Twyman-Green Interferometry Research Articles

Robust, motion-free optical characterization of samples using actively-tunable Twyman–Green interferometry

Optical interferometry-based techniques are ubiquitous in various measurement, imaging, calibration, metrological, and astronomical applications. Repeatability, simplicity, and reliability of measurements have ensured that interferometry in its various forms remains popular—and in fact continues to grow—in almost every branch of measurement science. In this paper, we propose a novel actively-controlled optical interferometer in the Twyman–Green configuration. The active beam control within the interferometer is a result of using an actively-controlled tunable focus lens in the sample arm of the interferometer. This innovation allows us to characterize transparent samples cut in the cubical geometry without the need for bulk mechanical motion within the interferometer. Unlike thickness/refractive index measurements with conventional Twyman–Green interferometers, the actively-tunable interferometer enables bulk-motion free thickness or refractive index sample measurements. With experimental demonstrations, we show excellent results for various samples that we characterized. The elimination of bulk motion from the measurement process promises to enable miniaturization of actively-tunable Twyman–Green interferometers for various applications.

Simple and fast calibration method for phase-only spatial light modulators.

Phase-only spatial light modulators (SLMs) are widely used to engineer the phase of light in various applications. However, liquid-crystal-on-silicon SLMs have undesirable spatial variations in phase response and optical flatness across the SLM panel, which must be compensated for accurate phase control. Here, we introduce a simple and fast way to simultaneously extract these two types of SLM nonuniformities at single-pixel resolution using Twyman-Green interferometry without a piezoelectric transducer. By modulating the interference intensity via the SLM gray level, our approach requires N times fewer interferograms than typical N-step phase shift interferometry (PSI), while providing flatness correction as accurate as PSI. In practice, our calibration method works well with as few as 18 interferograms, which can be quickly acquired without concern for phase drift. We detail the calibration procedure and discuss the performance of our calibration.

Tear Film Metric Derived from Twyman-Green Interferometry of Tear Films

Purpose An in-vivo tear surface interferometer has been developed to measure the dynamic tear surface shape (TSS). A corresponding prelens tear film (PLTF) metric has been developed. The purpose of this work was to examine whether the metric could distinguish PLTFs by different lens types.

Wavefront correction for spatial nonuniformity of the liquid crystal on silicon based spatial light modulator

Liquid crystal on silicon (LCoS) based spatial light modulator (SLM) is widely used in many research areas, such as holography display, optical measurement, and optical tweezers, due to its programmable capability of phase modulation. However, a certain aberration term to the incident wavefront is introduced due to the spatial nonuniformity of the LCoS backplane, and the correction of this phase aberration is important to improve the accuracy of phase modulation by the SLM, which will then influence the SLM applications, e.g., the image quality in holography display, or the beam shaping for optical tweezers and free-space communications. Fortunately, the LCoS based SLM has the programmable capability of phase modulation; therefore, the spatial nonuniformity can be compensated by a wavefront correction. Conventionally, this aberration is corrected by adjusting the Zernike coefficients of the generated wavefront manually. However, this procedure is time-consuming and subjective, since there are dozens of coefficients to be adjusted, and the adjustment is evaluated manually by monitoring the interferogram. In this paper, we propose to use several phase retrieval techniques to measure the corresponding wavefront directly and employ the obtained wavefront to correct the aberration introduced by the spatial nonuniformity. A Twyman-Green interferometry is used, and an interferogram acquisition technique based on the self wavefront modulation of SLM is proposed. Besides, a method for determining the corresponding active area of the LCoS backplane and parameters of 2-D projective transformation is proposed to transform the active area to the fitted resolution of the SLM. RMS value of the aberration to approximately λ/32 can be obtained by the proposed method. Consequently, more accurate phase modulation by SLM for applications such as holography display and beam shaping is achievable. Effectiveness of the proposed method is validated by experiment, and the correction results by different phase retrieval techniques are evaluated and compared.

