Abstract:
A structure for a chip or chip package is disclosed, with final passivation and terminal metallurgy which are mechanically decoupled but electrically coupled to the multilayer on-chip interconnects. This decoupling allows the chip to survive packaging stresses in the final passivation region, with strain relief from the decoupling region and compliant leads therein, so that on-chip interconnect levels do not feel these external packaging or other stresses. This structure is particularly preferred for on-chip interconnects consisting of Cu and low-k dielectric, the latter having inferior mechanical properties relative to SiO 2 . The decoupled region extends over all chips on the wafer. It may also extend into the edgeseal or dicing channel region so as to allow chip dicing and retention of this mechanical decoupling all around every chip on the wafer.

Description:
BACKGROUND OF INVENTION 
   This invention relates generally to interconnect structures for high-speed microprocessors, application specific integrated circuits (ASICs), and other high speed ICs, and more specifically, to a compliant passivated edge seal for low-k interconnect structures. This invention provides computer chips with improved mechanical integrity in the assembly and packaging, and also provides many additional advantages which shall become apparent as described below. 
   In semiconductor fabrication, insulating, semiconducting, and conducting layers are formed on a substrate. The layers are patterned to create features and spaces, forming devices, such as transistors, capacitors and resistors. These devices are then interconnected to achieve a desired electrical function, thereby producing an integrated circuit (IC). The formation and patterning of the various device layers are achieved using conventional fabrication techniques, such as oxidation, implantation, deposition, epitaxial growth of silicon, lithography, etching and planarization. 
   To increase throughput, a plurality of ICs are fabricated on a wafer in parallel. Thus, for example, a wafer may contain multiple IC units which have been formed on the planar surface area of the wafer. Each IC is a self contained entity surrounded by its own boundary region using orthogonal axes that are referred to as dicing channels or scribe lanes. Generally, these channels may have a width of about 50 to 100 μm. The ICs are typically separated into individual chips or “die” by cutting in this channel in a process known as “dicing” or “singulation”. Conventional dicing techniques include sawing with a diamond wheel, laser cutting, and “scribe and break”. As the dicing tool cuts or scribes the wafer, chips and cracks in the surface and substructure often result. Such cracks can propagate into critical areas within the IC in response to packaging stresses, and may cause permanent circuit failure. 
   To increase chip performance, wiring capacitance is reduced by changing from the industry-standard dielectric material SiO 2  (having a relative dielectric constant of about 4.0) to lower dielectric constant (“low-k”) insulators surrounding the interconnects. These low-k materials come at a significant disadvantage of loss of mechanical properties such as hardness and elastic modulus, and other forms of robustness and resistance to failure due to internal and external stresses. In particular, it has become a problem to probe, dice, and package chips containing low-k insulators without causing fracture, chipping or cracking of the dielectric material, and pullout of the terminal metal pads. There is therefore a need to isolate these weak on-chip materials from the harsh external environment and stresses associated with assembly and packaging. 
   Typical prior art approaches have adopted a hard dielectric passivation layer such as a silicon nitride material. For example, U.S. Pat. No. 5,742,094 discloses a sealed semiconductor chip. A hermetic seal consisting of a thin SiN passivation layer and a Ni passivation layer is selectively deposited on the chip surface. It has been observed, however, that when a low-k dielectric material is used as the inter-metal dielectric within the active area of the chip, such hard passivation layers do not adequately protect the device from fracture, chipping or cracking of the dielectric material, and pullout of the terminal metal pads. 
   Other prior art approaches combine a hard dielectric passivation layer such as a silicon dioxide or silicon nitride with an overcoat of a more compliant material such as a polyimide. For example, U.S. Pat. No. 6,383,893 discloses a hard passivation layer ( 124 ) consisting of inorganic insulators such as SiO 2  or SiN covering the wafer, and a soft passivation layer ( 125 ) consisting of polyimide overlying the hard passivation layer. U.S. Pat. No. 6,271,578 discloses a similar structure. Again, it has been observed that when a low-k dielectric material is used as the inter-metal dielectric within the active area of the chip, a hard passivation layer in contact with the active device area does not adequately protect the device from fracture, chipping or cracking of the dielectric material, and pullout of the terminal metal pads. The soft passivation layer overlying the hard passivation layer fails to alleviate this problem. 
