High-Frequency Acoustic Microscopy Studies of Buried Interfaces in Silicon

*Shriram Ramanathan Components Research, Intel Corporation

Hillsboro, Oregon 97124

Janet E. Semmens and Lawrence W. Kessler Sonoscan, Inc.

2149 E. Pratt Boulevard, Elk Grove Village, Il 60007 T: 847.437.6400 • Jsemmens@sonoscan.com • Lkessler@sonoscan.com

Abstract

Acoustic microscopes of the pulse-echo type produce resolution within solid materials that is determined by the acoustic frequency employed as well as the lens design parameters of focal length and beam diameter. At very high frequencies (>≈ 200 MHz), however, where the attenuation in the coupling fluid is significant, the prominent frequency of the acoustic pulse is significantly downshifted such that the resolution is reduced accordingly. In addition, spherical aberrations due to velocity mismatch between the different media would be expected to play a significant role in limiting the resolution.

While there have been several studies on surface imaging using acoustic microscopy methods, detailed reports on resolution capabilities while imaging buried interfaces are still lacking. In this paper we report for the first time a systematic study on experimentally determined resolution limits from an assortment of transducers having a nominal frequency of 230 MHz and focal lengths ranging from 0.15 inches to 0.75 inches. A resolution test wafer was designed for these experiments that consisted of a 525 micron thick silicon wafer anodic bonded to a 400 um glass plate. Another test target was designed that used a 750 micron thick silicon wafer direct bonded to another similar thickness wafer. In each case at the interface between the two materials is a series of varied spaced etched lines and spaces whose visibility to the acoustic beam became the indicator of resolution achieved.

The lateral resolution in silicon appears to be nearly the half wavelength limit as predicted by diffraction theory. While imaging through the glass layer, however, it appears that even better resolution is being achieved. In this paper we also report observations on silicon thickness effects on the observed resolution.

Introduction High frequency ultrasound has been in use for inspecting

microelectronic components since the late 1980s and has now become the standard method for finding package defects such as delaminations, cracks and voids. In most cases these defects are not easily seen, if at all, with other imaging modalities such as optical, x-ray and infrared. Ultrasound is uniquely sensitive to air gaps between layers making it very useful as a diagnostic tool. However, the lack of transmission of ultrasound through air necessitates a fluid couplant (typically water) to be used to deliver the acoustic energy to the sample.

Today’s typical acoustic microscope is employed to visualize internal structural features in the 20 to 200 micron

size range and frequencies typically used range from 15 to 230 MHz depending on the penetration depth needed. Higher frequency operation, e.g., 300 MHz and beyond, is less common but will likely become essential for inspecting future semiconductor devices and advanced packaging architectures.

High frequency operation has been shown to be practical but there are trade-offs that must be made. Based on the seminal work of C. F. Quate [1, 2] at Stanford University, ultrasonic microscopes were commercially available operating in the GHz frequency range from Olympus Corporation (Japan) in the 1980’s. However, these instruments had very short focus and penetration depth. Therefore, applications were restricted to visualizing surface and near-surface (within a few microns) structures in solids. Another type of instrument, the Scanning Laser Acoustic Microscope (SLAM), developed by Korpel and Kessler in the 1970s operated up to 500 MHz in a shadowgraph mode and, therefore, was not suitable for obtaining focused detail deep within a sample [3].

Resolution and Focus The fundamental limit to resolution in an imaging system

is governed by diffraction and is roughly ½ the wavelength of the inspection energy within the material being inspected. This principle holds true for optical, infrared, x-ray, ultrasound, etc. Therefore, in an acoustic microscope higher frequency insonification is needed to increase resolution.

In the pulse-echo acoustic microscopy technique in which a single spherical lens is used to focus the ultrasound, the numerical aperture limits the resolution attainable, similar to that of an optical microscope. Simplifying the concept of numerical aperture, we usually describe the lens by its F#, the ratio of the focal length (f) to acoustic beam diameter (d). The F# is directly proportional to the focal spot size of the ultrasound beam within the sample. A small F# is desired which translates to a high numerical aperture and high resolution. Since the F# is a ratio, any combination of focal length and beam diameter having the same ratio could be equivalent with respect to resolution; however, there is a complicating factor of acoustic absorption in the coupling medium. Acoustic absorption increases with the square of the frequency resulting in limited coupling of the ultrasound pulse spectrum being delivered to the sample. This limits both the absolute signal level and the resolution at high frequency operation [4]. This is further illustrated graphically below. In addition to resolution limitations due to absorption in the coupling fluid (pulse-echo systems only) there is an additional problem of spherical aberrations due to acoustic

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velocity mismatch between the coupling medium and the sample.

