensitive to this failure mechanism and require very careful engineering of this critical interface for long-term survival of the part. One technique is to add adhesion-enhancing design features near the chip edge, but this may consume chip area that could be used for circuitry or pose constraints on die size. The integrity of underfill to passivation adhesion in the package is currently a critical problem in the industry. Needed are additional ways to improve the durability and integrity of the passivation to encapsulant bond.
SUMMARY OF THE INVENTION
According to one embodiment, the present invention is a structure that includes a base structure over which a passivation layer is formed. The passivation layer is composed primarily of an organic polymer material, which may be polyimide, in which is suspended silicon-containing particles. The particles can have an average diameter between about 0.01 and 0.3 microns, and can be formed of an aminosilane or other adhesion promoter compound. A roughened surface can be formed by removing a portion of the organic polymer to at least partially expose a portion of those particles. The height variation of the roughened surface can be in the range of about 0.01 micron to 1.0 micron. An encapsulation layer can bond to the roughened surface to form an improved interface. The base structure can be an integrated circuit assembly comprising a) a semiconductor substrate, b) one or more dielectric layers formed on said substrate, and c) at least one device and an interconnect structure formed within said one or more dielectric layers. Furthermore, the base structure can be one of a plurality of such structures formed on a semiconductor wafer.
According to one embodiment, the above structure may include a package assembly bonded to the encapsulation layer, where the at least one device of the integrated circuit assembly is in electrical contact with a portion of the package assembly.
In another aspect of the invention a method is disclosed. The method includes a step of providing a base structure on which a passivation layer has been formed, where the passivation layer is composed of 1) an organic polymer material which may be polyimide and 2) silicon-containing particles which may be formed of an aminosilane compound; and another step of applying an encapsulation material to bond with the passivation layer. The base structure can be an assembly comprising 1) a semiconductor substrate having one or more devices formed thereon, and 2) a metallization layer comprising an interconnect structure formed within one or more dielectric layers. The method may include, prior to the providing step, applying a formulation over the base structure to form the passivation layer. Such formulation may include an aminosilane-like compound and over about 2% moisture, or may be formed by combining a quantity of Si-containing particles to a polymeric passivation layer formulation, wherein the quantity constitutes between 1 to 7 volume percent or between 1 to 100 particles per cubic micron of said combination.
The method may include a step of forming a roughened surface on the passivation layer prior to applying the encapsulation material, where such forming is achieved by removing a portion of the organic polymer material to expose a portion of one or more of the particles. The removing process can operate to remove a portion of the organic polymer material, and can form a roughened surface with a height variation of between 0.01 to 1.0 micron. The removing step can be achieved by any etch process that is selective for organic materials including RIE exposure or O2 plasma ashing.
The base structure can be one of a plurality of such structures formed on a single wafer, in which case the method may include a step of dicing the wafer to form individual chips, either before or after the applying step.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be understood and appreciated more fully from the following detailed description of the invention, taken in conjunction with the accompanying drawings.
FIG. 1 is an elevation view of an integrated circuit assembly.
FIG. 2 illustrates chip delamination.
FIG. 3A illustrates a 2-phase passivation layer.
FIG. 3B illustrates the profile of a 2-phase passivation layer after exposure to etching.
FIG. 4A is an SEM photograph showing the 2-phase polyimide layer according to an embodiment of the invention.
FIG. 4B is magnified view of the roughened surface of the 2-phase polyimide layer of FIG. 4A.