   Another prior art approach disclosed in U.S. Pat. No. 5,665,655 involves the use of a crackstop structure, specifically a groove surrounding the active region on a chip. In this structure, a dielectric material ( 3 ) which can be polyimide is deposited over the substrate including the active device regions, and then a hard passivation layer ( 11 ) of, for example, silicon nitride is deposited over the structure. When the dicing operation is performed, however, the sidewalls of the chip remain in contact with the substrate material, which is typically a semiconductor material such as silicon. Microcracks occurring in silicon substrates tend to propagate very rapidly, and would therefore lead to failures in the adjacent low-k dielectric material. Thus, this structure also fails to protect the device from fracture, chipping or cracking of the dielectric material, and pullout of the terminal metal pads. 
   Therefore, a need remains in the art for a structure and process to isolate the weak on-chip materials from the harsh external environment and stresses associated with assembly and packaging. 
   SUMMARY OF INVENTION 
   The aforementioned problems are addressed by the structure and method of this invention. Specifically, in one aspect this invention is directed to a semiconductor wafer comprising a substrate; a plurality of integrated circuits fabricated on the substrate; a dicing channel disposed between adjacent ones of the integrated circuits, the channel exposing sidewalls of the integrated circuits; a layer of first dielectric material disposed on a top surface and sidewalls of the integrated circuits; and a layer of second dielectric material disposed on the layer of first dielectric material, wherein the first dielectric material has a critical strain energy release rate, G c  at least about 10 times greater than the second dielectric material. The first dielectric material preferably also has a tensile strength of about 20 to 100 MPa. 
   In another aspect, this invention is directed to a method of forming an edge seal structure on an integrated circuit chip formed on a substrate. The method comprises: etching a channel in a kerf region surrounding the integrated circuit chip, thereby exposing sidewalls of the integrated circuit chip; depositing a planarizing layer of first dielectric material on the integrated circuit chip and in the channel; and depositing a second dielectric material over the first dielectric material, wherein the first dielectric material has a G c  value at least about 10 times greater than the second dielectric material. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The drawings are for illustration purposes only and are not drawn to scale. Furthermore, like numbers represent like features in the drawings. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows, taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  illustrates an embodiment of the present invention in which final passivation and compliant layers terminate perpendicular to and onto the substrate; and 
       FIG. 2  illustrates another embodiment of the present invention in which the compliant layer terminates perpendicular to and onto the substrate, and the final passivation layer terminates adjacent to a previously diced substrate. 
       FIG. 3  illustrates another embodiment of the present invention in which the final passivation layer comprises a plurality of layers. 
   

   DETAILED DESCRIPTION 
   The invention involves a structure and method to generate a mechanical isolation region between the final on-chip interconnect level and the terminal pads and dicing channel of the chip, while still providing electrical continuity through this region via flexible connections. The isolation material and flexible electrical feed-throughs allow for strain relief and shock absorption without permanent damage to the chip wiring levels. 
     FIG. 1  illustrates one embodiment of the invention. In the structure shown in  FIG. 1 , a plurality of integrated circuits (IC)  11  have been formed on substrate  10 , with dicing channel  18  therebetween. Each IC  11  comprises the active device regions of the chip (not shown), interconnect wiring  12 , and metal pads  13 . Surrounding each IC is a layer  16  of energy absorbing material which is capable of absorbing mechanical stresses imparted onto the IC  11  during assembly and packaging. A final passivation layer  17  is disposed over the layer  16  to hermetically seal the chip prior to dicing. Embedded in layer  16  are a plurality of conductive leads  14  connecting the metal pads  13  with bonding pads  15 . Leads  14  may be jogged or staggered, as shown, or they may be straight. The energy absorbing material  16  contacts the sides of IC  11 , in addition to the top surface of IC  11 . 
