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Hua Younan
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Proceedings Papers
ISTFA2016, ISTFA 2016: Conference Proceedings from the 42nd International Symposium for Testing and Failure Analysis, 287-290, November 6–10, 2016,
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In wafer fabrication (Fab), Fluorine (F) based gases are used for Al bondpad opening process. Thus, even on a regular Al bondpad, there exists a low level of F contamination. However, the F level has to be controlled at a lower level. If the F level is higher than the control/spec limits, it could cause F-induced corrosion and Al-F defects, resulting in pad discoloration and NSOP problems. In our previous studies [1-5], the theories, characteristics, chemical and physical failure mechanisms and the root causes of the F-induced corrosion and Al-F defects on Al bondpads have been studied. In this paper, we further study F-induced corrosion and propose to establish an Auger monitoring system so as to monitor the F contamination level on Al bondpads in wafer fabrication. Auger monitoring frequency, sample preparation, wafer life, Auger analysis points, control/spec limits and OOC/OOS quality control procedures are also discussed.
Proceedings Papers
ISTFA2015, ISTFA 2015: Conference Proceedings from the 41st International Symposium for Testing and Failure Analysis, 278-281, November 1–5, 2015,
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In the authors' previous papers, the failure mechanism and elimination solutions of galvanic corrosion (Al-Cu cell) on microchip Al bondpads in the Al process (0.18un and above) have been studied [1-2]. In this paper, the authors will further study the failure mechanism and root cause of galvanic corrosion (Al-Cu cell) on microchip Al bondpads in the Cu process (0.13um and below) with Ta barrier metal. Based on our results, the root cause of galvanic corrosion (Al-Cu cell) in the Al process is only one way and Al-Cu cell is from Al alloy (Al + 0.5%Cu) on Al bondpads. However, in the Cu process it may be from two ways and Al-Cu cell can be from both Al alloy (Al + 0.5%Cu) on Al bondpads and the Cu metal layer below the barrier metal Ta when Ta has weak points or pinhole. As such, the pinhole defects on Al bondpad caused by galvanic corrosion (Al-Cu cell) in the Cu process might be more serious than that in the Al process. In this paper, TEM is used for root cause identification. Based on the TEM results, galvanic corrosion was due to the weak point/pinhole at the Ta barrier metal layer and Al-Cu diffusion.
Proceedings Papers
ISTFA2015, ISTFA 2015: Conference Proceedings from the 41st International Symposium for Testing and Failure Analysis, 295-297, November 1–5, 2015,
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In authors’ previous paper, an OSAT [Optical, SEM (Scanning Electron Microscopy), Auger (Auger Electron Spectroscopy) and TEM (Transmission Electron Microscopy)] methodology was developed for qualification of microchip aluminum (Al) bondpads. Using the OSAT methodology, one can qualify microchip Al bondpads. In this paper, we will further study the NSOP (Non-Stick On Pad) problem on microchip Al bondpads. A new qualification methodology, OSSD [(Optical, SEM, and Surface and Depth profiling X-ray Photoelectron Spectroscopy (XPS)] will be proposed, in which XPS surface analysis is used to check the contamination level of fluorine and carbon on bondpad surfaces instead of Auger analysis. XPS depth profiling analysis will also be used to measure Al oxide thickness instead of TEM analysis. By using OSSD, Al bondpads can be qualified with both reduced costs and shortened turnaround times versus OSAT.
Proceedings Papers
ISTFA2014, ISTFA 2014: Conference Proceedings from the 40th International Symposium for Testing and Failure Analysis, 215-217, November 9–13, 2014,
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It is well-known that underetch material, contamination, particle, pinholes and corrosion-induced defects on microchip Al bondpads will cause non-stick on pads (NSOP) issues. In this paper, the authors will further study NSOP problem and introduce one more NSOP failure mechanism due to Cu diffusion caused by poor Ta barrier metal. Based on our failure analysis results, the NSOP issue was not due to the assembly process, but due to the wafer fabrication. The failure mechanism might be that the barrier metal Ta was with pinholes, which caused Cu diffused out to the top Al layer, and then formed the “Bump-like” Cu defects and resulted in NSOP on Al bondpads during assembly process.
