Hiding In Plain Sight. How Ultrasonics Can Help You Find the Smallest Bonded Wafer and Device Defects. A Sonix White Paper

Transcription

1 Hiding In Plain Sight How Ultrasonics Can Help You Find the Smallest Bonded Wafer and Device Defects A Sonix White Paper

2 If You Can See It, You Can Solve It: Understanding Ultrasonic Inspection of Bonded Wafers The need for smaller and smarter devices means that bond rings, through-silicon vias (TSVs) and other structures are becoming smaller and more complex. To verify the integrity of these structures and to provide diagnostic information for optimizing the bonding process bonded wafer manufacturers require efficient, high-resolution, non-destructive inspection platforms properly tuned for their bonding techniques and production methods. Sonix has worked with leading manufacturers around the world to optimize inspection recipes and techniques for all types of bonded wafers. This white paper offers our insights into the advantages of ultrasonic inspection, along with best practices for obtaining the clearest possible inspection results. Nondestructive Inspection Technologies The major inspection methods used today include: Optical inspection Optical systems are widely used to verify proper structure, alignment and absence of contaminants before bonding. However, because they can only reveal surface detail, they are of limited use in post-bonding inspection. Infrared Inspection (IR) Although IR can penetrate the wafer surface to reveal structures such as metal patterns, bond voids can only be detected by imaging IR diffraction at the void boundary. This prevents reliable detection of voids at scales below about 0.5mm (500μm). X-ray inspection X-ray systems can penetrate through bonded samples to clearly show metallic features, but they can t provide usable images of voids and delaminations in bond rings and TSVs. Wafer manufacturers may also wish to avoid these systems due to the potential health hazards of X-ray exposure. Ultrasonic inspection Ultrasonic sound frequencies have the ability to penetrate the bonded wafer structure, but will not travel through the entrapped air, gas or vacuum of a void. This makes scanning acoustic microscopy the preferred method for clearly imaging bond voids, cracks and delaminations at any wafer layer or depth. 2 Hiding In Plain Sight: How Ultrasonics Can Help You Find the Smallest Bonded Wafer and Device Defects A Sonix White Paper

3 How Ultrasonic Inspection Works At the heart of any scanning acoustic microscope is a transducer that generates high-frequency sound waves. Transducers used for wafer inspection range from 50 MHz to ultra-high frequencies (UHF) of 200 MHz or more. Lower frequencies offer deeper penetration and greater depth of field, while higher frequencies offer finer spatial resolution of microscopic details. At the boundaries between different materials, changes in acoustic impedance cause a reflection of ultrasonic energy. A digitized visualization of structures within the wafer can be created by measuring the amplitude and time-of-flight of these reflections. Because virtually all energy is reflected at the boundary of a void, crack or delamination, these defects can easily be seen as bright spots in the image. Because ultrasound waves cannot cross an air gap, water is used as a coupling medium between the transducer and inspection target. Ideally, the inspection system is designed to minimize the amount of water required to ensure a distortion-free image. Choosing the Right Frequency, Focal Length and Scan Settings You can t eliminate defects until you can see them clearly. Optimum ultrasonic frequency and focusing combined with advanced tools for defining regions of interest and interpreting results can provide the information you need not only to reject failures, but also to optimize bonding processes to increase yield and quality. Based on our experience creating scanning recipes for all types of wafer bonds, we have found the following guidelines to be effective. We re always happy to consult with you to help determine the best approach for your particular needs. 3 Hiding In Plain Sight: How Ultrasonics Can Help You Find the Smallest Bonded Wafer and Device Defects A Sonix White Paper

4 Silicon-on-Insulator (SOI) Bonding Because silicon and silicon dioxide have similar impedance to ultrasound, ultrasonic energy passes through SOI bonds with virtually no reflection. As a result, a properly bonded interface will essentially be invisible on the inspection image but a defect enclosing air, gas or a vacuum will be easily visible. The main inspection challenge here is choosing the right transducer and adjusting the focal length accurately to account for the thickness of the silicon layer and the depth of the bonds under inspection. Transducer choices range from 110 MHz for optimum penetration to ultra-high frequency (UHF) for optimum resolution. Although UHF transducers provide the clearest imaging of the smallest defects, precise focusing is crucial. Anodic Bonding Because anodic bonding requires atomic contact between the surfaces to be bonded, any misalignment, irregularity or contamination can cause voids, delaminations, cracks and incomplete bond rings. Transducer frequency and focal length must be carefully chosen for balanced penetration and resolution, especially when inspecting glass-silicon interfaces in triple-stacked wafers. Device-level inspection also requires good pattern recognition and analysis techniques to reliably distinguish defects in regions of interest, such as bond rings, from regions where voids are to be expected, such as MEMS cavities. Figure 1 Ultrasonic image of an SOI bonded wafer pair at 110 MHz, 8mm, 100μm scan resolution. The white areas indicate voids. Round voids are usually caused by particles, while irregularly shaped voids are usually caused by trapped air. Figure 2 Ultrasonic image of an anodic bonded wafer pair at 110 MHz, 8mm, 20μm scan resolution. White areas indicate voids. 4 Hiding In Plain Sight: How Ultrasonics Can Help You Find the Smallest Bonded Wafer and Device Defects A Sonix White Paper