It will be appreciated that for simplicity and clarity of illustration, elements shown in the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a simplified elevation view of an integrated circuit structure 100 which includes an integrated circuit chip 10 attached to a packaging structure 20. The integrated circuit chip 10 has a device layer 1 which is the output of the “front end of the line” process (FEOL) and includes a semiconductor substrate on which have been formed microelectronic devices and dielectric layers (not shown). Integrated circuit chip 10 has a metallization layer 2 which serves to electrically connect the devices of layer 1 to each other or to an external circuit or both. The FEOL semiconductor substrate is typically a semiconductor wafer such as epitaxial silicon, and may optionally include one or more oxide layers such as in ‘silicon on insulator’ technology. Back end of the line (BEOL) processing creates metallization layer 2 which can include several insulator layers, each typically having a metal interconnect structure formed therein. To protect it from abrasion, contamination, and/or corrosion, the metallization layer is usually coated with a passivation layer 30. Common passivation materials include inorganic materials such as silicon dioxide or silicon nitride, and organic polymeric materials, for example, polyimide. Passivation layer 30 of the present invention is an organic polymer formulation. Typically in flip-chip technology, passivation layer 30 is a polyimide layer applied over a layer of hard dielectric (e.g. silicon nitride and/or oxide) (not shown in FIG. 1). Interface 60 is the bottom surface of passivation layer 30 and in contact with the hard dielectric layer if present, and surface 50 is the top of passivation layer 30.
Packaging structure 20 facilitates the connection of circuits on the IC chip 10 to external circuits through e.g., metal legs (not shown) that can be plugged into a circuit board. Contact structure 40 enables electrical connection between a contact such as a metal pad within the metallization layer 2 to a terminal embedded within packaging structure 20. Contact structure 40 can be a solder ball in the case of a flip-chip, and numerous other contact structures are known including gold wire. Encapsulant 70 fills the void space between contact structures 40. Materials identified as ‘underfill’, “molding compound”, or “dam and fill” are all encapsulants which are formulated with a base resin, such as an epoxy or cyanate ester, a hardener which may be amine or anhydride based, and a catalyst, and may include other additives such as stress absorbers, fillers, coupling agents, and pigments.
It may be appreciated that during fabrication and continuing through assembly and actual use, integrated circuit structure 100 can experience significant mechanical and thermal stresses. Such stress makes IC chips susceptible to delamination, particularly along the interface at surface 50 between passivation layer 30 and encapsulant 70. FIG. 2 illustrates a cross-section of intergrated circuit structure 100, along surface 50. Delamination 52 tends to start at the corner of the structure within the chip/packaging interface and can propagate a crack into critical chip structures such as through a solder bump and even through BEOL metallization.
One example of a passivation material is HD4004 which is a photosensitive polyimide (PSPI) formulation available from HD Microsystems. Common polymeric passivation layer formulations such as HD4004 include a small concentration of a bulk film adhesion promoter. The adhesion promoter, also known as a bonding agent, can be part of the passivation material formulation or blended in separately prior to spin-application to the chip. A common adhesion promoter is gamma-aminopropyltriethoxysilane. The adhesion promoter is believed to promote adhesion between the polyimide and the chip by forming a transitional bond between the hard dielectric (SiOx, SiNx) on the chip side, and the polyimide itself, and therefore is generally an aminosilane-like molecule having a SiO2-friendly (Silicon-containing) group on one end and a polyimide friendly (NH2-containing group) on the other. The adhesion promoter is normally not distinguishable from the bulk polyimide film with cross-sectional scanning electron microscopy (SEM). That is, a typical cured layer of a polyimide formulation with bulk adhesion promoter appears to be a single-phase film.
The present inventors have found that at least the Si-containing end of the adhesion promoter can form particles, that is, a second phase, when a polyimide formulation is produced in the presence of excess moisture. Specifically, a second phase particle can form if the formulation includes moisture in the range of between 1.5% and 5% or greater than about 2%, such as between 2% and 3%. Apparently, partial hydrolysis of the bulk adhesion promoter causes formation and precipitation in the bulk film of small particles of Si-containing material, these particles having an average diameter within the range of 0.01 micron to 0.3 micron, or between 0.05 micron and 0.2 micron, or about 0.1 micron. Solid particles could alternatively be added to a single phase passivation formulation to form a 2-phase passivation material at a density of about 5% or about 10 particles per cubic micron if the average particle diameter is about 0.1 micron. FIG. 3A depicts such a two phase layer 30 comprising a first phase 34 with Si-containing particles 32 dispersed therein.
Preferential etching of the organic portion of the passivation layer tends to expose inorganic moieties. When the etch process is inherently selective for organic materials relative to inorganics, the presence of Si-containing particles near the exposed surface can create a roughened surface. This so-created “roughness” modification contributes significantly to the ability of this polyimide surface (on the package side) to form a strong adhesive bond with the underfill packaging material.
Any conventional etching process can be used, as long as it has a high removal rate for organic polymeric material and a very low or nonexistent rate for inorganic or SiOx-like materials. Oxygenated plasma ashing is known to have a high “selectivity” for removal of organic materials relative to inorganic materials (e.g., SiOx particles). Similarly, a fluorinated RIE process can preferentially or selectively remove the organic polymer. In the context of integrated circuit fabrication, such a selective etch can be applied during the wafer level process either after passivation layer cure or during (pre or post) the bumping operation, or it could also be done after the wafer is diced. For purposes of the present invention, it is preferred but not required that this etch roughening occur shortly before attaching the chip to the packaging structure. The etch depth can be several microns or more, but deep etching is not required to achieve the improved bond integrity of the present invention.
FIG. 3B illustrates how the bulk presence of minute particles 32 can act as ‘micromasks’ whereby removal of a portion of material 34 to a nominal depth of h1 at least partially exposes a portion of such particles 32 to form bumps 32A or 32C, with a height up to that of a particle diameter, or even spikes of height h2 formed by a height of first phase material 34 and capped by a second phase particle 32B. The resulting roughened surface has a relatively smooth field 54, at depth h1 below the initial film surface. The height variation of the roughened surface for purposes herein means the average h2 height of the top of the exposed particles relative to the relatively smooth field 54.
EXAMPLE 1
Large die-size test vehicle parts were formulated with hydrolyzed PSPI material according to one embodiment. The PSPI material was applied to wafers in the normal fashion. After standard curing, the wafers were exposed to an oxygen-plasma ash surface clean process. Some were subjected just to O2 plasma, whereas others were exposed to both RIE followed by O2 plasma processing. The PSPI layer can be removed to some degree by the RIE process, but typically the etch selectivity for this step is optimized for removing the inorganic hard dielectric relative to organics. However, the PSPI layer is strongly etched by the O2 plasma.
The O2 plasma ash selectively removed the polyimide locally around SiOx particles, lowering the PSPI average field height and exposing those SiOx particles near the surface, creating a localized surface “roughening” consisting of prominent SiOx spheres standing slightly above the surrounding polyimide. The presence of these exposed SiOx spheres significantly improved the durability of the polyimide/encapsulant interface. Improved adhesion was found with a gentle etch to a depth of only 0.1 micron, as well as at larger depths such as between 0.3-0.5 micron, or as much as 1.0 micron.
FIGS. 4A and 4B are SEM photographs of a cross section through a portion of an integrated circuit apparatus according to an embodiment of the invention. FIG. 4A shows a 2-phase PSPI layer corresponding to passivation layer 30 overlying metallization layer 2. As depicted, interface 60 is the bottom surface of passivation layer 30. The upper surface 50 was etched by RIE followed by O2 plasma ashing to form a roughened surface, shown in FIG. 4B. Particles 32 prevented the underlying polyimide from being etched resulting in the formation of bumps and spikes rising from the relatively smooth field 54.
After dicing, a significant sample of parts built into modules and tested per JEDEC-standard DTC stressing [−40 to 125 C, 2 cph], survived in excess of 1500 cycles without showing any sign of corner wear, film delamination or electrical failure. This is in contrast to a control population of standard polyimide modules which exhibited clear sign of corner film delamination at 1250 cycles and a high electrical fail rate at 1500 cycles. This stress involved a 26×21.5 mm chip in a 50 mm organic laminate package.
The foregoing description is intended to be illustrative and not limiting. While the invention has been described and demonstrated for flip-chip/solder bump packaging, those skilled in the art will appreciate that various changes can be made therein without departing form the spirit and scope of the invention. For example, the invention is equally applicable to wirebond packaging, for a standard assembly in which a mold compound forms the packaging interface with the chip-level polyimide.

