Sue Ann Bidstrup-Allen

Silver metallization for advanced interconnects

Silver metal has the highest room-temperature electrical conductivity of any substance; however, it has found limited acceptance in the electronic industry (e.g., silver filled epoxy) due to the high rate of metal corrosion and migration causing dendrites and electrical failures. With decreasing transistor feature sizes, device-operating voltages have scaled down considerably. In this paper, the reliability of silver and potential benefits of silver metallization are discussed in terms of future trends in microelectronic interconnections. Experimental data supports existing reliability models indicating that electrochemical migration failure modes may not be operative at low voltages. Silver metal corrosion and migration are studied under accelerated test conditions to obtain a qualitative understanding of the failure mechanism.

Stresses in thin film metallization

Stresses in conductors used in microelectronic interconnections are a critical processing and reliability issue. This work examines: 1) the temperature-dependent stress behavior of sputtered and electroplated silver and gold films on silicon substrates; 2) the use of wafer curvature using multiple substrates for the simultaneous determination of coefficient of thermal expansion (CTE) and modulus for thin films. The stress-temperature behavior of gold films on gallium arsenide and aluminum substrates was measured to determine its CTE and modulus. It is shown that electroplated noble metal films have lower stresses than sputtered films, due to larger grain sizes.

Air‐Gaps for Electrical Interconnections

The fabrication of air-gap structures for electrical interconnections has been demonstrated using a sacrificial polymer encapsulated in conventional dielectric materials. The air-gap is formed by thermally decomposing the sacrificial polymer and allowing the byproducts to diffuse through the encapsulating dielectric. The diffusivity of the polymer decomposition products is adequate at elevated temperatures to allow the formation of an air-gap. The decomposition of a 5 μm thick polymer film results in less than 100 A of residue. Electromagnetic simulation shows that the effective dielectric constant of silicon dioxide (e = 4.2) can be lowered to 2.4-2.8 for relevant structures.

Next-generation microvia and global wiring technologies for SOP

As microsystems continue to move toward higher speed and microminiaturization, the demand for interconnection density both on the IC and the package levels increases tremendously. The 2002 ITRS roadmap update identifies the need for sub-100-/spl mu/m area array pitch and data rates of 10 Gb/s in the package or board by the year 2010, requiring much finer lines and vias than the current microvias of 50 /spl mu/m diameter and lines and spaces of 25 /spl mu/m. After a brief description of the future need for high-density substrates, the historical evolution of microvia technologies worldwide is summarized. With the move toward highly integrated and higher performance system-on-a-package (SOP) technology, the demand for micro via wiring density in the package is increasing dramatically requiring new innovations in fine line, ultralow-loss, and ultrathin-film dielectrics. The low-cost needs of this technology are driving research in high throughput and large area processes in dielectric and conductor deposition. The third section of this paper describes in detail some of the key emerging global microvia research and development in the fabrication of microminiaturized, multifunction SOP packages including rapid curing of low-loss dielectric thin films on organic substrates, environmentally friendly high-speed electroless copper plating, ultrafine lines, and spaces down to 5 /spl mu/m and low-cost stacked via structures without chemical-mechanical polishing. This paper concludes with a perspective on future directions in dielectrics and conductor materials and processes leading to ultrahigh-density and low-cost microvia technologies for build-up SOP implementation.

Variable frequency microwave curing of photosensitive polyimides

Variable frequency microwave (VFM) curing was investigated as a means of rapid curing of two photosensitive polyimides. The properties of two polymers, PI 2734 and Ultradel 7501, cured by convective heating and VFM curing were compared. The results of this study indicate that rapid VFM curing of these polymers is feasible. Complete imidization was possible. The most significant differences in properties between VFM and thermally cured films were in the electrical properties due to the slow evolution of chemical products.