Full-field deflection measurement of the flexible transparent sheets

Because flexible transparent sheets (FTSs) have high transmittance and large deflection, their full-field deflections are difficult, or even impossible, to obtain experimentally. This study proposes a methodology based on Twyman-Green interferometry to measure the full-field deflection of FTSs. Several polyethylene terephthalates (PETs) with known deflections were prepared to verify the measurement precision of the proposed methodology. Results show that the proposed methodology can be used to measure full-field deflection up to 6.5 mm. The existing fringe of the null field significantly influences the measurement results under small deflections. To the authors' best knowledge, this is the first full-field and non-contact method of measuring the deflection of FTSs.

A study on thermal cycling (T/C) reliability of anisotropic conductive film (ACF) flip chip assembly for thin chip-on-board (COB) packages

In this work, thermal cycling (T/C) reliability of anisotropic conductive film (ACF) flip chip assemblies having various chip and substrate thicknesses for thin chip-on-board (COB) packages were investigated. In order to analyze T/C reliability, shear strains of six flip chip assemblies were calculated using Suhir’s model. In addition, correlation of shear strain with die warpage was attempted.The thicknesses of the chips used were 180μm and 480μm. The thicknesses of the substrates were 120, 550, and 980μm. Thus, six combinations of flip chip assemblies were prepared for the T/C reliability test. During the T/C reliability test, the 180μm thick chip assemblies showed more stable contact resistance changes than the 480μm thick chip assemblies did for all three substrates. The 550μm thick substrate assemblies, which had the lowest CTE among three substrates, showed the best T/C reliability performance for a given chip thickness.In order to investigate what the T/C reliability performance results from, die warpages of six assemblies were measured using Twyman–Green interferometry. In addition, shear strains of the flip chip assemblies were calculated using measured material properties of ACF and substrates through Suhir’s 2-D model. T/C reliability of the flip chip assemblies was independent of die warpages; it was, however, in proportion to calculated shear strain. The result was closely related with material properties of the substrates. The T/C reliability of the ACF flip chip assemblies was concluded to be dominatingly dependent on the induced shear strains of ACF layers.

Residual Strain Measurement of Thin-Layer Cured Adhesives and Their Effects on Warpage in Electronic Packaging

Residual strains (stresses) of thin-layer adhesives are important and need be determined and further analyzed for better reliability design of the joints or bump joints in electronic or optoelectronic packaging. The purposes of this paper are to quantify residual strains of two adhesives: A (paste type) and B (film type) due to chemical shrinkage, stress relaxation, and thermal-loadings and moisture-loadings, and to investigate their effects on the warpage of die attachment assembly. At the beginning, the mechanical properties of the two adhesives in terms of temperatures are measured by dynamic mechanical analyzer (DMA) and thermomechanical analyzer (TMA). The residual strains are documented by testing fully cured adhesive/silicon bi-material plates under thermal and moisture loading using Twyman-Green (T/G) interferometry system associated with Timoshenko's bi-material theory and finite element method (FEM). Furthermore, the warpage of silicon/adhesive/Cu tri-material plates (similar to die attachment assembly) with both adhesives under thermal and moisture loading are also investigated by T/G experiments and confirmed by Suhir's solution and FEM results. For adhesive-A, the results of the bi-material plate suggest that the residual strains are only induced by mismatch of the coefficients of thermal expansion during thermal loading, rather than other factors, right after the fully cured adhesive cooling down to room temperature. On the other hand, for adhesive-B, it was found that the additional residual strain of this film adhesive caused by chemical shrinkage plus stress relaxation, is about 2.26 ? 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , which accounts for 85% of thermal-residual strains in the bi-material plate from curing temperature cooling down to room temperature. Experimental results also indicated that moisture-absorption expansion of the both adhesives is the major cause of decreasing warpage for the bi-material plates at 29?C/55% RH, other than stress relaxation. In contrast to the results of the bi-material plate, the warpages of the tri-material plates are found to be insensitive to the moisture-absorption, chemical, and thermal strains of adhesives, but not to elastic moduli of adhesives, due to their relatively thin-layer and compliant nature. This paper has successfully laid down an approach for determining residual strain of thin-layer adhesive and provided information about their effect on warpage of die attachment assembly.