Acoustic Attenuation in the Coupling Fluid At high frequencies a primary concern is the efficient

delivery of the ultrasound to the sample. A variety of coupling fluids can be employed; however, water seems to be the most common from the standpoint of having low acoustic losses relative to other liquids. Acoustic attenuation in water is a function of temperature wherein higher temperatures cause less signal loss. For example, by increasing the temperature from 25° C to about 50° C, the absorption drops by a factor of two [5]. At 100 MHz and room temperature, for example, there is a loss of about 3db signal level for every millimeter of travel between the lens and sample surface. Attenuation losses in silicon are relatively small compared to that in water.

Acoustic absorption in water is also frequency dependent, being more severe at higher frequencies than at low and governed by a square law dependence. For pulse-echo methods the frequency dependent absorption causes resolution degradation because the higher frequency components of the pulse are attenuated more than the low frequency components of the pulse thereby decreasing the average effective pulse frequency used in imaging. The effect is termed “frequency downshifting” and is shown graphically below in Figure 5 [6]. The parameters of concern are the transducer working distance (space between the transducer and the sample when the transducer is at focus within the sample), the attenuation of the coupling medium and the bandwidth (frequency spectrum) of the ultrasound pulse.

In Figure 1 the frequency of a received pulse is shown relative to the initially generated pulse for a transducer having 50% bandwidth, typical of common transducers. In this figure it is assumed that there is a 3mm water coupling path at 30° C. Clearly, at low frequencies (below 75 MHz) the received pulse has similar frequency content as the initial pulse and very little resolution degradation would be observed. However, there is severe degradation at high frequency. The return frequency is limited to about 150 MHz even as the incoming frequency is about 250 MHz due to high frequency attenuation.

In Figure 2 the effect of minimizing the water path is shown. Here a similar analysis is performed assuming a 0.1mm working distance and there is much less degradation of the pulse frequency content. It is seen that the return echo would have almost exactly the same frequency content as the incoming acoustic frequency and it is possible to obtain higher resolution images due to this effect.

Resolution Test Wafer Results The test target used for acoustic calibration purposes consists of a glass plate that is anodically bonded to a silicon wafer and has various size air gap features etched into the silicon at the glass interface [5]. This simulates some of the conditions in which high frequency acoustic micro imaging systems are used in semiconductor applications. The glass wafer side allows for direct visual viewing of the test structures and comparison with the acoustic images. Acoustic images can be produced from either side of the test sample, i.e., through the

glass or through the silicon. Figure 3 shows a schematic of the test wafer and the various etched patterns are marked in the figure.

Figure 1: Return echo frequency of ultrasound as a function of initial acoustic frequency. The water path between the lens and sample is 3mm.

Figure 2: Return frequency of ultrasound as a function of initial acoustic frequency. The water path between the lens and sample is 0.1mm.

Figure 3: Schematic of cross-section of test target consisting of silicon wafer anodically bonded to glass wafer.

Glass wafer (B ili l )

Silicon wafer525

400

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Figure 4: Acoustic image of the bonded interface across the entire test wafer through the glass side.

Figure 4 shows an acoustic image of the entire sample through the glass side and focused at the interface for an overview of the feature layout. Figures 5 and 6 display enlargement of details of the resolution lines. Notice that the 3 um resolution lines are visible using a 230 MHz transducer whose equivalent downshifted frequency is 150 MHz. The acoustic data were obtained on a Sonoscan D9000 C-SAM® acoustic microscope. This result goes beyond the resolution that would be expected at this frequency through glass.

Figure 5: Close-up view of etched line groups with spacing ranging from 25 to 100 microns. The acoustic image is taken through the glass side.

Figure 6: Close-up enlarged view of etched lines spaced 7 and 10 um apart. Acoustic image is taken through the glass side.

When imaging through the silicon wafer the resolution pattern is mirror reversed and typical image data indicate about 20 micrometers to be the minimum level of detail that can be resolved with a 300 MHz transducer. This is shown in Figure 7 which shows etched lines spaced 20 um apart to be barely visible.

Figure 7: Close-up view of etched line groups having spacing from 17 to 25 microns. Acoustic image is taken through the silicon side.

