Jiali Wu

Evaluation and characterization of reliable non-hermetic conformal coatings for microelectromechanical system (MEMS) device encapsulation

The thrust of this project was to evaluate commercial conformal encapsulation candidates for low cost aerospace applications. The candidate conformal coatings evaluated in this study included silicone elastomers, epoxies, and Parylenes with bi-layer or tri-layer designs. Properties characterized in this study included mobile ion permeation and moisture ingress resistance, interfacial adhesion variation through thermal shock cycling and 85/spl deg/C/85% RH aging. Surface Insulation Resistance (SIR), Triple Track Resistance (TTR) and die shear strength were used for the corresponding electrical and physical property characterizations. Parylene F displayed excellent properties for environmental protection. Silicone elastomers displayed less resistance to the harsh environment as compared to the Parylene family (N, C, D types), but it could provide advantages for low residual stress applications. The change in adhesion strength between Parylene C and silicone elastomers after exposure to thermal shock cycling or 85/spl deg/C/85%RH aging for different time periods were conducted from die shear test in terms of the interfacial failure. SIR values of all the candidate materials after 1000 h exposure to 85/spl deg/C/85%RH, with 100 V dc for resistance measurement, range from 1/spl times/10/sup 8/-1/spl times/10/sup 9/ /spl Omega/. Leakage current values after 1000 h exposure to 85/spl deg/C/85%RH, 175 V bias, are in the range of 10/sup -9/ to 10/sup -11/ Amp. The bi- or tri-layer conformal coating combination investigated in this study showed significant promise for encapsulation of the microelectromechanical system (MEMS) devices.

Novel bi-layer conformal coating for reliability without hermeticity MEMS encapsulation

A flexible, smooth, and low profile conformal coating was developed to accomplish the encapsulation of a microelectromechanical system (MEMS) device that will be applied to sense the static pressure on aircraft during real flight testing. The encapsulant should be able to protect the MEMS device and the multichip module (MCM) from adverse environmental conditions, i.e., mechanical shock, temperature fluctuation, engine fuel and oil contamination, and moisture/mobile ion permeation. Presently, conventional packaging schemes for electronics cannot satisfy this specific outdoor application, and a new encapsulation combination has been designed in accord with the requirement of reliability without hermeticity (RWOH). A bi-layer structure was selected because of property limitations of a single material. Pliable elastomeric silicones are typically flexible, water repellent, and abrasion resistant. The silicone encapsulant will be first applied to planarize the MEMS surface and function as durable dielectric insulation, stress-relief, and shock/vibration absorbers over a wide humidity/temperature range. To compensate for the deficiency of silicone on engine fuel/oil contamination, Parylene C is to be deposited afterward. This bi-layer coating can achieve excellent bulk properties, such as moisture and mobile ion barrier resistance, chemical compatibility, and electrical insulation characteristics. However, the poor adhesion of Parylene C to silicone greatly restricts its application. To address this problem, silane coupling agents were used as an adhesion promoter. Significant adhesion improvement was achieved by placing an interlayer silane coupling agent to provide interfacial bonding to the silicone elastomeric surface and the Parylene C film. Furthermore, a possible mechanism of adhesion enhancement will also be presented in this study.

Interface-adhesion-enhanced bi-layer conformal coating for avionics application

A flexible, smooth, low profile conformal coating was developed for encapsulation of a MEMS applied to sense static pressure on aircraft during flight testing. The encapsulant should protect the MEMS and the MCM from environmental conditions, i.e. mechanical shock, temperature fluctuation, engine fuel and oil contamination, and moisture/mobile ion permeation. Conventional electronics packaging schemes cannot satisfy this specific outdoor application, and a new encapsulation combination was designed for the requirement of reliability without hermeticity (RWOH). A bi-layer structure was selected due to the limitations of single materials. Pliable elastomeric silicones are flexible, water repellent, and abrasion resistant. The silicone encapsulant is applied to planarize the MEMS surface and function as durable dielectric insulation, stress-relief, and shock/vibration absorbers over a wide humidity/temperature range. To compensate for the deficiency of silicone on engine fuel/oil contamination, parylene C is deposited afterward. This bi-layer coating has excellent bulk properties, e.g. moisture and mobile ion barrier resistance, chemical compatibility, and electrical insulation characteristics. However, the poor parylene C-silicone adhesion greatly restricts its application. To address this problem, silane coupling agents were used as adhesion promoters. Significant adhesion improvements were achieved by placing an interlayer silane coupling agent to provide interfacial bonding to the silicone elastomeric surface and parylene C film. Another possible adhesion enhancement mechanism is also presented.

Use of compliant adhesives in the large area processing of MCM-D substrates

Large area substrate processing is a key solution for improving the productivity of multichip module deposition (MCM-D) technology. This project is focused on high temperature polymeric adhesives for attachment of silicon tiles to suitable pallets to facilitate large area film processing of MCM structures. Current polymeric high temperature adhesives are predominately polyimide-based that are not reworkable, which places an obstacle to remove the coated substrates and to reuse the high cost pallets. However, an approach will be presented in this paper to address this demand by introducing thermally cleavable links in the thermoset polyimide-amide resin. A series of novel reworkable high temperature (in excess of 350–400 °C) adhesives have been developed, that can meet the requirements of adhesion, viscosity, thermal stability, and reworkability of the MCM-D production. Furthermore, scanning electron microscopy (SEM) microstructure images are presented for intuitive reworkability analysis.

