Surface Flatness and Bond Thickness Measurement Methods Using the Acoustic Microscope

Janet E. Semmens

Sonoscan, Inc. 2149 E. Pratt Boulevard

Elk Grove Village, IL 60007 USA jsemmens@sonoscan.com

ABSTRACT Acoustic micro imaging has been used in the microelectronics industry for years to evaluate the quality of devices by imaging the internal structures and/or defects in the devices. Lately methods have been developed to investigate other properties of the samples such as the flatness/warpage of devices, substrates, or wafers, and to measure the bondline thickness for die attach or thermal interface materials. AMI (Acoustic Micro Imaging) is a non-destructive test method that utilizes high frequency ultrasound in the range of 5 MHz to 500 MHz [1]. Ultrasound is sensitive to variations in the elastic properties of materials and is particularly sensitive to locating air gaps (delaminations, cracks and voids). AMI typically involves producing images at specific depths in a device based on the location of reflected ultrasound echoes from the internal interfaces. The initial echo information is presented in the A-scan which represents a single point depth profile in the part. The time difference between the echoes corresponds to the depths within the device. The echo of interest is selected using an electronic gate and an x, y plot of the echoes for that selected depth (z) is produced. The shape and amplitude of the echoes at the interface give the condition at the interface. However, the information contained in the A-scan can also be used for other types of analyses. The distance of the surface echo from the sample relative to the transducer can be used to measure the flatness of the sample surface. Internal to the device the distance between echoes can be used for thickness measurements. This paper will discuss the acoustic surface flatness (ASF) and bondline thickness measurement methods as well as present examples of these applications. Key words: Acoustic Micro Imaging (AMI), Warpage Measurement, Bondline Thickness INTRODUCTION Surface flatness, warpage and tilt are an increasing concern in the microelectronics industry for applications such as bonded wafers, printed circuit boards, package on package and more. Irregularities in the surface topography of the boards or devices would cause problems with surface mount assembly and be an important issue for 3D packaging and integration.

The layer thickness of certain materials such as thermal interface materials (TIM) or die attach adhesives is also of interest where the layers are important to ensuring the thermal conductivity performance of the devices. The primary use for Acoustic Micro Imaging has been to investigate the internal structures in materials and components for the presence of defects that can affect the functioning of the devices but information also can be obtained from the acoustic signals to measure the surface flatness or topography of a sample and to measure the thickness of layers. Basic AMI Principles A-Scan In reflection mode Acoustic Micro Imaging the fundamental information is contained in what is called the A-Scan. The A-Scan displays the echo depth information in the sample at each x, y coordinate. Echoes displayed in the A-Scan correspond to different interfaces in the device being examined. There is a time / distance relationship between the echoes related to their depth in the device and the ultrasonic velocity in the materials. In conventional AMI an echo corresponding to a specific depth within the sample is selected using an electronic gate (Figure 1) and the amplitude and phase (polarity) information of the echoes is used to characterize the condition at the interface and is dependent on the acoustic impedance value of the materials involved. The equation that describes the pulse reflection at an interface between materials is as follows: Z2 - Z1 R = I -------- Z2 + Z1

Where R is the amplitude of the reflected pulse, I is the amplitude of the incident pulse, Z1 is the intrinsic acoustic impedance of the material through which the pulse is traveling and Z2 is that of the next material which is encountered by the pulse.

Figure 1: A-scan with gated echo for a reflection mode image.

As mentioned earlier there is a time/distance relationship between the echoes based on the acoustic velocities in the materials that can be used to predict the positions of the interface echoes for the various levels. Velocity = 2 x distance/time For the purposes of surface flatness and thickness measurements this time / distance relationship between the echoes on the A-scan is the significant factor. When measuring surface flatness the electronic gate is used to track the distance variation of the sample surface echo relative to the gate (Figure 2). The height variation is calculated based on the ultrasonic velocity of the signal in the fluid couplant, typically water.

Figure 2: The gate for acoustic surface flatness tracks variations in position of the front surface echo. When measuring the thickness of an internal layer the gate defines the two echoes from the upper and lower boundaries of the interface material to be measured (Figure 3). The upper boundary echo is used as the reference position and the distance that the second echo travels with respect to the reference echo is measured. In this case the distance is calculated based on the acoustic velocity of the material being measured.

Figure 3: The gate tracks the change in distance between two selected echoes.