Thermal-Mechanical Process Simulation of an Advanced NCF Technology

The paper introduces an advanced nonconductive film (NCF) typed FC technology employing a novel compliant composite interconnect structure. The interconnect reliability and bondability of the technology are demonstrated through experimental thermal humidity (TH) test in conjunction with a two-point daisy chain resistance measurement. The alternative goal of the study aims to look into the insight of the thermal-mechanical behaviors of the novel packaging technology during NCF bonding process and thermal testing through numerical modeling and experimental validation. For effectively simulating the bonding process, a process-dependent finite-element (FE) simulation methodology is performed. The validity of the proposed methodology is verified through several experimental methods, including a Twyman-Green (T/G) interferometry technique for warpage measurement, and a four-point probe method for contact resistance measurement. At last, a design guideline for improved process-induced thermal-mechanical behaviors is presented through parametric FE analysis. Both numerical and experimental results demonstrate the feasibility in applying the novel compliant interconnects to achieve a proper contact stress at various temperature environments so as to hold a low and stable connection resistance at elevated temperature. Most importantly, the novel interconnects survive the 85degC/85%RH TH test for 500 hours.

Effects of Thermal Cycling on Material Properties of Nonconductive Pastes (NCPs) and the Relationship Between Material Properties and Warpage Behavior During Thermal Cycling

In this paper, the effects of thermal cycling on material properties such as coefficient of thermal expansion (CTE), modulus, and glass transition temperature (Tg) of nonconductive pastes (NCPs) for flip chip applications were investigated. Using a thermomechanical analyzer, the dimensional changes of NCPs and an underfill material were measured. The dimensional changes of all materials during the first cycle rapidly increased near Tg. However, the rapid increase of dimensional change near Tg was not observed during the second and third cycles. Furthermore, using a dynamic mechanical analyzer, the modulus and Tg were measured. The modulus in the first cycle was smaller than that in the second cycle for all materials. After the first cycle, the modulus curves followed the second cycle curve. Next, the warpage behavior of the flip chip assemblies was observed using the Twyman-Green interferometry method to investigate how material property changes affect warpage behavior during thermal cycling (T/C) and it was found that the warpage of the flip chip assembly decreased after the first cycle. However, after the first cycle, the amount of warpage was constant for the following five cycles. As a result, it was verified that the material properties of NCPs and the underfill material change after the first thermal cycle, and the material property changes are closely related to the warpage hysteresis behavior during T/C. Finally, warpage hysteresis was understood as shear strain.

Transient Stress Distributions in NCF-Bonded COG Packages due to Moisture Diffusion

The nonconductive film (NCF) adhesives have potentials to replace the anisotropic conductive film (ACF) adhesive in the chip-on-glass (COG) or other packages in liquid crystal displays (LCDs) due to their feasible features of finer bump pitch and cost reduction. Prior to applying the NCF to the COG packages, reliability issues, such as interfacial delamination and increase of bump contact resistance under thermal and moisture loads, have to be overcome. The objective of this paper is to investigate the effect of moisture ingression on stress field of the COG packages with NCF bonding. The coefficients of moisture diffusion and the hygro-strains of the NCF adhesive have been determined by an innovative approach of measuring the out-of-plane deformation of the bimaterial specimens during moisture absorption at the condition of 30degC/85%RH using Twyman-Green interferometry, associated with a finite element analysis and bimaterial theory. In this study, the results show that the average of the moisture diffusion coefficients and saturated hygro-strains of the NCF under 30degC/85%RH are 1.09 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /s and 1.51 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , respectively. After being validated by the diffusion theory, the moisture- transient finite element analysis reveals that the moisture absorption of the COG package under 30degC/85%RH is nearly saturated after 1000 h. It is also found that there are large tensile stresses at pad corner and mediate ones across bump joint, and these tensile stresses gradually increase with moisture absorption and up to the maximum at the state of moisture saturation. Through the parametric study by the validated finite element model, the results show that the tensile bump-joint and interfacial stresses increase nonlinearly with the NCF elastic modulus and linearly with the saturated moisture-induced strain (SMIS) of the NCFs. Furthermore, the COG package with polyimide bumps is found to have lower moisture-induced tensile stresses than that with gold bumps. It is further shown that the tensile stresses from the longitudinal-section and transverse-section models are similar only with a few percent difference in values. It can be concluded that the stress states of the NCF-bonded COG packages induced by transient moisture ingression have been fully addressed through experimental, theoretical and numerical approaches in this study.