Development of new low stress epoxies for MEMS device encapsulation

In this study, a series of new low stress epoxies was introduced as conformal encapsulants, which show a high promise to meet all the requirements for the protection of the pressure sensor system. Mechanical properties such as initial Youngs modulus, toughness and ultimate tensile stress were evaluated. The more critical issue of materials contamination resistance to the jet fuel was improved. And the mechanism behind materials low stress and toughness behaviors was investigated from the viewpoint of microstructure.

Study on self-alignment capability of electrically conductive adhesives (ECAs) for flip-chip application

This study is focused on a feasibility study of the self-alignment capability of electrically conductive adhesives (ECAs) for flip-chip interconnection applications. The effect of the low melting point alloy (LMA) filler on the self-alignment capability of ECAs was investigated. The surface energy of the epoxy resin was studied in terms of the various additives and their loading level by using a goniometer with a built-in environmental chamber. The curing profile of the epoxy resin and the melting point of the LMA filler were measured using a differential scanning calorimeter (DSC). The ECAs filled with the LMA over 85 wt% showed self-alignment during the heating process. However, when the silver flakes were added into the ECA formulation, the self-alignment did not take place. It is thought that the LMA filler depleted into the silver flakes can adversely affect the self-alignment of ECAs. The key parameters required for self-alignment capability of ECAs were presented. This paper discusses the feasibility of the application of ECA with self-alignment capability for flip-chip interconnection. Initial results from these series studies indicate that incorporation of the LMA into ECAs is an efficient way to make ECAs with self-alignment capability.

Characterization of reworkable high-temperature adhesives for MCM-D application

The need to have a high-temperature adhesive that can withstand temperatures in excess of 350°C for MCM-D silicon substrate process application, yet which can be reworkable at slightly high temperature ∼ 400°C for the removal from the glass pallet, is important. A novel, reworkable, high-temperature adhesive based on polyimide–amide–epoxy (PIAE) copolymer was developed and investigated using modulated differential scanning calorimetry (MDSC), thermal gravimetric analysis (TGA), Fourier Transform Infrared Spectroscopy (FTIR) and solid-probe pyrolysis mass spectroscopy (MS). Compared with commercial polyimide–amide (PIA) adhesives, FTIR spectra reveal that the thermally degradative ester groups contribute to the reworkability of the PIAE adhesive at a specific temperature (400°C), yet they remain thermally stable at a lower working temperature (350°C). FTIR spectrum comparison of the residuals of PIAE and PIA are similar after exposure to 400°C. MS spectra of outgassed products identify that the components of radical fragmentation from PIAE are due to polymeric chain degradation at 400°C, while only volatile trace water and N-methyl pyrolidone (NMP) are evolved from the commercial PIA adhesive. TGA results suggest a complementary explanation for the variation of total ion current (TIC) curves on these two adhesives. MDSC curves further verify that the reworkable PIAE adhesive is a copolymer. Furthermore, a reasonable thermal degradation mechanism is presented on the adhesive reworkability.

Development of high performance interfill materials for system chips technology

An innovative precisely interconnected chip (PIC) technology is currently under development at IBM to seek more effective means of creating system chips. The objective of this research is developing fabrication methods to permit the realization of high yielding large area chips, as well as chips that may contain very diverse technologies. This paper reports the use of a high-performance interfill material based on epoxy resin, which is used to connect the different chip sector macros that make up the system chip. This novel interfill material remains thermally stable through the subsequent processing temperature hierarchies during the interchip interconnection fabrication. Spherical SiO/sub 2/ powders are incorporated into the epoxy resin to improve its mechanical properties, reduce coefficient of thermal expansion, and increase thermal conductivity. Adhesion and rheology of the formulated interfill materials are evaluated. Microstructure of SiO/sub 2/ filled epoxy system is also investigated to confirm the reliability of the composite before and after thermal aging. Initial results indicate that the formulated EPOXY A resin composite is qualified for the system chip manufacturing process in terms of the dispensing processability, structural and mechanical integrity, and reliability.

Evaluation and characterization of high-performance filling encapsulants for system chips application

An innovative Precisely Interconnected Chips (PIC) technology is currently under development at IBM to seek more effective means of creating system chips. This development focuses on developing fabrication methods to permit the realization of high yielding large area chips, as well as chips that may contain very diverse technologies. Another focus is to realize system chips which have their basic chip characteristics compromised as is the case in many of todays system chip concepts. This paper reports on the use of a high performance filling encapsulant based on epoxy resin, which is used to connect the different chip sector macros that make up the system chip. This novel encapsulant remains thermally stable through the subsequent processing temperature hierarchies during the system chips fabrication. Spherical SiO/sub 2/ powders (with special morphology and size distribution) are incorporated into the epoxy resin to improve its mechanical properties, reduce coefficient of thermal expansion, and increase thermal conductivity. Adhesion and rheological properties of the formulated materials are evaluated. Microstructure of the filled Epoxy system is investigated to confirm the thermal reliability of the encapsulants. The formulated EPOXY A resin is qualified for manufacturing process based on the filling process, mechanical integrity, and thermal reliability.

Formulation of underfill materials for simultaneous underfilling and stack bonding

Die stacking is one of the next-generation 3D IC packaging methods, but its stringent material requirements are unlikely to be met by traditional underfills. Moreover, filler trapping is becoming an increasingly serious issue in no-flow and wafer-level underfills. We have previously reported an underfilling technology to reduce filler trapping using no-flow underfills and surface modified Cu bond pads. In such process, underfilling and interconnect bonding are carried out simultaneously, which calls for new underfill materials that would meet process requirements while leaving minimal residue filler on the bonding pads. Here we report a development epoxy composite material that is compatible with the bonding profile for eutectic SnAg solder.