Proceedings Papers
ISTFA2012, ISTFA 2012: Conference Proceedings from the 38th International Symposium for Testing and Failure Analysis, 203-206, November 11–15, 2012,
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The back-end-of-line (BEOL) structure of current IC devices fabricated for advanced technologies is composed of film stacks with multiple interfaces. The requirement of high interfacial strength is therefore necessary between the different layers in the BEOL stacks to ensure device reliability. To enhance the IC performance for new technologies, inter-level dielectric (ILD) made of SiO2 is replaced by low-k and ultra low-k (ULK) dielectrics, which possess a low dielectric constant but have poor mechanical strength. Therefore, the challenge in maintaining BEOL film stack integrity and reliability becomes even greater for advanced technologies. In this paper, we show failure analysis results on a case study of ULK adhesion failure during the IC manufacturing process. The symptoms of the BEOL failure are due to debris dropping on the wafer during chemical mechanical polishing (CMP) after Cu thin film deposition and failure of focusing at wafer extreme edge during the subsequent photolithography process. Extensive mechanical and chemical analyses were conducted on the ULK and adjacent thin films. It was revealed that the interface of ULK and Silicon Nitride from a suspected problematic machine showed abnormally low adhesion energy and high carbon composition. Troubleshooting on that suspected machine found a clog in the foreline. Based on the failure analysis and machine troubleshooting results, the failure mechanism of the case was discussed.
Proceedings Papers
ISTFA2012, ISTFA 2012: Conference Proceedings from the 38th International Symposium for Testing and Failure Analysis, 293-296, November 11–15, 2012,
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In this work, delineation of crystal defects in Si by preferential chemical etching (Wright etch) is discussed. Investigation of defects in Si wafers by preferential chemical etching enables the study of various types of crystal defects for large area defect distribution (up to full wafer) and root cause analysis. In the case of dislocation defects, the shapes of etch pits are different for different etching duration. We show the mechanism of the pit shape evolution under preferential etching and suggested the appropriate etching duration for defect type identification with inspection by optical microscopes. The dislocation delineation method has been applied to a case of functional failure of devices caused by abnormal process in Laser Scanning Annealing (LSA). It was shown that the distribution of dislocation defects depends largely on the direction of LSA scan direction. We discuss the relationship between dislocation defect distribution and the density and uniformity of the active-Si patterns as well as possible solutions for elimination of dislocation defects in LSA process.
Proceedings Papers
ISTFA2012, ISTFA 2012: Conference Proceedings from the 38th International Symposium for Testing and Failure Analysis, 305-309, November 11–15, 2012,
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A case study of Fluorine (F)-outgassing is presented in this paper that caused the corrosion of Aluminum bond pad. It will be shown that the source of F-contamination is not the typical residue left behind after the passivation etch with Fluorine-based gas chemistry and the subsequent removal of the etch polymer generated with solvent (chemical) clean. Rather, it is introduced as a result of F-outgas over a period of time from the intermetallic dielectric (IMD) film, fluorosilicate glass (FSG), during the post-fab wafer storage. The methodology used in our failure analysis (FA) lab to identify and characterize this type of failure mode is presented in the paper.
Proceedings Papers
ISTFA2011, ISTFA 2011: Conference Proceedings from the 37th International Symposium for Testing and Failure Analysis, 112-117, November 13–17, 2011,
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In this paper, a comprehensive analysis methodology for gate oxide integrity (GOI) failure using combined FA techniques is proposed. The current method integrates the failure analysis flow we previously reported with a new flow proposed in this paper. The method is applicable to a wide range of GOI failure cases and has been used in analyzing many product wafers with GOI failure. In particular, there is one wafer with GOI failure that results from known failed process machines. This wafer could be readily analyzed with this new method to identify the root causes. The newly proposed flow is based on our previous report on GOI failure analysis, but the detection limit of contamination elements was significantly improved. The enhancement of detection limit is mainly attributable to the utilization of Vapor Phase Decomposition and Inductively Coupled Plasma Mass Spectrometry (VPD ICP-MS). The ICP-MS technique is highly sensitive and capable of simultaneously measuring a large number of elements at very low concentration level in the range of ppb (part per billion) to ppt (part to trillion). This enhanced sensitivity enables effective investigation of contamination caused by specific machines. A case study of GOI failure investigated by the proposed new method will be discussed in detail. In the study, Al, Fe, Mo and Sn contamination from a suspected tool were detected by ICPMS, followed by confirmation by Secondary Ion Mass Spectrometry (SIMS) on the affected product wafers. Failurepart isolation investigations of the affected diffusion furnace revealed that the root cause of the failure is due to a defective gas flow valve.