   The energy absorbing layer  16  and the final passivation layer  17  should each be passivated, i.e., formed of dielectric material. In addition, the energy absorbing material  16  should be compliant yet tough. This material may be characterized by fracture toughness, which may be defined as a material&#39;s ability to resist the propagation of a crack within itself. The parameter G c  is known as the critical strain energy release rate, or the energy at which a crack will propagate. G c  has units of kJ/m 2 , and is often referred to as the fracture toughness of a material. Another parameter that is frequently encountered when quantifying the fracture toughness of a material is the critical stress intensity factor, K c . The relationship between G c  and K c  is as follows: G c =K c   2 /E for plane stress and G c =K c   2 (1−ν 2 )/E for plane strain, where ν is Poisson&#39;s ratio. K c  has units of MPa-m 1/2 . Low values of G c  and K c  are indicative of low fracture toughness. Materials having low fracture toughness typically will exhibit brittle failure. High values of G c  and K c  are indicative of high fracture toughness, where failure modes are likely to be ductile. 
   One may record the stress-strain curve for a given set of materials in a static tensile test at constant temperature and strain rate as performed in accordance with ASTM methods. Those materials that exhibit low fracture toughness typically exhibit brittle failure and will fail at very low strains in the region of 1–3%. Those materials that exhibit some degree of yield or elongation to break will fail at high strains &gt;10% and are typical of materials with high fracture toughness and ductile failure modes. Thus, the energy absorbing material  16  preferably has a fracture toughness which is at least about 10 times greater than the fracture toughness of the final passivation layer  17 . The energy absorbing material  16  preferably has a G c  greater than about 0.1 kJ/m 2 , and more preferably in the range of about 0.5 to 2.5 kJ/m 2 . The final passivation layer  17  preferably has a G c  less than about 0.05 kJ/m 2 , and more preferably in the range of about 0.005 to 0.05 kJ/m 2 . 
   The energy absorbing material  16  also may be characterized by tensile strength. The tensile strength or ultimate tensile strength of a material may be defined as the maximum stress the material under load can attain prior to failure. Tensile strength is typically expressed in units of MPa (MN/m 2 ). One may record the stress-strain curve in a static tensile test at constant temperature and strain rate as performed in accordance with ASTM methods, and from this plot mark the tensile strength as previously defined. Those materials that exhibit low tensile strength typically experience brittle failure and will fail at very low strains in the region of 1–3%. Those materials that exhibit some degree of yield or elongation to break will fail at high strains &gt;10% and typically have high tensile strength and ductile failure modes. The yield stress, σ y , may be reported to indicate a material&#39;s strength and is a very different point on the stress-strain curve. Some materials yield or “neck down” when under load; this is exhibited as a maximum in the stress-strain curve followed by a slight drop in stress for constant strain. The material may then continue to carry load with only a gradual increase in stress with increase in strain (known as elongation) until failure occurs. The energy absorbing material  16  preferably has a tensile strength of about 20 to 100 MPa, while the final passivation layer  17  preferably has a tensile strength of about 700 to 10,000 MPa. 
   Any material exhibiting the above described characteristics may be used for the energy absorbing material  16 . Preferred materials are largely organic in nature, and include: polyesters, phenolics, polyimides, polysulfones, polyether ether ketones, polyurethanes, epoxies, polyarylene ethers, polyethylene terepthalates. For example, polystyrene has a G c  value of 1–2 kJ/m 2 , and a tensile strength of 0.08 GPa. Other examples include polymethyl methacrylate with a G c  value of 0.2–0.6 kJ/m 2  and a K c  of 1.5 MPa-m 1/2 , and polyethylene with tensile strength of 0.05 GPa. A particularly preferred energy absorbing material is polyarylene ether known as SiLK™, available from Dow Chemical, and having a fracture toughness, K c , of 0.62 MPa-m 1/2 . An additional benefit of SiLK and some of the polyimides is thermal stability to relatively high temperatures such as from 350C to 450C; these materials would survive all subsequent processing involved in the chip fabrication or packaging. 
   The final passivation layer  17  may be formed of any material exhibiting the above described characteristics. Preferred materials are largely inorganic in nature, and include: silicon-based glasses such as SiN and SiO 2 , SiC, tetraethylorthosilicate (TEOS), fluorinated TEOS (FTEOS), fluorinated silicate glass (FSG), and organosilicate glass (OSG). A particularly preferred material for the final passivation layer is SiO 2 , having a tensile strength of 5900 MPa. 
