Variable Frequency Microwave Research Articles

Variable Frequency Microwave and Convection Furnace Curing of Polybenzoxazole Buffer Layer for GaAs HBT Technology

Photosensitive polybenzoxazole (PBO) film has been used in GaAs heterojunction bipolar transistor (HBT) technology for stress buffer and mechanical protection layer applications. However, this film needs to be cured at high temperatures for a long period of time in order to obtain its desired excellent material characteristics. High-temperature curing can result in degradation to the electrical characteristics and performance of the underlying GaAs devices due to limited thermal budget. In this paper, we have characterized the effects of curing the PBO film on GaAs HBT wafers using a conventional convection furnace and using a variable frequency microwave (VFM) furnace. The results show that a VFM cure can achieve similar excellent physical, mechanical, thermal, and chemical material characteristics at a lower curing temperature and in a much shorter time, as compared to convection furnace curing, therefore resulting in minimal GaAs device degradation. Based on these results, an optimum curing condition using the VFM method can be obtained that satisfies both stress buffer layer material and device requirements for GaAs HBT technology.

Curing Dielectric Layers for Microelectronics with Microwaves: Chemistry, Mechanisms, and Applications

Recent work in curing new polyimide (PI) and polybenzoxazole (PBO) films on silicon device wafers with Variable Frequency Microwaves (VFM) rapidly (total cycle times less than 15 minutes) and also below 200ºC has produced equal or better film properties than conventional curing at 350ºC and in a fraction of the standard cycle time. The nature of the microwave heating mechanism will be discussed as well as the results of experiments that enable the custom synthesis of polymers and additives that take advantage of this unique heating process. Emphasis will be placed on the changes in chemical polymerization that appear to occur under microwave energies and the resultant network structures and mechanical property predictions for the films. Parallels to the curing of polymer adhesives at lower temperatures will also be examined with respect to the effects of unique network formation and cross-linking recently discovered with the use of VFM.

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

Variable Frequency Microwave Synthesis of Silver Nanoparticles

Synthesis of silver nanoparticles based on a polyol process and variable frequency microwave (VFM) was investigated. Comparing to a thermal method, the reaction by VFM radiation was much faster. The effects of silver nitrate concentration, poly(N-vinylpyrrolidone) (PVP) concentration, reaction time and reaction temperature were studied. It was found that the higher concentration of silver nitrate, longer reaction time and higher temperature increased the particle size while the higher concentration of PVP decreased the particle size.

PRODUCTIVITY IMPROVEMENT OF COMPOSITES PROCESSING THROUGH THE USE OF INDUSTRIAL MICROWAVE TECHNOLOGIES

This paper starts with the characteristics and advantages of microwaves processing. The shortcomings of fixed frequency, typically at 2.45 GHz were also mentioned. On account of this, the newly developed variable frequency microwave (VFM) fabrication was mentioned and adopted in place of the fixed frequency process. Two cases of fixed frequency microwave processing of materials were described; the characteristics, pros and cons of each case was mentioned and commented. Two cases of processing materials using variable frequency microwave facility (VFMF) were mentioned; the advantages and limitations of each case were discussed. The microwave processing of materials provides improved mechanical, physical and electrical properties with much reduced processing time. Furthermore, variable frequency microwave processing is more superior to its fixed frequency counterpart except that the cost of the facilities of the former is much higher than the latter at this point in time but it appears that the price will drop in the coming ten years.

Chemothermal modeling and finite-element analysis for microwave cure process of underfill in flip-chip packaging

The reaction kinetics of microwave cure process of underfill materials in flip-chip packaging was investigated with nonisothermal kinetic method and compared with that of the thermal cure. Three-dimensional (3-D) nonlinear cure kinetic and transient heat transfer coupled model was solved by finite-element method (FEM) to simulate the microwave cure process. The accuracy of the program was verified using a simple heat conduction case by commercial FEM software. Temperature and conversion inside underfill during microwave cure process were evaluated by solving the nonlinear anisotropic heat conduction equation including internal heat generation produced by exothermic chemical reactions. Numerical results show that the iteration calculations are very sensitive to small changes in time step sizes. It was also found that variable frequency microwave can process underfill materials with uniform conversion under different curing temperatures.