Proceedings Papers
ISTFA2010, ISTFA 2010: Conference Proceedings from the 36th International Symposium for Testing and Failure Analysis, 117-121, November 14–18, 2010,
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Fluorine-induced corrosion is one of the well-known failure modes of Al bondpad leading to non-stick on pad (NSOP) issues. Exposure to moisture (H2O) and atmosphere (O2) play an important role in determining the shape and size of the Al-F corrosion defects on the Al bondpad surface. In this paper, we will propose a laboratory simulation methodology that can reproduce F-induced defects observed either at the wafer fab or the assembly house. The methodology, known as SLAT ( S helf L ifetime A cceleration T est), is used to study the relationship between the F-corrosion defects and the relative temperature (T) and humidity (RH %). It is observed that Al-F corrosion defects simulated are similar to the real defects found in wafer fab and assembly house in our previous studies. A relatively higher T and lower RH % results in the formation of the “crystal-like” defects, but if a relatively lower T and higher RH % condition is used, the “oxide-like” defects were formed. In this paper, we will compare the simulation results to the real defects found in the previous cases and discuss the failure mechanism. From the present study, the importance of controlling T and RH % during wafer storage to eliminate F-induced defects will be highlighted.
Proceedings Papers
ISTFA2010, ISTFA 2010: Conference Proceedings from the 36th International Symposium for Testing and Failure Analysis, 249-253, November 14–18, 2010,
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Wire bonding continues to remain as the dominant chip interconnect technology in the far backend process, regardless of the shrinkage of microchip Al bond pad size and the increase in the number of I/O connections in the modern ICs. The reliability of IC devices is directly affected by the quality of adhesion between wire bond and microchip Al pad. Many factors, such as the wafer fab process residue and corrosion, creep-induced wire breakage and electrostatic damage, may result in poor adhesion. In this paper, we show a p-channel Field-Effect Transistor (pFET) failure caused by a mismatch in the bond pad size and the wire bond diameter as well as electrostatic damage during wire bonding. The failure analysis results, failure mechanism and the design rule of microchip Al bond pad in wafer fabrication are discussed. FA investigations were performed on the high gate leakage (nA to mA level) issue in the packaged pFET. It was found that two major factors contributed to the failure, namely mechanical and electrostatic damage. The mechanical damage was mainly due to incompatible Al pad size and bond wire diameter. More specifically, in the failed device, the bond wire diameter was larger than half size of the bond pad opening, contrary to the general design rules of wire bonding. The failure to adhere to the design rule resulted in the device failure. In addition, the electrostatic damage during wire bonding resulted in defects of poly Si/gate oxide and thus the high gate leakage. In this paper, the FA results, failure mechanism of the high gate leakage and the bond pad design rule will be discussed. Also, it will be demonstrated that to achieve good bonding quality and eliminate mechanical and ESD damage the diameter of the bond wire should be equal to or smaller than half of the bond pad opening.
Proceedings Papers
ISTFA2009, ISTFA 2009: Conference Proceedings from the 35th International Symposium for Testing and Failure Analysis, 177-181, November 15–19, 2009,
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As device feature size continues to shrink, the reducing gate oxide thickness puts more stringent requirements on gate dielectric quality in terms of defect density and contamination concentration. As a result, analyzing gate oxide integrity and dielectric breakdown failures during wafer fabrication becomes more difficult. Using a traditional FA flow and methods some defects were observed after electrical fault isolation using emission microscopic tools such as EMMI and TIVA. Even with some success with conventional FA the root cause was unclear. In this paper, we will propose an analysis flow for GOI failures to improve FA’s success rate. In this new proposed flow both a chemical method, Wright Etch, and SIMS analysis techniques are employed to identify root cause of the GOI failures after EFA fault isolation. In general, the shape of the defect might provide information as to the root cause of the GOI failure, whether related to PID or contamination. However, Wright Etch results are inadequate to answer the questions of whether the failure is caused by contamination or not. If there is a contaminate another technique is required to determine what the contaminant is and where it comes from. If the failure is confirmed to be due to contamination, SIMS is used to further determine the contamination source at the ppm-ppb level. In this paper, a real case of GOI failure will be discussed and presented. Using the new failure analysis flow, the root cause was identified to be iron contamination introduced from a worn out part made of stainless steel.