5 Glass Frit Bonding Without correct alignment and precise control over pressure, temperature and time, glass frit can spread insufficiently, leaving voids in bond rings. Or, it can overspread into hermetic cavities and compromise the function of MEMS structures. Other common defects include misalignment, delaminations and cracks. Beyond choosing the right transducer and focal length, the primary inspection challenge is identifying the correct regions of interest to clearly differentiate between bond line defects and the hermetic cavities enclosed by the bonds. Eutectic Bonding At the very small scale typical for eutectic bonds, any variation in process pressure, temperature or bonding time can cause incomplete bonding or uncontrolled flow into MEMS cavities. Manufacturers may also be concerned about variations in the composition, density and distribution of bond materials that could lead to premature failure. Ultrasonic inspection can reveal these defects in addition to voids, delaminations, cracks and misalignment. With bond rings that may be 50μm or smaller, clear imaging of eutectic bonds generally requires precisely focused UHF transducers along with advanced pattern recognition techniques to reliably characterize variations in material properties. Figure 3 Ultrasonic images of hermetically encapsulated MEMS devices in a glass frit bonded wafer pair at UHF, 5.9mm, 10μm scan resolution. Bond areas are seen as dark frames around the devices. Voiding can be seen in the bottom image. Figure 4 Ultrasonic image of a eutectic bonded wafer pair at 110 MHz, 8mm, 20μm scan resolution. Variations in the consistency of the bond layer can be seen. The black area is likely a kissing bond, where the two wafers are touching but there is no bonding material. 5 Hiding In Plain Sight: How Ultrasonics Can Help You Find the Smallest Bonded Wafer and Device Defects A Sonix White Paper

6 Metal-to-Metal Diffusion Bonding Extremely thin metal traces and 3D stacked circuits call for the most careful inspection techniques using very high-resolution images. Different regions of interest may exist at different focal lengths, and the inspection system must be able to clearly differentiate metal traces from the BCB fill used to support and protect them. Transducers in the UHF range or even higher are required to resolve bond lines and detect misalignment at scales of 50μm or less. Precise adjustment of focal length is crucial, and analysis software that provides accurate identification of complex bond patterns can substantially improve defect detection while accelerating the inspection process. Figure 5 Ultrasonic images of metal-tometal diffusion bonded wafers at 110 MHz, 8mm, 50μm scan resolution. The irregularly shaped voids indicate trapped air. The left image shows the results of the initial bonding process, while the right image shows the improvement achieved after optimizing the bonding process. Figure 6 Ultrasonic images of a patterned metal-to-metal diffusion bonded wafer at 110 MHz, 8mm, 50μm scan resolution. The left image shows significant voiding across the full wafer. The right image shows the void patterning in a zoomed-in portion of the wafer. 6 Hiding In Plain Sight: How Ultrasonics Can Help You Find the Smallest Bonded Wafer and Device Defects A Sonix White Paper

7 Sonix Advantages Sonix has been the technology leader for ultrasonic inspection since 1986, with more than 15 years of continuous innovation devoted specifically to bonded wafer inspection. With the industry s largest installed base of ultrasonic wafer inspection systems, we know how to help you improve processes, increase yields and assure product quality. Our patented water coupler simplifies water management while providing distortion-free images. Our patented water heater minimizes signal attenuation to optimize imaging at UHF frequencies and even into the GHz range as we prepare to serve the industry s future requirements. Superior analytics enable precise capture of regions of interest to speed analysis and drive sustainable process improvements. We design Sonix transducers in-house to provide the clearest possible imaging specifically for the semiconductor industry. Our transducer mounting systems are designed for easy changeover and focusing in under a minute an especially useful feature when using UHF inspection and multiple inspection recipes. Our wafer handling, scanning, pass/fail sorting and SECS/GEM communications technologies speed productivity. We can help integrate Sonix equipment within fully automated assembly lines, eliminating the need for a scanning system operator. Sonix experts understand bonded wafer manufacturing and work with customers to optimize their inspection processes, recipes and user training. For more information today, visit our website at And be prepared to share with us your most difficult inspection challenges today and tomorrow. We look forward to helping you solve them. 7 Hiding In Plain Sight: How Ultrasonics Can Help You Find the Smallest Bonded Wafer and Device Defects A Sonix White Paper

8 About Sonix Established in 1986, Sonix is solely focused on leading the industry in the development of non-destructive ultrasonic testing solutions for microelectronics. We are the world s largest supplier of automated ultrasonic solutions for nondestructive wafer inspection, with over 70 percent market share. Sonix is solely owned by Danaher ($18B, NYSE: DHR), providing financial stability with access to resources for investment and information sharing across sister company platforms and technologies Morrissette Drive Springfield, VA T F Sonix, Inc. All Rights Reserved.