Static and dynamic characterization of AlN-driven microcantilevers using optical interference microscopy

We have developed an optomechanical methodology, combining interferometric deflection data, the nanoindentation technique and analytical modeling to perform the characterization of piezoelectrically driven microcantilevers operating as MEMS actuators. Here, the association of standard Twyman–Green interferometry (TGI) with time averaged and stroboscopic techniques permits the evaluation of the 3-D out-of-plane deflections of microdevices and provides feedback of measurements that helps us to optimize MEMS structures and improve the reliability and stability of microcantilevers. The goal of the presented study was investigation of high-quality cantilevers composed from silicon beam and a transducer including the aluminum nitride (AlN) layer. It is a material with piezoelectric properties, which can be an alternative for PZT films in micromachining technology. After presenting the fabrication process of the testing devices, the rest of the paper will focus on non-contact measurements of cantilevers deflection by interferometry: static data (e.g., initial shape, deformation, stress) and dynamic parameters of samples (e.g., resonance frequency and amplitude distributions in vibration modes). On the basis of these experimental data, parameters such as piezoelectric coefficient d 31 have been calculated taking into account multiple film stacking.

Experimental and Numerical Studies of Warpages of ACF-Bonded COG Packages Induced From Manufacturing and Thermal Cycling

Regarding the application of the chip on glass (COG) with anisotropic conductive film (ACF) to the liquid crystal displays (LCDs), the problems with the warpage of COG packages, interfacial delamination, and increasing contact resistance of bumps, during or after thermal and moisture loading, are major reliability issues encountered in the industry. The goal in this paper is to investigate the effect of the parameters, such as bonding pressure and temperature during manufacturing, thermal and moisture expansion of the ACF, and its elastic modulus and fillets, associated with thermal cycling (from room temperature to 85 ), on the warpages of the ACF-bonded COG packages. The full-field Twyman-Green interferometry is used for measuring the warpages of the COG packages due to the fabrication with the various bonding pressure and temperature and during thermal cycling. Three-dimensional finite-element models (FEM) are used for calculating the warpage in terms of such parameters, and those results are compared with experimental observations in order to understand the mechanics. The results show that the tremendously saddle-warped shapes are found at the COG packages after manufacturing, and their maximum warpages increase with the bonding pressure but are insensitive to the bonding temperature. However, these large warpages have been obviously relaxed as the temperature goes beyond 50 during the first thermal cycling. After the first cycling, the warpages of the COG packages become much less severe and remain stably, then slightly increase with the temperature. The deformed behaviors of the COG package under the thermal loads have been resolved by comparing the FEM results with the experimental observations. The moisture-induced expansion strain of the saturated ACF in the condition of 29 /85%RH was determined to be 0.147% by means of the combination of experimental and theoretical analyses. It is also found that, for relatively thin fillets, the coefficient of thermal expansion (CTE) and moisture absorption of the ACF do not have any obvious influence on the warpages of the COG packages, but for relatively thick fillets they do somewhat. Moreover, the warpages of the COG package affected by ACF fillet thickness, elastic modulus, and CTE have been thoroughly studied by the finite-element analysis and the results have been interpreted in detail. Overall, for reducing the warpage of the COG package, two approaches are suggested: giving the packages at 85 preheated after manufacturing and precisely controlling low mismatch of CTE between chip and glass substrate, since both play significant roles.