C-Scan (Interface Scan) The C-scan output is a two-dimensional image consisting of an x-y plot of one specific plane in the z dimension. The intensities in the image are coded to reflect the amplitude and phase of the acoustic signal at the interface. This is the most common imaging method used to evaluate devices for voids and delaminations between layers. The two dimensional plot is also used to display the initial ASF information where the color or grey level in the image relates to the height variations on the sample. The image can be converted to a three-dimensional contour plot to better view the surface topography and direct height measurements can be made from the image. The accuracy of the measurements is down to plus or minus one micron. In the case of the C-Scan for thickness variations in a layer, changes in the intensity levels in the image indicate variations in the thickness of the layer. Direct measurements can be taken at any point(s) on the image. Resolution The concern in AMI with resolution is to be able to achieve the best image definition of the internal features. In general a higher frequency transducer emits sound with a smaller wavelength (shorter pulse duration) and, therefore, affords better resolution. (For a detailed description of resolution using AMI please refer to the reference material [2] [3].) For ASF and TMM the shorter pulse duration is important as it will allow better distinction between the echoes to be measured. This is particularly important for the thickness measurements as the layers to be measured are relatively thin. APPLICATIONS BGA Strip Acoustic Surface Flatness A strip of four BGA parts prior to singulation are shown in the first example of ASF applications. In Figure 4 a conventional 30 MHz reflection mode C-Scan of the surface of the BGA strip is shown. Some small superficial defects in the molding compound are seen in the image. The grey scale variation across the surface of the encapsulation already suggests that the strip of parts is warped.

Figure 4: 30 MHz acoustic image of the surface of a strip of BGAs The 3D contour plot of the surface flatness (Figure 5) clearly illustrates the curvature of the surface. Quantitative measurements of the surface height can be made from the scale shown with the 3D projection. In this example the surface is warped by 1.4 mm from the center to the outer edges of the strip. The warping of the individual BGAs will be less after singulation but some degree of warping was observed on singulated devices as well. This may be a concern for further SMT assembly processes.

Figure 5: Acoustic surface flatness 3D contour plot of the BGA strip Bonded Wafer Acoustic Surface Flatness This example application shows acoustic surface flatness of a bonded wafer. The base wafer is the full original thickness (>700 microns) but the top wafer was back thinned to approximately 100 microns. The two dimensional ASF plot (Figure 6) shows color variation across the wafer image that

indicates a variation of the surface height of the wafer. The 3D projection of the information (Figure 7) further illustrates the non-uniformity of the surface height.

Figure 6: Two dimensional surface flatness image of a bonded wafer.

Figure 7: 3D contour image of the bonded wafer. Die Attach Bondline Thickness Measurement The evaluation of die attach bonding in various microelectronic devices is an established application for AMI. Typically the purpose has been to detect voids and delaminations in the bond that can lead to premature failure of the device. The thickness of the die attach material may also be a concern particularly when heat dissipation is accomplished through the die attach such as in power devices. Figures 8 and 9 show an example of a solder die attach to a metal heat sink. In this case 100 MHz ultrasound was introduced through the back of the heat sink to the solder bond. In the conventional interface scan some voids are seen in the center of the bond (Figure 8). The time difference scan of the bond interface (Figure 9) displays a

variation in colors across the bond in the image that corresponds to thickness variations in the bondline.

Figure 8: 100 MHz acoustic image of solder die attach to heat sink with voids (white spots) in the bond.

Figure 9: Time difference image showing non-uniform thickness of the material as indicted by the color variation in the image. TIM Bondline Thickness Measurement Thermal interface materials (TIM) are used to conduct heat away from the back of a silicon die to a heat sink/lid on the package. Uniform coverage and thickness of the thermal material insures uniform dissipation of the heat which is particularly important in high power devices. Figure 10 shows an acoustic image of the metal heat sink/lid to TIM interface over the back of a flip chip die. The die coverage

appears complete in the image with a thin void (white strip) in the excess material along the upper edge of the die. The chart below the image lists the thickness measurements corresponding to the five points indicated on the image.

Figure 10: The acoustic image shows the coverage of the TIM material over the back of the die. The chart below the image gives the thickness measurements at five selected points on the image. CONCLUSION The main use for AMI is and will continue to be the evaluation of internal structures and interfaces for defects in packages and materials. However additional information is available from the acoustic echoes that can be used to monitor other important properties such as surface warpage and topography or layer thickness in devices. REFERENCES [1] L.W. Kessler, “Acoustic Microscopy” in Metals Handbook, Ninth Edition, Vol. 17 - Nondestructive Evaluation and Quality Control - ASM International, Materials Park, OH, 1989, pp. 465-482. [2] S. Canumalla, “Resolution of Broadband Transducers in Acoustic Microscopy of Encapsulated ICs – Transducer Selection,” IEEE Transactions on Components and Packaging Technology, Vol.22, No. 4, December 1999. [3] J.E. Semmens, “Advancements in High Frequency, High Resolution Acoustic Micro Imaging for Thin Silicon Applications”, Pan Pacific 2011, Hawaii, HI.