Proceedings Papers
ISTFA2009, ISTFA 2009: Conference Proceedings from the 35th International Symposium for Testing and Failure Analysis, 289-292, November 15–19, 2009,
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Non-stick on pad (NSOP) is a yield limiting factor that can occur due to various reasons such as particle contamination, galvanic corrosion, Fluorine-induced corrosion, process anomalies, etc. The problem of NSOP can be mitigated through a careful process characterization and optimization. In this paper, a bondpad qualification methodology (OSAT) will be discussed. It will be argued that by employing different physical analysis techniques in a failure analysis of wafer fabrication, it is possible to perform comprehensive characterization studies of the Aluminum bondpad so as to develop a robust far backend of line process. A good quality Al bondpad must meet the following four conditions-OSAT: (i) it should be no discoloration (using Optical inspection); (ii) should be defect free (using SEM inspection); (iii) should be with low contamination level (such as fluorine and carbon contamination should be within a control limit) (using Auger analysis) and (iv) should have a protective layer on bondpad surface so as to prevent bondpad corrosion (using TEM).
Proceedings Papers
Studies of Fluorine-Induced Corrosion and Defects on Microchip Al Bondpads and Elimination Solutions
ISTFA2008, ISTFA 2008: Conference Proceedings from the 34th International Symposium for Testing and Failure Analysis, 285-290, November 2–6, 2008,
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In this paper, a comprehensive study of Fluorine-induced Aluminum bondpad corrosion will be presented. Fluorine corrosion is detrimental to Al pad quality resulting in non-stick on pad (NSOP). In wafer fabrication, NSOP refers to lift-off of Au wire-bond from the surface of Al bondpad due to its poor adhesion to the contaminated pad surface. It will be shown that besides the well-known corrosion mechanism that causes NSOP, namely, via the formation of Al fluoride defect ([AlFx](x-3)-), it can also happen due to Al fluoride oxide defect, AlxOyFz. Unlike the Al fluoride defect, which has a unique “flower-like” shape, Al fluoride oxide defects exist in a variety of shapes: “Crystal-like”, “Oxide-like” and “Cloud-like”. The physical dimensions of these defects (including Al fluoride) can be dramatically different, varying all the way from micron to nanometer. In this paper, each of these morphological shapes and their respective failure mechanisms will be covered. Solutions to mitigate F-corrosion and ways to control/monitor contamination on Al pad surface in wafer fab will be presented.
Proceedings Papers
ISTFA2008, ISTFA 2008: Conference Proceedings from the 34th International Symposium for Testing and Failure Analysis, 291-293, November 2–6, 2008,
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Electron beam induced radiation damage presents great challenges for the electron microscopy analysis of low k and ultra low k dielectrics due to their beam sensitive nature. In order to minimize the radiation damage, it is necessary to understand the mechanisms behind the damage. This work presents detailed studies regarding the mechanisms behind the effects of probe currents, accelerating voltage and anticharging coating layers on the radiation damage to low/ultralow K dielectrics. The results indicate that the probe current shows a stronger dependence on the size of the condenser lens aperture than the accelerating voltage. Therefore, in terms of the probe current, the condenser lens aperture plays a decisive role in affecting the radiation damage process. In order to minimize the radiation damage, SEM imaging should be conducted with not only a low accelerating voltage but also a small condenser lens aperture to reduce probe current. Based on simulation results, the effects of a coating layer and accelerating voltage are related to the interaction volume and the penetration depth of the electron beam. Pt coating can act as not only an anti-charging layer, but also an effective barrier layer for reducing electron flux that interacts with the low/ultra-low dielectrics.
Proceedings Papers
ISTFA2007, ISTFA 2007: Conference Proceedings from the 33rd International Symposium for Testing and Failure Analysis, 193-196, November 4–8, 2007,
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Galvanic corrosion (two metal corrosion) on microchip Al bondpads may result in discolored or non-stick bondpad problem. In this paper, a galvanic corrosion case at bondpad edge will be presented. Besides galvanic corrosion (Al-Cu cell), a concept of galvanic corrosion (Al-Ti cell) is proposed, which is used to explain galvanic corrosion at bondpad edge with layers of TiN/Ti/Al metallization structure. A theoretical model of galvanic corrosion (Al-Ti cell) is proposed to explain chemically & physically failure mechanism of galvanic corrosion at bondpad edge. According to the theoretical model proposed in this paper, galvanic corrosion on microchip Al bondpads could be identified into two corrosion models: galvanic corrosion (Al-Cu cell) occurred mostly at the bondpad center and galvanic corrosion (Al-Ti cell) occurred specially at bondpad edge with TiN/Ti/Al metallization structure. In this paper, a theoretical model of galvanic corrosion (Ai-Ti cell) will be detail discussed so as to fully understand failure mechanism of galvanic corrosion the bondpad edge. Moreover possible solutions to eliminate galvanic corrosion (Al-Ti cell) are also discussed.