Hygro-Thermal Warpages of COG Package With Nonconductive Paste Adhesive

The use of nonconductive paste (NCP) or film adhesives to replace the anisotropic conductive film in chip on glass (COG) packages may be a possible solution for the low-cost and finer-bump-pitch application to the liquid crystal displays. However, when the NCP is applied, the reliability-related issues, such as excessive warpage, interfacial delamination, and increasing contact resistance of bumps, occur in the COG packages. The goal of this paper is to experimentally and numerically study the warpage of NCP-bonded COG packages with and without fillets after being cured in the chamber with 150degC for 2 h, under thermal and moisture loading. Twyman-Green interferometry is used for measuring the warpages (out-of-plane deformations) of the COG packages due to such loading conditions. A 3-D finite element method (FEM) is used for calculating the warpage and the obtained results are compared with experimental observations in order to provide an insight to it in terms of mechanics. The full-field out-of-plane deformations (warpages) of the COG packages with and without fillets have been measured under thermal cycling and moisture loading. The results indicate that, right after manufacturing, the very (apparently) concave-up-shape (smiling-shape) warpages are found at the COG packages with fillet, but slightly concave-down (crying) shapes are at those without fillet. And the warpages decrease with the heating temperature increasing. The warpages of the COG package, determined from the experiment and FEM, are quite consistent for the case either with fillet or without fillet. Prior to investigating the moisture effect on warpages of the COG packages, the moisture-induced strain of the NCP at the condition of 29degC/85%RH for 12 h has been obtained to be epsiv <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sup> =0.3% by testing a NCP/silicon bimaterial plate with a combination of Twyman-Green experiment and Timoshenko's bimaterial theory. This strain value has been incorporated into the FEM analysis, and the obtained warpage distributions of COG packages with and without fillets are compared with those from Twyman-Green experiment. It is found that moisture-absorption expansion strain leads to dramatically reduce the warpage of the COG package with fillet, but not for that without fillet. The results regarding the maximum warpages of the COG in terms of the parameters, such as fillets, the coefficient of thermal expansion, and elastic modulus of the NCP are also thoroughly discussed in this paper.

Effects of the Functional Groups of Nonconductive Films (NCFs) on Material Properties and Reliability of NCF Flip-Chip-On-Organic Boards

Nonconductive films (NCFs) are one of the polymer interconnect materials for flip-chip interconnection. NCFs containing no conductive particles play several roles such as adhesion, insulation, and underfilling at the same time. The most important issue of NCF flip-chip-on-board (FCOB) assemblies is thermal cycling reliability. Thermomechanical properties of cured NCFs, such as glass transition temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> ), storage modulus (E'), and coefficient of thermal expansion (CTE), significantly affect the thermal cycling reliability of NCF FCOB assemblies. In this paper, the improvement of thermomechanical properties of NCFs was investigated by controlling the number of functional groups of NCF resins. To compare the reliability of conventional and modified NCF FCOB assemblies after thermal cycling test, electrical analysis and scanning acoustic microscopy investigation were performed. Thermal deformations of NCF FCOB assemblies under thermal cycling environment were also investigated, and quantitatively compared using high sensitivity Twyman-Green interferometry and portable engineering Moire interferometry. According to these results, the functional groups of NCFs have significant effects on thermomechanical properties of cured NCFs, thermal deformation, and thermal cycling reliability of NCF-bonded FCOB assemblies. As a result, the functionality modified NCF FCOB assemblies showed significantly enhanced reliability compared conventional NCFs in thermal cycling test.