Rapid curing of positive tone photosensitive polybenzoxazole based dielectric resin by variable frequency microwave processing

High performance polymer dielectrics such as polyimides and polybenzoxazoles are used for several applications in the semiconductor industry due to their excellent dielectric and thermomechanical properties. However, these materials require curing at high temperatures for long periods of time in order to achieve the desired properties. High temperature exposure for long periods of time can be detrimental to device characteristics and reliability. In this study, rapid low temperature curing of a positive tone photosensitive polybenzoxazole based dielectric resin by variable frequency microwave (VFM) processing was investigated. The chemical changes occurring in the film during the condensation reaction and the percent conversion achieved as a function of cure condition were monitored by Fourier transform infrared spectroscopy. The effectiveness of rapid VFM curing was studied by characterizing the optical, electrical, and thermomechanical properties of VFM cured films with thermally cured films. The thermal stability of cured films was investigated by thermal gravitational analysis (TGA) and mass spectrometry (MS) studies. The results showed that a higher percent conversion and higher thermal stability can be achieved by using VFM processing than can be obtained using conventional thermal curing at the same cure temperature. However, the complete removal of photopackage related residual products requires slow ramp rates and long cure times

Packaging-compatible wafer level capping of MEMS devices

A cost-effective, wafer-level package process for microelectromechanical devices (MEMS) is presented. The movable part of MEMS device is encapsulated and protected while in wafer form so that commodity, lead-frame packaging can be used. A polymer epoxycyclohexyl polyhedral oligomericsilsesquioxanes has been used as a mask material to pattern the sacrificial polymer as well as overcoat the air-cavity. The resulting air-cavities are clean, debris-free, and robust. The cavities have substantial strength to withstand molding pressures during lead-frame packaging of the MEMS devices. A wide range of cavities from 20@mmx400@mm to 300@mmx400@mm have been fabricated and shown to be mechanically stable. These could potentially house MEMS devices over a wide range of sizes. The strength of the cavities has been investigated using nano-indentation and modeled using analytical and finite element techniques. Capacitive resonators packaged using this protocol have shown clean sensing electrodes and good functionality.

Polynorbornene for Low K Interconnection

Within the microelectronics industry, there is an ongoing trend toward miniaturization coupled with higher performance. The scaling of transitors toward smaller dimensions, higher speeds, and lower power has resulted in an urgent need for low dielectric constant interlevel insulators. Low dielectric constant interlevel dielectrics have already been identified as being critical to the realization of high performance integrated circuits in the SLA Roadmap. Thus, there exists a need in the microelectronics industry for a thermally stable, noncorrosive low dielectric constant polymer with good solvent resistance, high glass transition temperature, good mechanical performance and good adhesive properties, particularly to copper. In addition, the desired dielectric material should be capable of being processed in environmentally friendly solvents, and the final thermal and electrical performance should not be affected by manufacturing or post environmental conditions. High glass transition temperature polynorbornenes are being developed which provide many of these desired features. This polymer family is produced via a new transition metal catalyzed polymerization. Attributes which make polynorbornene particularly attractive in microelectronics include: (i) excellent thermal performance, (ii) adhesion to conductors without the use of adhesion promoters or barrier layers, (iii) very low moisture absorption (

Novel technique for measuring through-plane modulus in thin polymer films

Polymer thin films are widely used as coatings and interlevel dielectrics in microelectronic applications. In multilayer structures, stresses generated in the films due to interaction with adjacent layers and solvent evaporation induced shrinkage, causing the polymer chains to orient in the plane of the film resulting in anisotropic film properties. Characterization of properties in all directions is essential for accurate electrical and mechanical design and modeling. A new technique has been developed to measure, in-situ, the through-plane (z) stress-strain behavior of thin polymer films. A parallel plate capacitor device and an interdigitated electrode structure were used as sensors to detect changes in dielectric constant and thickness of thin polymer films under compression, Results are reported for 8-14 micrometer thick, Dow Chemical Cyclotene 3022 benzocyclobutene (BCB) films, The dielectric constant was found to change linearly with stress. Using this result, the through-plane stress-strain curve was obtained.

An investigation of fundamental factors influencing the permittivity of composite for embedded capacitor

The effect of the polymer permittivity on the permittivity of the polymer/lead magnesium niobate-lead titanate (PMN-PT) composite was investigated using epoxies, polyimides, polycarboxylic acids and PMMA. The permittivity of the polymers (without ceramic loading) ranged from 2.8 to 4.6. The permittivity of the composites was found to agree with the values predicted by the Lichtenecker and Smith equations except for epoxies. Polyacrylic acid (PAA) was found to be present on PMN-PT due to an acid-base neutralization reaction. Novolac epoxy was not found on PMN-PT. Because of the neutralization reaction, polycarboxylic acids could act as a surfactant to deagglomerate PMN-PT powder while epoxies may not. An electrostatic field simulation showed that the finer the particle size, the higher the permittivity of composite. These results could account for the lower permittivity of composites using epoxies than those of other polymers.