Conventional Scanning Acoustic Microscope Research Articles

High-Order Pulse-Echo Ultrasound

Multiple reflections between transducer and imaged object can naturally occur in ultrasound imaging and other acoustic sensing applications such as sonar. The repeated interaction of the emitted wave front with the imaged object is traditionally regarded as an undesired reverberation artifact, often misinterpreted as fictitious acoustic boundaries. We introduce high-order reflection pulse-echo (HOPE) ultrasound, a method that leverages high-order reflections to improve on several aspects of conventional ultrasound imaging. HOPE is experimentally demonstrated to resolve submicrometer features by breaking through the sampling limit. The major contrast enhancement of the high reflection orders allowed defects within materials invisible to conventional scanning acoustic microscopy to be revealed. The technique is further shown to improve accuracy of frequency-dependent ultrasound attenuation measurements from biological tissues. HOPE ultrasound requires no additional hardware and is easy to implement, underscoring its potential to boost imaging performance in biomedical imaging, nondestructive testing, and other acoustic sensing applications.

High sensitivity detection of ultrasonic signal for nondestructive inspection using pulse compression method

Semiconductor parts are currently being stacked and miniaturized, and as device structures become more complicated, nondestructive inspections are increasingly needed for improving yield and securing reliability. Scanning acoustic microscopy (SAM) is used to detect delamination and voids. However, when the frequency of the ultrasonic probe is increased to improve the spatial resolution, conventional SAM cannot detect defects in deep parts because ultrasonic attenuation increases. In this research, we developed high sensitivity detection technique of ultrasonic signal using a pulse compression method, examined a modulation method, and designed the waveform of the transmitted signal to achieve high defect detection sensitivity. Conventional and developed detection techniques of ultrasonic signal were applied to a 3-layer stacked specimen, and their ultrasonic images, noise reduction effect, and defect detection sensitivity were compared.

Frequency domain scanning acoustic microscopy for power electronics: Physics-based feature identification and selectivity

In the evaluation of power electronic components non-destructive test methods (NDT) such as scanning acoustic microscopy (SAM) are valuable tools in packaging and failure analysis. As power modules become more compact and power devices thinner, echo overlap and interference can hamper the imaging capability and analysis of the conventional SAM time domain-based imaging techniques. Frequency domain analysis offers increased resolution and contrast but the interpretation and feature identification of waveforms is less obvious and interpretation guidelines have received little attention. In this paper a physics-based analysis of the frequency domain response is presented in a power module case study. The analysis is used to demonstrate physics-based feature selective contrast in such systems and offers guidelines for feature prediction. The approach is verified on full scan datasets from acoustic scans of a hybrid multilayer stack. Distinct features in the echo frequency domain are identified as resonances associated with multiple internal reflections in the layer structure. Where such features may hamper analysis in the conventional approach these can, if properly assigned, be exploited to yield selective and improved contrasts and allow accurate structural and material analysis.

Scanning acoustic GHz-microscopy versus conventional SAM for advanced assessment of ball bond and metal interfaces in microelectronic devices

The current paper describes the application of acoustic GHz-microscopy in comparison to conventional scanning acoustic microscopy for the investigation of ball bond and metal interfaces of microelectronic devices. The non-destructive ultrasonic inspection method is based on a back-side approach with the ultrasonic pulses applied through the back side of the exposed and thinned Si chip. A direct comparison between the innovative ultra-high frequency setup using special highly focusing acoustic lenses enabling the application of up to 1GHz acoustic frequency and a standard SAM setup based on a conventional 300MHz transducer is presented for several samples. It is demonstrated that the lateral resolution can be drastically increased from 10–15μm for the conventional setup to about 1μm for the ultra-high frequency setup which thereby allows for new applications such as a reliable 2-dimensional quality assessment even of small ball bond interfaces with dimensions of a few 10μm.