Comparison of linear and non-linear sweep rate regimes in variable frequency microwave technique for uniform heating in materials processing

Materials processing using microwave technology have numerous advantages compared to traditional materials processing techniques; however, conventional microwave processing technology suffers from variable product quality due to uneven processing or heating. The variable frequency microwave (VFM) technique is an emerging technology that overcomes the inherent difficulties of uneven heating experienced in conventional microwave facilities and was developed for the purpose of processing many of today's advanced materials including polymers, composites and fibre-reinforced materials and is a rapid and controlled approach to uniform heating. By sweeping through a bandwidth of frequencies, the VFM method creates multiple hot spots within the processing cavity that lead to time-averaged uniform heating. The current work focuses on the sweep-rate regime used in VFMs and the effect of varying this parameter. A “recipe” for a non-linear sweep-rate regime is introduced and compared with the current linear sweep regime and conventional fixed-frequency processing in terms of heating uniformity. Theoretical modelling of the linear and non-linear sweep regimes are presented and are experimentally verified by heat-mapping of a multimode cavity with the aid of thermally sensitive material to quantify improvements in the uniformity of heating.

Lead-free interconnect technique by using variable frequency microwave

A novel lead-free interconnect technique using variable frequency microwave (VFM) was investigated. The lead-free solder interconnection between the component chips and the metal pads through VFM was first demonstrated. Comparison between the microstructures of the lead-free solder joints on Cu and Sn surfaces formed by a conventional thermal reflow process and VFM was conducted. The VFM heating technique successfully created the lead-free solder/Cu and Sn joints through their intermetallic compounds (IMCs), while maintaining the substrate temperature as low as 210°C.

Contrast Joints of Glass Fiber with Carbon Fiber-reinforced Low-density Polyethylene Composite Bonded by Microwave Irradiation

This paper contrasts the loss tangent, durability of reinforcement and, lap shear strengths of 33% by weight random glass fiber-reinforced low-density polyethylene matrix composite [LDPE/GF (33%)] with 33% by weight random carbon fiber reinforced low-density polyethylene matrix composite [LDPE/CF (33%)] bonded using microwave irradiation. Fixed (2.45 GHz) and variable (2-18 GHz) frequency microwave (VFM) facilities are used to bond the two composites. With a given power level, the composites are exposed at various exposure times to microwave irradiation. The primer or coupling agent used for joining the glass fiber-reinforced composite is a 5-min two-part adhesive, Araldite. No filler was used in joining the carbon fiber-reinforced composite.

Low Temperature Rapid Curing of Polymer Dielectrics on Metallized Organic Laminates by Variable Frequency Microwave Processing

The curing of polymer dielectrics on organic laminates by variable frequency microwave (VFM) processing was studied. Polymer dielectrics, such as polyimides, normally require long cure times at elevated temperatures resulting in processing incompatibility with epoxy-based laminates due to the low thermal stability of the organic laminates. In this study, VFM processing was investigated as an alternative, low temperature technique for curing polyimide and benzycyclobutene (BCB) on metallized bismaleimide triazine/epoxy (BT) board. The VFM heating characteristics of the metallized (BT) boards were studied. The VFM heating rates of the BT boards strongly depended on the configuration, size, and shape of the copper patterns on the boards. The VFM curing of polyimide and BCB on the metallized BT boards was also performed. The curing condition was optimized to minimize board degradation. At the same board degradation level, the chemical, thermo-mechanical and electrical properties of the VFM cured film were compared with a film thermally cured using a conventional convection furnace. A higher degree of curing and lower dielectric constant was achieved for benzocyclobutene (BCB) films on BT boards processed using VFM curing than was obtained using conventional curing. VFM cured polyimide (PI 2611) showed a similar degree of curing and Young's modulus to conventionally cured PI 2611. However, the VFM cured PI 2611 showed a higher residual content solvent and a higher dielectric constant than conventionally cured PI 2611.