Figure 8: Optical image of 20 micron square copper bond pads separated by lines that are 8 microns wide. The smaller squares are 15 microns separated by 6 micron lines.

Figure 9: Acoustic image of the test pattern as seen by focusing the ultrasound directly upon the pre - bonded surface.

The second resolution test sample consisted of 2 silicon wafers (each 750 microns thick) bonded together. Copper bond pads on the wafers were aligned and bonded using thermo-compression bonding. The bond pad geometry

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consists of square copper pads of various sizes that are separated by lines of known width. An optical image of the structure prior to being bonded is shown below in Figure 8. Prior to bonding the wafers one side was imaged acoustically. The acoustic image of this sample, shown in Figure 9, clearly demonstrates acoustic resolution that is sufficient to resolve 6 micron features.

Figure 10: Acoustic image of the test pattern as seen through 20 microns of silicon and after being bonded to the substrate wafer.

Discussion From diffraction theory the lateral resolution in a pulse-

echo imaging system is determined by ½ the Airy disc diameter (measured to the first null) multiplied by a factor 0.707 due to double focusing by the acoustic lens on transmit and on receive actions. In the limit, the smallest Airy disc that can be produced by a lens focusing system is ½ the wavelength in the material. With these factors and the velocity of sound in silicon being about 8480 m/s, the anticipated maximum resolution in a pulse-echo acoustic microscope would be about 23 microns at 150 MHz. We observed resolution of 25 microns as determined by the resolution test wafer described in Figure 4. The table below summarizes resolution obtained with a variety of transducers having nominal frequencies of 230 MHz and 300 MHz. The focal length of each transducer is indicated in parentheses. The material through which the ultrasound pulse traveled in getting to the interface was either glass or silicon with thickness indicated. The frequency downshifting of the ultrasonic pulse, due to the working space between the transducer and the top surface of the test wafer, caused the effective frequency (F max) of the return pulse to be less than the nominal frequency of the initially generated acoustic pulse.

Conclusions A detailed investigation of C-SAM resolution while

imaging through buried interfaces has been conducted using suitable test wafers. It has been found that the lateral resolution generally agrees with that predicted by diffraction theory for pulsed-echo systems. No investigation of axial resolution has been conducted in this work, however, spherical aberrations do not appear to play a significant role

in degrading the resolving power of the acoustic microscope under conditions employed in this study.

TRANSDUCER

MATERIAL

RESOLUTION

F Max

300 (6.25 mm)

525um Si

20 microns

175

300 (6.25 mm)

400um Glass

20 microns

175

230 (3.0 mm)

400um Glass

5 + microns

175

230 (8.0mm)

525 um Si

25 microns

150

230 (12.5mm)

525 um Si

35 microns

75

230 (3.0mm)

20 um Si

5

175

References 1. Kessler, L.W., and Semmens, J.E., “Acoustic Micro

Imaging Failure Analysis of Electronic Devices”, Chapter 7, Electronic Failure Analysis Handbook, Perry Martin, editor, McGraw Hill (1999).

2. Semmens, J.E., and Kessler, L.W., “Application of Acoustic Micro Imaging for the Evaluation of Advanced Microelectronic Packaging: An Overview of Experiences From 1973-1996”, Proceedings of the Second International Symposium on Electronic Packaging Technology, Shanghai, China, co-sponsored by IEEE CMPT, December 1996.

3. Lemons, R.A., and Quate, C.F., “Acoustic Microsope – Scanning Version”, Applied Physics Letters, Vol. 24 (4), 1974, pp. 163-165.

4. Korpel, A., Kessler, L.W., and Palermo, P.R., “Acoustic Microscope Operating at 100 MHz”, Nature, Vol. 232 (5306), 1970, pp. 110-111.

5. Canumalla, Sridhar, “Resolution of Broadband Transducers in Acoustic Microscopy of Encapsulated ICs: Transducer Selection”, IEEE Transactions Components and Packaging Technology, Vol. 22, No. 4, December 1999, pp. 582-592.

6. Pinkerton, J.M.M., “The Absorption of Ultrasonic Waves in Liquids”, Proceedings Physical Society of London, B62, 1949, pp. 129-141.

7. Nondestructive Testing Handbook, Vol. 7: Ultrasonic Testing”, edited by Paul McIntire, published by American Society of Nondestructive Testing, 1991, p. 225.

*Present address: Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138. E- mail: shriram@deas.harvard.edu.

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