Effect of High Glass Transition Temperature on Reliability of Non-Conductive Film (NCF)

Among many factors that influence the reliability of a flip-chip assembly using NCF interconnections, the most effective parameters are often the coefficient of thermal expansion (CTE), the modulus (E), and the glass transition temperatures (Tg). Of these factors, the effect of Tg on thermal deformation and device reliability is significant; however, it has not been shown clearly what effect Tg has on the reliability of NCF. The Tg of a conventional NCF material is approximately 110°C. In this study, a new high Tg NCF material that has a 140oC Tg is proposed. The thermal behaviors of the conventional and new NCFs between -40oC to 150oC are observed using an optical method. Twyman-Green interferometry and the moiré interferometry method are used to measure the thermal micro-deformations. The Twyman-Green interferometry measurement technique is applied to verify the stress-free state. The stress-free temperatures of the conventional and new Tg NCF materials are approximately 100oC and 120oC respectively. A shear strain at a part of the NCF chip edge is measured by moiré interferometry. Additionally, a method to accurately measure the residual warpage and shear strain at room temperature is proposed. Through the analysis of the relationship between the warpage and the shear strain, the effect of the high-Tg NCF material on the reliability is studied.

The Effect of Epoxy Molding Compound on Thermal/Residual Deformations and Stresses in IC Packages During Manufacturing Process

Thermal/residual deformations and stresses in plastic integrated circuit (IC) packages caused by epoxy molding compound (EMC) during the manufacturing process are investigated experimentally (only for deformations), theoretically, and numerically. A real-time Twyman-Green interferometry is used for measuring the out-of-plane thermal and residual deformations of die/EMC bi-material specimens. Dynamic mechanical analysis (DMA) and thermomechanical analysis (TMA) are for characterizing thermomechanical properties of the EMC materials. A finite element model (FEM) and theory associated with experimental observations are employed for understanding the thermal/residual deformations and stresses of IC packages due to EMC encapsulation. It is shown that EMC materials must be fully cured so that the material properties are stable enough for applications. Experimental results show that the EMC material experiences stress relaxation due to its viscoelastic behavior during the post mold curing (PMC) process. As a result, the strains (stresses) resulted from the chemical shrinkage of the EMC curing could be relaxed during the PMC process, so that the chemical shrinkage has no effect on the residual strains (stresses) for the plastic packages being post cured. Compared with numerical and theoretical analyses, the experimental results have demonstrated that die/EMC bi-material structure at high temperature (above Tg) warps less than expected, as a result of viscoelastic stress relaxation of EMC at high temperature (during solder reflow process). Meanwhile, this stress relaxation can also cause shifting this zero-stress temperature to the higher one, so that the residual deformations (stresses) of die/EMC bi-material specimens were found to increase by about 40% after the solder reflow process. The residual and thermal stresses have been resolved by FEM and theoretical analyses. The results suggest that the pure bending stresses (without shear and peel stresses) of the bi-material specimens are only limited in the region from x= 0 (the center) to x= 0.75 L due to the free edge effects, but this region is shrunk down to x= 0.4L at 200degC. And the maximum warpage and bending stress per unit temperature change is occurred around 165degC (Tg of the EMC). This study has demonstrated that the Twyman-Green experiment with associated bi-material plate theory and FEM can provide a useful tool for studying the EMC-induce residual/thermal deformations and stresses during the IC packaging fabrication

Study of Some Parameter Effects on Warpage and Bump-Joint Stresses of COG Packages