Scanning Acoustic Microscopy Beyond Conventional Applications

The ultimate aim in the development of concepts, methods and architectures in microelectronic technologies is the continuous reduction of lateral dimensions by increasing integration density, functionality and processing performance of electronic and microelectronic components. Recent trends therefore employ all three spatial dimensions for further increasing integration density and functionality. Novel challenges and additional requirements on the physical and electrical behavior arising from these integration concepts result in the need for specifically adapted compositions and materials. Also existing testing methods are likely compromised due to challenges of either their resolution capabilities or the underlying contrast mechanisms. As novel architectural concepts require the development of appropriate processing technologies the robustness, reliability and performance of the individual processing steps are of superior importance with respect to their global applicability and thus require comprehensive evaluation prior to introduction into industrial manufacturing. In process development, quality assessment, failure analysis and others inspection methods operating non-destructively are of major importance. The application of multiple tests employing a variety of contrast mechanisms is extremely valuable for assessment, defect localization and evaluation. Scanning acoustic microscopy (SAM) employ elastic waves that interact with a specimens mechanical properties. With the elastic waves capability of penetrating optically opaque materials acoustic microscopy enables the assessment of internal structures and features and allows the depth specific assignment. The contrast mechanism is based on the wave interaction with gradients in either elastic properties or mass density and can be evaluated both quantitatively and qualitatively. The properties of the elastic waves employed by the acoustic microscope can be adjusted and specifically adapted to the requirements of the inspection task. This provides a high degree of freedom for acoustically based analyses and makes SAM a valuable and powerful tool for non-destructive evaluation and assessment. Limitations and challenges of current conventional acoustic microscopy for the inspection of novel 3D-integration concepts mainly have their origin in the lateral resolution required for imaging feature sizes in the low micrometer range. However, in some cases phenomena are required that cannot be exploited easily and may require an adapted set-up for acoustic insonation. In a broad range of applications additional and specifically adapted signal analysis can be of high values for the detection and identification of defects and deviations. The current paper focusses on scanning acoustic microscopy and its applications and challenges as non- and semi-destructive testing method for 3D integration but also for microelectronic and wafer bonding technologies. The development and modification of an acoustic GHz-microscope and its application for inspection of 3D-relevant technologies is described. With lateral imaging resolutions of 1 µm and below TSVs of 10 µm diameter have been inspected for filling-voids and rim-delaminations. Due to the large numerical apertures employed in the GHz-SAM wave modes like Rayleigh- or shear modes can be excited in addition to the pure compressional wave component potentially providing access to new contrast phenomena. For assisting result interpretation however, modelling and numerical simulation of the complex wave propagation became necessary and will be described. The high sensitivity to surface and near-surface structures is illustrated by the detection and localization of sub-surface µ-cracks describing the value of acoustic GHz-microscopy for non-destructive crack detection in near-surface regions of highly integrated microelectronic components. In additionally to GHz-SAM conventional scanning acoustic microscopy supplemented by signal analysis approaches operating in time- and spectral domain and their value for defect localization and evaluation will be presented.

Scanning acoustic gigahertz microscopy for metrology applications in three-dimensional integration technologies

Current trends in microelectronics focus on three-dimensionally integrating different components to allow for increasing density and functionality of integrated systems. Concepts pursued involve vertical stacking and interconnecting technologies that employ micro bumping, wafer bonding, and through silicon vias (TSVs). Both the increasing complexity and the miniaturization of key elements in three-dimensional (3-D) components lead to new requirements on inspection and metrology tools and techniques as well as for failure analysis methodologies. For metrology and quality assessment in particular, methods operating nondestructively are of major importance. Scanning acoustic microscopy has the ability of illuminating optically opaque materials and, thus, allowing the assessment and imaging of internal structures. Conventional scanning acoustic microscopy (SAM) equipment can be applied to analyze the quality of wafer-bonded interfaces in 3-D integration but may reach its limitations when structures shrink in size and gain complexity. A new concept of acoustic inspection in the gigahertz (GHz) frequency band is explored for its applicability to 3-D integration technologies. Extending the acoustic inspection frequency allows for lateral resolutions in the 1-μm range and also enables the inspection of microbumps and TSVs in addition to wafer bonded interfaces, which exceed the applicability of conventional SAM. Three case studies are presented here ranging from conventional SAM on a full wafer scale to acoustic GHz microscopy on thin films and TSVs.