Application of Variable Frequency Microwave (VFM) to Adhesives

Microwave processing of adhesives is a relatively new technology alternative that provides new approaches for enhancing material properties as well as economic advantages through energy savings and accelerated product development. Alternative in the sense that most adhesives are normally cured in ambient conditions or in ovens. However, the most commonly used facilities for microwave processing of materials operate on fixed frequency microwaves (FFM), e.g., 2.45 GHz. This paper presents a review of microwave technologies, processing methods and industrial applications, using variable frequency microwave (VFM) facilities. The technique offers rapid, uniform and selective heating over a large volume at a high energy coupling efficiency. This is accomplished using a preselected bandwidth sweeping around a central frequency by employing tunable frequency sources. Successful applications of these modern facilities include finding out the optimum cavity conditions of glass or carbon fibre reinforced thermoplastic matrix composites, and of adhesives, e.g., two-part five-minute Araldite, and the joining of the above-mentioned composite materials with, or without, primers. Finding out the optimum cavity conditions of a material has helped identify the best frequency range to process the material using microwave energy and by means of the VFM facility. Microwave energy has been used to rapidly cure several types of two-part epoxy based adhesives, e.g., Araldite. Bond strengths obtained using variable frequency microwave (VFM) techniques are compared with adhesive joints cured in fixed frequency microwave (FFM) conditions.

Low Temperature Curing of Polyimide Precursors by Variable Frequency Microwave

Polyimides are commonly used as a thermally stable insulator for semiconductors. To assure adequate chemical or physical properties, a layer of their precursors has to be cured in a convection oven at more than 300 °C. In this paper, we compare variable frequency microwave (VFM) heating with convection heating for curing various polyimide precursors and a photosensitive polyimide precursor. Then, we demonstrate that the use of VFM enables us to cure the precursor lower than the standard convection cure temperature while maintaining necessary film properties such as cyclization, Tg, Td, and elongation.

Finite element analysis for microwave cure of underfill in flip chip packaging

A 3D transient heat transfer and cure kinetic coupled finite element model was developed to simulate the microwave curing process of underfill material in flip-chip packaging. Temperature distribution inside underfill was evaluated by solving the an-isotropic heat conduction equation including internal heat generation produced by exothermic chemical reactions. A non-isothermal cure kinetic method was used to validate the model. The cure kinetic parameters of the model were determined by differential scanning calorimeter (DSC) from the underfill precursor. Numerical results show that variable frequency microwave (VFM) oven can process underfill materials with uniform temperature distribution and conversion.

Wafer bonding using microwave heating of parylene intermediate layers

This paper describes a novel wafer bonding technique using microwave heating of parylene intermediate layers. The bonding is achieved by parylene deposition and thermal lamination using microwave heating. Variable frequency microwave heating provides uniform, selective and rapid heating for parylene intermediate layers. The advantages of this bonding technique include short bonding time, low bonding temperature, relatively high bonding strength, less void generation and low thermal stress. In addition, the intermediate layer material, parylene, is chemically stable and biocompatible. This bonding technique can be used for structured wafers also because parylene provides a conformal coating. Therefore, this is a very attractive bonding tool for many MEMS devices. The bonding strength and uniformity were evaluated using diverse tools. Fracture mechanisms and the effects of bonding parameters and an adhesion promoter were also investigated. The bonding with a structured wafer was also successfully demonstrated.