The purpose of this paper is to investigate the parameters such as thermal loads, adhesive material properties and fillets, bump materials, and moisture, affecting the warpage and bump-joint stresses of the chip-on-glass (COG) packages with anisotropic conductive film (ACF). Two- and three-dimensional finite-element models (FEMs) are employed for calculating the warpage and bump- joint stresses in terms of such parameters. In addition, the real-time full-field Twyman-Green interferometry is used for measuring the out-of-plane deformations of the specimens and providing validation of the warpage calculated from finite-element models. Also, Suhir's close-form solution of a die-substrate assembly is applied for looking into insight of thermomechanics, especially for adhesive stresses and correlated with finite-element results. The results show that the fillets strongly affect the warpage of the COG specimens by changing the convex shape of warpage for those without fillets to the saddle shape for those with fillets. However, unlike those with fillets, the warpage of the COG is insensitive to those with or without (gold or polymer) bumps. It is also found that the two-dimensional (2-D) FEM in consistence with Suhir's solution can be used for approximately addressing both warpage and bump-joint stress issues. The COG warpage results for thermal loads, from 2-D FEM, indicate that the maximum warpage is very sensitive to the modulus of the ACFs (especially for low modulus) rather than the thermal expansion coefficients of the ACFs, which are consistent with Suhir's solution. The stress results show that the COG with gold bumps provides higher bump-joint stresses (especially for compressive transverse normal stress, which is important for maintaining good electrical contact between bumps and tracks through the conductive particles in the ACF) than that with polymer bumps for the isothermal loaded case. Note that negative isothermal load here applied represents the cooling temperature change from an uniform elevated temperature of the ACF curing in the COG packaging. Furthermore, the transverse normal bump-joint stress increases three fold, when the fillets are present for the COG either with polymer bumps or with gold bumps. The similar mechanism is found in the case of the COGs under thermal-gradient loads. It is also seen that the higher the elastic modulus (E) and thermal expansion coefficient (CTE) of the ACF, the larger the compressive transverse normal bump-joint stress. It is also interestingly found that the volume swelling of the ACF due to moisture absorption provides very relatively large tensile transverse normal bump-joint stresses for the COGs with bumps. These tensile stresses could be one order of magnitude larger than the compressive values resulted from other parameters discussed here. As a result, the approaches to well controlling ACF moisture absorption and selecting low-moisture-expansion ACF materials are very critical to improving the reliability of the COG packages

Measuring and compensating for 5-DOF parasitic motion errors in translation stages using Twyman–Green interferometry

A new metrological method is presented that performs simultaneous real-time measurements and compensates for 5-DOF parasitic motion errors in translation stages for precision profiling of optical surfaces. Two plane mirrors are used to obtain motion-dependent interferometric fringes generated by the optical principles of Twyman–Green interferometry. The fringes generated are monitored using high-speed 2D photodiode arrays and analyzed to determine the five separate motion error components in real time. Simultaneously, null control is performed to suppress the measured motion errors independently using piezoelectric actuators through real-time feedback control while the machine axis is moving. The experimental results demonstrate that 5-DOF parasitic motion errors are effectively measured and compensated to within 10 nm for translational motions and 0.15 arcsec for angular motions.

Development and instrumentation of an integrated three–dimensional optical testing system for application in integrated circuit packaging

Measurement of hygrothermomechanical deformation is critical for the failure analysis and reliability evaluation of integrated circuit (IC) packages. Conventional methods, however, provide one– or two–dimensional measurements, and these measurements are not enough for accurate analysis and evaluation in many cases. In this paper, the development of a compact three-dimensional (3D) optical testing system based on moiré interferometry and Twyman-Green interferometry is described. Furthermore, a novel technique to simultaneously record the fringe patterns of three deformation fields with a single CCD camera is developed based on this 3D testing system. Some well-established measurement methods like thermal-mechanical-analyzer (TMA) and Taylor-Hobson stylus profilometer are used to verify the system performance. The results show good comparisons. In general, hygrothermomechanical deformation of IC packages during their fabrication and service can be measured comprehensively and dynamically using our 3D optical testing system, some drawbacks of the conventional optical methods and/or systems in the application of the IC packaging are overcome.

PMMA melting under proton beam exposure

Experiments on 50μm thin PMMA sheets have been performed using a homogeneous 2MeV proton beam of 5mm diameter. The irradiations have been carried out at the Debrecen nuclear microprobe facility. For the characterization of the samples DIC microscopy and Twyman–Green interferometry have been used. It was found that the samples melted at a beam intensity dependent critical dose. Numerical calculations including heat transport and heat radiation corroborated the experimental results.