Elastic properties determination of CuInSe2 polycrystalline thin films via a dynamic method

Developing and using a simulation program based on the spectrum angular model, we first determine reflectance functions and acoustic signatures for bulk as well as for different thickness of CuInSe2 films. For bulk material, it is found that the longitudinal and Rayleigh modes are excited at incidence angles of 23.4° and 47°, respectively. This result reveals the great difficulties to characterize CuInSe2 with a conventional scanning acoustic microscope that uses a lens half- opening angle of 50° and water as a coupling liquid. Hence, Freon is used as alternative coupling liquid. Consequently, the effect of thickness on reflection coefficient and acoustic signature variations are quantified for both bulk and thin material. It is shown that as the thickness increases: (i) the critical angle of mode excitation increases, (ii) the periods of acoustic signature curves decrease and (iii) the Rayleigh velocity, VR, mode shifts towards lower values. Hence, a velocity dispersion curve is established in terms of VR as a function of film thickness; it decreases initially from the velocity value of the glass substrate then saturates when it reaches that of CuInSe2. The importance of such curve lies in the possibility of velocity determination by just knowing the thickness, and vice versa. Moreover, elastic constants are straight forward deduced from such a velocity.

Ultrasonic Tissue Characterization of Atherosclerosis by a Speed-of-Sound Microscanning System

We have been developing a scanning acoustic microscope (SAM) system for medicine and biology featuring quantitative measurement of ultrasonic parameters of soft tissues. In the present study, we propose a new concept sound speed microscopy that can measure the thickness and speed of sound in the tissue using fast Fourier transform of a single pulsed wave instead of burst waves used in conventional SAM systems. Two coronary arteries were frozen and sectioned approximately 10 microm in thickness. They were mounted on glass slides without cover slips. The scanning time of a frame with 300 x 300 pixels was 90 s and two-dimensional distribution of speed of sound was obtained. The speed of sound was 1680 +/- 30 m/s in the thickened intima with collagen fiber, 1520 +/- 8 m/s in the lipid deposition underlying the fibrous cap, and 1810 +/- 25 m/s in a calcified lesion in the intima. These basic measurements will help in the understanding of echo intensity and pattern in intravascular ultrasound images.

Microelectronic package characterisation using scanning acoustic microscopy

In a highly competitive market, reliable techniques for manufacturing quality control of electronic devices are demanded. Characterisation of modern microelectronic package integrity becomes more difficult due to the continued miniaturisation of electronic device and the complexity of advanced micro-assembling technologies such as chip-scale packages and 3D IC stacks. In this paper, sparse representations of acoustic signals are sought to improve the scanning acoustic microscopy (SAM), a common non-destructive tool for failure analysis of microelectronic packages. Sparse representation of an ultrasonic signal is obtained by decomposing it in an overcomplete dictionary. Detection and location of ultrasonic echoes are then performed on the basis of the resulting redundant representation. The method offers a solution to the deconvolution problem for restoration of the ultrasonic reflectivity function. It can restore closely space overlapping echoes beyond the resolution of the conventional SAM system. It also produces high resolution and accurate estimates for ultrasonic echo parameters, i.e., time-of-flight, amplitude, centre frequency, and bandwidth. These merits of the proposed method are explored in various potential applications for microelectronic package characterisation.