## Claims

1. A method comprising steps of:
providing a packaging structure and an integrated circuit chip, the integrated circuit chip including at least a device layer, a metallization layer and a passivation layer;
etching the passivation layer to roughen a top portion of the passivation layer thereby exposing one or more silicon-containing precipitates extending from the passivation layer;
electrically connecting the integrated circuit chip to the packaging structure through at least one contact structure; and
applying an encapsulation material to bond the integrated circuit chip and the packaging structure, wherein the encapsulation material bonds directly to the passivation layer;
wherein the one or more silicon-containing precipitates are created by hydrolysis during passivation layer formation or added to the passivation layer.

2. The method according to claim 1, wherein the etching removes at least a portion of an organic polymer material to create the roughened surface.

3. The method according to claim 2, wherein said roughened surface has a height variation in the range of 0.01 to 1.0 micron.

4. The method according to claim 1, whereby the etching step is achieved by RIE exposure.

5. The method according to claim 1, whereby the etching is optimized by application of an O2 plasma ash process.

6. The method according to claim 1, wherein said integrated circuit chip is an assembly comprising:
the device layer having one or more devices formed thereon; and
the metallization layer comprising an interconnect structure formed within at least one of a plurality of dielectric layers.

7. The method according to claim 1, wherein the integrated circuit chip is one of a plurality of such chips formed on a single wafer, said method further comprising:
dicing said wafer to form individual chips after said applying step.

8. The method of claim 1, wherein the passivation layer comprises a photosensitive polyimide material.

9. The method of claim 1, wherein the passivation layer comprises an adhesion promoter.