George Frederick Walker

Adhesion and interface studies between copper and polyimide

The adhesion and interface structure between copper and polyimide have been studied. Polyimide films were prepared by spinning a polyamic acid solution (Du Pont PMDA-ODA) in an NMP solvent onto a Cu foil, followed by thermal curing up to 400°C. The adhesion strength was measured by a 90° peel test. The peel strength of 25 μm thick Cu foil to 25 μm thick polyimide substrate was about 73 g/mm with the peel strength decreasing with increasing polyimide thickness. Cross-sectional TEM observation revealed very fine Cu-rich particles distributed in the polyimide. Particles were not found closer than 80-200 nm from Cu boundary. These Cu-rich particles were formed as a result of reaction of polyamic acid with Cu during thermal curing. We attribute the high peel strength to interfacial chemical bonding between Cu and polyimide. This behavior is in contrast to vacuum-deposited Cu onto fully cured polyimide.

Enabling technologies for wafer-level bonding of 3D MEMS and integrated circuit structures

In this paper, we describe several critical aspects of wafer scale or die level bonding to demonstrate: (1) low temperature bonding for planar layer interconnections; (2) low temperature bonding for non-planar layer sealing; (3) alignment and transfer of process sub-assemblies such as BEOL wiring, MEMS cavity or active device structures; and (4) integration methodology for fabrication of these layer stacks into 3D circuits and MEMS. We also show examples of how layer stacking protocols using wafer bonding technology provides a capability to integrate mixed materials and technologies potentially adaptable to many other applications. In addition, we demonstrate that in order to evaluate the influence of bonding on the electrical integrity of the transferred ICs, state-of-the art circuits, such as short channel length MOSFETs or ring oscillators, should be tested as they are most sensitive to environmental/processing changes.

Adhesion and interface investigation of polyimide on metals

The interface and adhesion of PMDA-ODA and PAA-ethyl ester polyimides to several metals including Cr, Ni, Cu, and Au have been studied by transmission electron microscopy (TEM) and a 90° peel test. The adhesion strength of polyimide on metals is generally very high compared with that of metals on polyimide. The trend in both cases is the same, i.e., Cr has the highest adhesion strength followed by Ni, Cu, and Au. Cross-sectional TEM studies showed that very fine particles were precipitated in the polyimide near the interface in the case of polyimide on Cu (or Ni). The precipitates were not found closer than 80-200 nm from the polyimide/Cu (or Ni) interface. Some large particles were identified as Cu2O (NiO in the case of polyimide/Ni) by scanning transmission electron microscopy and microdiffraction analyses. The formation of these particles was ascribed to a process involving first the reaction of polyamic acid with Cu (or Ni) to form a polyamic acid complex. During subsequent thermal curing, the complex...

Polyimide on copper: The role of solvent in the formation of copper precipitates

The interaction at the polyimide‐copper (oxide) interface has been investigated by means of cross‐sectional transmission electron microscopy. By using a solventless molecular beam epitaxy growth technique, the role of solvent in the aggregation and diffusion of copper‐rich particles into the polyimide film and away from the interface was unambiguously demonstrated. The polyimide on copper growth sequence is contrasted to the copper on polyimide growth sequence, and the reasons for the differences between the two growth interfaces are discussed.

A novel elastomeric connector for packaging interconnections, testing and burn-in applications

Elasticon connectors have been developed for a wide variety of interconnection and test applications which include module-to-board, board-to-board interconnections; high density module, board and LCD testings; as well as chip/wafer testing and burn-in to produce Known-Good-Die. Depending on the applications, the Elasticon connectors can be fabricated into two basic types, the single-sided (integrated to a substrate) and the double-sided (interposer) structure. The single-sided structure, the Integrated Probe, is designed and fabricated for test and burn-in applications. The density, compliance, thickness, pattern and size of both the one- and two-sided structures can be easily tailored to meet the requirements of each specific application. The fabrication processes involve wire bonding, laser and polymer casting and curing. The electrical, mechanical and thermal properties of the connector have been fully characterized. To achieve high compliance with low contact force, a proprietary elastomeric material has been formulated to achieve not only high compliance but high thermal stability. The conductive element uses highly conductive, corrosion free, and oxidation resistant noble metals and their alloys. The reliability and durability of the elastomeric connector have been evaluated with mechanical cycling, thermal cycling, stress relaxation, outgassing, and temperature and humidity rest. The thermal stability of the connector, including both the polymer and the conductive element, has been measured to exceed the burn-in temperatures, which range from 125 to 180/spl deg/C.

Adhesion of poly(arylene ether benzimidazole) to copper and polyimides

Poly(arylene ether benzimidazole) (PAEBI) is a high-performance thermoplastic polymer with imidazole functional groups which form the polymer backbone structure. It is proposed that upon coating PAEBI onto a copper substrate, the imidazole groups of PAEBI form a bond with or chelate to the copper surface, resulting in strong adhesion between the polymer and copper. The adhesion of PAEBI to other polymers such as poly(biphenyl dianhydride-p-phenylene diamine) (BPDA-PDA) and 6F photosensitive polyimide was also quite good and stable. The resulting locus of failure as studied by X-ray photoelectron and external reflectance infrared spectroscopy indicates that the failure is in the polyimide layer for the case of polyimide/PAEBI/Cu adhesion. Owing to its good adhesion and mechanical properties, PAEBI can be used in the fabrication of thin film semiconductor packages such as multichip module dielectric (MCM-D) structures. In these applications, a thin PAEBI coating is applied directly to the wiring layer for e...

Cu passivation: A method of inhibiting copper‐polyamic acid interactions

The use of passivation metals to protect Cu from interacting with polyamic acid precursor has been investigated by means of cross‐sectional transmission electron microscopy, Rutherford backscattering spectroscopy, and resistivity measurements. Palladium or platinum, each 100 nm in thickness, was electron‐beam evaporated onto Cu surface. Although not a good diffusion barrier against Cu outdiffusion, these metals can still effectively inhibit the Cu‐polyamic acid interactions and, consequently, prevent the formation of Cu oxide precipitates in the polymer film. The trade‐off is that, since Pd and Pt both diffuse into Cu and form solid solution alloys, the resistivity of Cu is raised significantly due to the enhanced electron‐impurity scattering. In contrast, Cr serves both as a passivation metal and a good diffusion barrier without raising Cu resistivity. This is probably due to the lack of mutual solubility between Cr and Cu.

A new direction for elastomeric connectors

Elastomeric connectors are a relatively new technology compared with conventional connector systems. A wide variety of elastomeric connectors are available today to meet the interconnection requirements for many different electronic packaging applications. Multichip modules are one of the many applications that benefit from the high density interconnection capabilities of elastomeric connectors. The ELASTICON connector is a new high performance elastomeric connector that was developed to address some of the key limitations of existing MCM and land grid array connectors. The ELASTICON connector uses gold or gold alloy wires for the conductive elements embedded in an elastomer material. The size, shape and spacing along with the elastomer material properties can be tailored to specific application requirements. The processes that have been developed for fabricating the ELASTICON connector represent a new direction for elastomeric connector manufacturing. Besides LGA packaging applications, ELASTICON connectors can be used for board-to-board and cable-to-board interconnections as well as high density PCB and IC chip testing applications.<<ETX>>