Twenty years’ development of acoustic microscopy for medicine and biology

Since 1985, scanning acoustic microscope (SAM) systems for medicine and biology has been developed and have been equipped for investigation of acoustic properties of various organs at Tohoku University. There are three objectives of SAM for biomedicine. First, it is useful for intraoperative pathological examination because staining is not required. Second, it helps understanding lower frequency ultrasonic images such as echography or intravascular ultrasound because SAM provides microscopic acoustic properties. Third, it provides information on biomechanics of tissues and cells. The most important feature of our SAM is to measure attenuation and sound speed of soft tissues by analyzing the frequency-dependent characteristics of amplitude and phase. Recently, a new-concept acoustic microscopy using a single pulsed wave instead of burst waves used in conventional SAM systems was proposed. Application of SAM has also been broadened and it covers not only myocardium and atherosclerosis in cardiology but also skin, bone, cartilage, tendon, and muscle in dermatology and orthopedic surgery. Higher resolution acoustic microscopy with over 500 MHz enabled us to visualize cultured cells and it can be applied for quality assessment of regenerated skins and cartilage. Impedance acoustic microscopy and 3-D acoustic microscopy are developed for in vivo tissue imaging.

Ultrasonic speed microscopy for imaging of coronary artery

We have been developing a scanning acoustic microscope (SAM) system for medicine and biology featuring quantitative measurement of ultrasonic speed and attenuation of soft tissues. In the present study, we will propose a new concept ultrasonic speed microscopy that can measure the thickness and ultrasonic speed using fast Fourier transform of a single pulsed wave instead of continuous waves used in conventional SAM systems. Six coronary arteries were frozen and sectioned approximately 10 μm in thickness. They were mounted on glass slides without cover slips. The scanning time of a frame with 300 × 300 pixels was 121 s and two-dimensional distribution of ultrasonic speed was obtained. The ultrasonic speed was 1720 m/s in the thickened intima with collagen fiber, 1520 m/s in lipid deposition underlying fibrous cap and 1830 m/s in calcified lesion in the intima. These basic measurements will help understanding echogenecity in intravascular ultrasound (IVUS) images. Imaging of coronary artery with the ultrasonic speed microscopy provides important information for study of IVUS coronary imaging.

High-resolution picosecond acoustic microscopy for non-invasive characterization of buried interfaces

Non-destructive investigation of buried interfaces at high-resolution is critical for integrated circuit and advanced packaging research and development. In this letter, we present a novel non-contact microscopy technique using ultrahigh frequency (GHz range) longitudinal acoustic pulses to form images of interfaces and layers buried deep inside a silicon device. This method overcomes fundamental limitations of conventional scanning acoustic microscopy by directly generating and detecting the acoustic waves on the surface of the sample using an ultrafast pump-probe optical technique. We demonstrate our method by imaging copper lines buried beneath a 6-μm silicon wafer; the lateral spatial resolution of 3 μm is limited by the laser spot size. In addition to the high lateral spatial resolution, the technique has picosecond (ps) time resolution and therefore will enable imaging individual interconnect layers in multi-layer stacked devices.

Nonlinear Ultrasonic Method to Detect Micro-Delamination in Electronic Packaging

The linear ultrasonic technique has been extensively used as a powerful, non-destructive test tool for reliability testing and failure analysis of electronic packaging. This is used most often in the inspection of defects such as delaminations, voids, or cracks through use of a SAM (Scanning Acoustic Microscope). Then, as the reliability level that is required of electronic packaging becomes higher and the thickness of package becomes thinner, the possible defect which needs to be detected becomes smaller. In the conventional SAM, however it is very difficult to detect small defects less than m µ 1 . 0 , such as micro-delaminations. In order to solve such a problem, this paper proposes a nonlinear ultrasonic method, where the nonlinearity caused by the effect of crack-face interactions is considered. The basic concept of this method involves harmonic frequencies that are generated in the transmitted ultrasonic wave due to the partial contact at the interface of micro-delamination. As an evaluation index, the nonlinear parameter dependent on the amplitude of the second order harmonic frequency component is obtained by spectral analysis of the transmitted signal. Experimental results show that the nonlinear parameter has good correlation with the micro-gap and the proposed method can detect the micro-delamination even less than nm 1 .