   One skilled in the art will recognize that the energy-absorbing material and final passivation layer may be characterized by other material properties such as Young&#39;s modulus and hardness. Preferred materials for the final passivation layer include Si 3 N 4 , with modulus ranging from about 174 GPa to about 290 GPa and hardness of about 13.5 GPa; PECVD silane oxide with modulus of about 60 GPa and hardness of about 6.8 GPa; and fused silica with modulus of about 72 GPa and hardness of about 8.7 GPa. The energy-absorbing material should generally have modulus and hardness values approximately two orders of magnitude lower than those for the final passivation layer. For example, SiLK™ has a modulus of about 3.5 GPa and hardness of about 0.21 GPa. 
   The structure shown in  FIG. 1  may be formed by the following method. A wafer comprising a plurality of ICs  11  is completed through processing of the final metal level, for example by copper/low-k dual damascene processing. The final metal level includes metal pads  13  to connect subsequently to terminal pads  15  for wirebond, C4 or direct pin creation. 
   Next, a channel  18  is defined by conventionsi techniques, such as by photolithography, and is etched in the kerf region surrounding each chip  11 . The channel is etched Through the various layers residing on substrate  10 , but is not etched through substrate  10 . A planarizing layer of energy-absorbing material  16  is deposited, preferably spun on and cured, so as to at least partially fill the etched channels and to provide a layer of this material over all active chip areas  11 . Material layer  16  preferably has a thickness of about 1 to about 5 μm. 
   Contact holes are then created in the material layer  16  down to the metal pads  13 . These holes may be created, for example, by photolithography and etching. Alternatively, if the material  16  is a photosensitive polyimide, then these holes may be directly patterned and etched. S-shaped or spring-shaped compliant leads  14  may be created which make contact with the exposed metal pads and rise up the side of the tapered holes in the material  15 . For example, leads  14  may be created using the techniques disclosed by Hollie A. Reed et al. in “Compliant wafer level package (CWLP) with embedded air-gaps for sea of leads (SoL) interconnections,” Proc. of IEEE 2001 IITC, pp. 151–153, the disclosure of which is incorporated herein by reference. As another example, leads  14  may be formed using the techniques disclosed by Khandros et al. in U.S. Pat. No. 6,372,527 or U.S. Pat. No. 6,538,214 or U.S. Pat. No. 5,679,977, the disclosures of which are incorporated herein by reference. 
   Formation of leads  14  include the steps of sputtering a release layer and seed layer in the contact holes, forming a photomask over the material  16 , through-mask plating of the compliant leads  14 , removing the resist, and stripping the exposed seed layer and release layer. Optionally, a second polymer layer (not shown) may be reflowed into the contact holes to plug the holes around the compliant leads. Leads  14  may be formed of any suitable metal, such as copper, aluminum, or tungsten. 
   Jogged leads  14  must be fabricated in two sequential stages. After the material  16  has been deposited, a suitable hardmask scheme may be employed to enhance the lithography of the via level of the jogged leads  14 . The via level is defined using conventional lithography and etching techniques, followed by metallization and cap deposition. A second deposition of material  16  then may be applied and the line level of the jogged leads  14  may be defined using similar techniques, with an engineered offset as shown in  FIG. 1 . 
   Next, the final passivation layer  17  is deposited. Layer  17  may comprise, for example, about 0.5 μm each of SiO 2  and Si 3 N 4 . Contact holes for terminal pads  15  are then formed using, for example, photolithography and etching. Metallurgy for terminal pads  15  is deposited in the contact holes, contacting the compliant leads  14  and plugging the contact holes through the final passivation layer  17 . The wafer may then be diced and the individual chips may be packaged according to conventional processes. 
   In the structure of  FIG. 1 , the hard passivation layer  17  is terminated on the substrate  10 , forming a hermetic seal on the top surface of the substrate.  FIG. 2  differs from  FIG. 1  in that the hard passivation layer  17  forms an edge seal such that it encapsulates the entire substrate  10 . The structure of  FIG. 2  may be formed by a method similar to the method for forming the structure of  FIG. 1 , except that the channel  18  must be etched at least partially through substrate  10 , thereby exposing sidewalls of the substrate. 
   The structure and method of this invention may be used not only for die isolation, but also may be used for isolation of different macros on a die. The structure and method of this invention also may be used on the package rather than on the chip, providing similar mechanical isolation function. 
   While the present invention has been particularly described in conjunction with a specific preferred embodiment and other alternative embodiments, it is evident that numerous alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore intended that the appended claims embrace all such alternatives, modifications and variations as falling within the true scope and spirit of the present invention.