微細バンプ接合に適用する可変高周波加熱(VFM）によるCu-Sn拡散挙動の基礎検証

微細バンプ接合に適用する可変高周波加熱(VFM）によるCu-Sn拡散挙動の基礎検証

Investigation of Variable Frequency Microwave Processing for Electronic Packaging Applications

This paper summarizes the effects of the variable frequency microwave (VFM) technique for rapid cure of polymeric encapsulants (such as flip-chip underfills) on the performance and reliability of electronic devices. Initial electrical performance of selected commercial IC packages following the application of VFM radiation stress has been recorded, and the performance has been compared to packages that were treated with comparable temperature cycles as applied in a convection oven. Failure analysis was performed on packages that showed electrical degradation to identify likely degradation and failure modes of the packaged ICs. Overall, our results show that VFM technology can safely be applied as an electronic packaging technology. However, proper control of VFM operating parameters is needed to ensure favorable performance of electronic devices.

Variable Frequency Microwave Processing of Underfill Encapsulants for Flip-Chip Applications

This paper discusses an innovative technique for rapid cure of polymeric encapsulants such as underfills used in direct chip attach devices using variable frequency microwaves (VFM). VFM processing reduces the cure time for underfill encapsulants to 10 min or less, as compared to 30 or more minutes when using convection oven methods. We report here the results of our investigations measuring key material attributes of VFM and conventionally cured underfill encapsulant samples, and we also have characterized voiding and delamination characteristics of flip-chip with underfill test structures. Finally, particle settling in the flip-chip with underfill test structures has been characterized. Our results show that the VFM technique produces underfill attributes that are comparable to conventionally cured samples.

Variable frequency microwave (VFM) processing facilities and application in processing thermoplastic matrix composites

Microwave processing of materials is a relatively new technology advancement alternative that provides new approaches for enhancing material properties as well as economic advantages through energy savings and accelerated product development. Factors that hinder the use of microwaves in materials processing are declining, so that prospect for the development of this technology seem to be very promising [1]. The two mechanisms of orientation polarisation and interfacial space charge polarisation, together with dc conductivity, form the basis of high frequency heating. Clearly, advantages in utilising microwave technologies for processing materials include penetration radiation, controlled electric field distribution and selective and volumetric heating. However, the most commonly used facilities for microwave processing materials are of fixed frequency, e.g. 2.45 GHz. This paper presents a state-of-the-art review of microwave technologies, processing methods and industrial applications, using variable frequency microwave (VFM) facilities. This is a new alternative for microwave processing.

Lap shear strength comparison between different random glass fibre reinforced thermoplastic matrix composites bonded by adhesives using variable-frequency microwave irradiation

AbstractThis paper compares the lap shear strengths of three types of random glass fibre reinforced thermoplastic matrix composite joined by adhesives using microwave energy. Variable-frequency microwave (VFM) (2-18 GHz) facilities are used to join 33 wt % random glass fibre reinforced low-density polyethylene composite [LDPE/GF (33%)], 33 wt % random glass fibre reinforced polystyrene composite [PS/GF (33%)] and 33 wt % random glass fibre reinforced nylon 66 composite [nylon 66/GF (33%)]. With a given power level, the composites were exposed for various times to microwave irradiation. The primer or coupling agent used was a 5 min two-part adhesive.

A Review of the Variable Frequency Microwave Technology in Material Processing

The Variable Frequency Microwave (VFM) processing technology is an emerging technique that foregoes many of the inherent problems obtained in fixed-frequency microwave processing. By sweeping through a bandwidth of frequencies, a time-averaged uniformity is obtained which can eliminate heating non-uniformities as well as catastrophic thermal excursions sometimes experienced in conventional microwave technology Primarily aimed at processing advanced materials, advantages include uniform heating, arc-free processing, a higher degree of controllability, higher coupling efficiency and the ability to choose “specific” frequency or frequency bands suitable for particular applications. The review within provides an overview of the theory behind the technique as well as an outline of the advances regarding this method in both the academic and industrial stage. Current and future applications using this emerging technology as well as numerical modeling are also discussed.