Experimental Investigation on the Interface Properties Evaluation in Piezoelectric Layered Structures by Love Waves Propagation

The linear ultrasonic technique has been extensively used as a powerful, non-destructive test tool for reliability testing and failure analysis of electronic packaging. This is used most often in the inspection of defects such as delaminations, voids, or cracks through use of a SAM (Scanning Acoustic Microscope). Then, as the reliability level that is required of electronic packaging becomes higher and the thickness of package becomes thinner, the possible defect which needs to be detected becomes smaller. In the conventional SAM, however it is very difficult to detect small defects less than m µ 1 . 0 , such as micro-delaminations. In order to solve such a problem, this paper proposes a nonlinear ultrasonic method, where the nonlinearity caused by the effect of crack-face interactions is considered. The basic concept of this method involves harmonic frequencies that are generated in the transmitted ultrasonic wave due to the partial contact at the interface of micro-delamination. As an evaluation index, the nonlinear parameter dependent on the amplitude of the second order harmonic frequency component is obtained by spectral analysis of the transmitted signal. Experimental results show that the nonlinear parameter has good correlation with the micro-gap and the proposed method can detect the micro-delamination even less than nm 1 .

Quantitative scanning acoustic microscopy compared to microradiography for assessment of new bone formation

Recently, it has been shown that quantitative scanning acoustic microscopy (SAM) is a powerful tool to image the acoustic impedance of even inhomogeneous materials like bone. Therefore, the aim of our study was to compare SAM to conventional microradiography with respect to histomorphometrical assessment of undecalcified sections of newly formed bone. Forty specimens were harvested 12 weeks after implantation of either autogenous cancellous bone graft or 5.0 mg of Osteogenic Protein-1 (BMP-7) in a critical-sized defect model in sheep. Undecalcified transverse bone sections of 500 μm thickness were investigated with conventional microradiography and SAM. Linear regression analysis was carried out to compare the measurements of the area of new bone formation within the defect sites. Both methods allowed for good discrimination between newly formed bone and cortical bone at the edges of the former defect. Images obtained with SAM revealed a better resolution and sharpness compared to that of microradiographs since SAM imaging unlike microradiography does not depend on the thickness of bone sections. The results of quantitative histomorphometric analysis obtained by both methods showed no significant differences, and it was possible to predict 90% of the variability of each method (coefficient of determination r 2 = 0.90; P < 0.0001). In conclusion, SAM offers comparable quantitative histomorphometric information with a better spatial resolution than conventional microradiography. Thus, SAM is a promising new micro-visualizing technique for basic bone research.

Sound speed scanning acoustic microscopy for biomedical applications

Since 1985, we have been developing a scanning acoustic microscope (SAM) system for biomedical use and have been investigating the acoustic properties of various organs and disease states by using this SAM system. In biomedicine, SAM is useful for intraoperative pathological examination, study of low-frequency ultrasonic images, and assessment of biomechanics at a microscopic level. Recently, we have proposed a new concept -- acoustic microscopy -- using a single pulsed wave instead of continuous waves used in conventional SAM systems. In the present study, we compared two systems by measuring the same biological material. The sound speed image obtained by sound speed microscopy corresponded well to that obtained using a conventional SAM system. Lesions with hyaline degeneration showed a lower sound speed when compared with that of normal myocardium. Frequency domain analysis of amplitude and phase by both methods also showed similar characteristics. Although the data acquisition time of one frame was greater than that in conventional SAM, the total time required for calculation was significantly shorter. The SAM system can be applied to intraoperative pathological examination.

High-Precision Measurement of Surface Acoustic Wave Velocity by SAM.

An attempt has been made to measure precisely the leaky surface wave velocity, VR, within a very localized region using a scanning acoustic microscope (SAM). To achieve the relative precision of VR, e.g., 10-4, required for acoustoelastic stress measurement, two major modification of the conventional SAM were made. The defocus distance was measured with a high-resolution displacement sensor. For each measurement, the distance as well as the output of V(z) were averaged many times. The very low as well as very high frequency components in V(z) curves were eliminated by the digital filtering technique, and temperature of water was controlled within 0.02 K. Thus the relative standard deviation of VR of less than 10-4 was achieved for fused silica.

A Lamb wave lens for acoustic microscopy

In a conventional scanning acoustic microscope the excited leaky modes contribute significantly to the high contrast obtained in the images. However, all such modes exist simultaneously, and the interpretation of the images is not straightforward, especially in layered media. A lens geometry is proposed that can be used with acoustic microscopes to image layered solid structures. This lens can focus the acoustic waves in only one of the Lamb wave modes of the layered solid with a high efficiency. V(Z) curves obtained from this lens are more sensitive to material properties compared to that obtained from conventional lens. Measuring the return signal as a function of frequency results in another characteristic curve, V(f). The Lamb wave lens and the associated characterization methods for the layered structures are described. The results presented show that the Lamb wave lens is at least an order of magnitude more sensitive than the conventional lens and can easily differentiate between a good bond and disbond in a layered structure.

Improvement of anisotropy sensitivity in the scanning acoustic microscope

The response of the conventional scanning acoustic microscope (SAM) to anisotropic materials is theoretically investigated. For this purpose, the reflection coefficient of plane acoustic waves incident on a liquid-solid interface is numerically calculated for a general anisotropic solid oriented in any arbitrary direction. In general, the reflection coefficient depends on polar and azimuthal angles of incidence. For the case of a circularly symmetric acoustic microscope lens, a mean reflectance function can be defined that depends only on the polar angle. With this mean reflectance function, it is very easy to predict the anisotropic material response of the acoustic microscope. It is found that, under certain conditions, the amplitude response of the acoustic microscope can depend heavily on the orientation of the solid material under investigation. The amplitude of the acoustic microscope signal is influenced by the orientation of the material because there is a cancellation of acoustic rays reflected from the object surface at different azimuthal angles. This cancellation is revealed as a minimum in the mean reflectance function. It is shown by numerical simulation that the sensitivity to orientation can be increased by the use of a ring-shaped insonification at the back of the acoustic lens.

Akustische Rastermikroskopie als Methode zur Oberflächenuntersuchung

Techniques of scanning acoustic microscopy generally rely on local variations of such solid state parameters influencing generation or propagation of acoustic waves. Depending on the manner of impressing acoustic waves into the sample various methods are distinguished. In conventional scanning acoustic microscopy ultrasound is generated by a lens-transducer arrangement outside the sample and focussed onto or below its surface. Changes in the propagation of this ultrasound wave, like absorption and reflexion or temporal propagation delays, enable analysis of the mechanical or elastic response. At very high frequencies and with additional time-resolving detection techniques applications of this technique to surface analysis become possible. Other scanning acoustic microscopes imply the generation of sound or ultrasound directly within the sample itself due to the impact of temporarily modulated particle or photon beams. These are presently laser, electron, or ion beams. With these methods the acoustic signal as detected by a transducer attached to the sample is on principle affected by propagation properties, too, but it is dominated by local changes of the generation process for the acoustic wave, mainly because the frequency ranges used presently are associated with very long acoustic wavelengths. Depending on the physical nature of the primary probe used many sound generation mechanisms are given resulting in a large amount of different applications. By adjusting the probe parameters in a suitable manner the sound generation process can be confined to the direct vicinity of the specimen surface, which makes this technique feasible for surface characterization. The principles of the various techniques are described, and their usability for surface analysis is discussed.