Proceedings Papers
ISTFA2006, ISTFA 2006: Conference Proceedings from the 32nd International Symposium for Testing and Failure Analysis, 300-304, November 12–16, 2006,
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After wafer-die sawing process, sometimes silicon (Si) dust on microchip Al bondpads is difficult to be cleaned away by DI water, especially at pinhole/corrosive areas caused by galvanic corrosion, thus resulting in non-stick on pads (NSOP) problem in assembly process. To eliminate NSOP problem due to Si dust contamination, in this paper, we will study the mechanism of Si dust contamination and propose a concept of Si dust corrosion. A theoretical model will be introduced so as to explain Si dust contamination and corrosion problem during wafer die sawing process. Based on the mechanism proposed, Si dust contamination and corrosion is related to galvanic corrosion as OH- ions generated from galvanic corrosion will not only react with Al to cause Al corrosion, but also react with Si dust to cause Si dust corrosion. During Si dust corrosion, poly-H2SiO3 and Si-Al-O complex compounds will be formed on Al bondpads, especially at the pinholes/corrosive areas. Poly-H2SiO3 and Si-Al-O complex compounds are “gel-like” material and stick onto the surface of bondpads. It is insoluble in water and difficult to be cleaned away by DI water during or after wafer die sawing process and will cause bondpad discoloration or/and NSOP problem. Some eliminating methods of Si dust contamination and corrosion on Al bondpads during wafer die sawing process are also discussed.
Proceedings Papers
ISTFA2005, ISTFA 2005: Conference Proceedings from the 31st International Symposium for Testing and Failure Analysis, 274-282, November 6–10, 2005,
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A failure analysis flow is developed for surface contamination, corrosion and underetch on microchip Al bondpads and it is applied in wafer fabrication. SEM, EDX, Auger, FTIR, XPS and TOF-SIMS are used to identify the root causes. The results from carbon related contamination, galvanic corrosion, fluorine-induced corrosion, passivation underetch and Auger bondpad monitoring will be presented. The failure analysis flow will definitely help us to select suitable methods and tools for failure analysis of Al bondpad-related issues, identify rapidly possible root causes of the failures and find the eliminating solutions at both wafer fabrication and assembly houses.
Proceedings Papers
ISTFA2004, ISTFA 2004: Conference Proceedings from the 30th International Symposium for Testing and Failure Analysis, 474-481, November 14–18, 2004,
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In failure analysis of wafer fabrication it is difficult to identify possible sources of carbon-related contaminants as most of them are from polymers, organic and complex compounds. In this paper, the fingerprints of EDX, FTIR, XPS and TOFSIMS techniques will be introduced so as to identify sources of carbon-related contaminants. For example, Si peak (1.740 keV) can be used as a fingerprint of EDX technique to identify the ink-related contaminant from the other carbon-related contaminants. FTIR spectra of more than 10 possible materials from wafer fab and assembly processes are discussed, which may be used as the fingerprints of FTIR technique to identify carbon-related contaminants. The C=O functional group and the PDMS (PolyDimethylSiloxane) are recommended as the fingerprints of XPS and TOF-SIMS techniques to identify source of carbon-related contaminants, respectively. In this paper, some application cases will be also discussed.
Proceedings Papers
ISTFA1998, ISTFA 1998: Conference Proceedings from the 24th International Symposium for Testing and Failure Analysis, 269-272, November 15–19, 1998,
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Discolored bondpads failure was investigated. SEM & EDX techniques were used to identify the root causes. The results show that discolored bondpads are caused by many holes on bondpads due to galvanic corrosion. It is most severe at power-line pads and less severe on rest of pads in the die. These bondpad holes have led to poor contact between the wire & integrated circuits and faulty computer chips. The holes are most likely to be caused by galvanic corrosion as there is a nucleus centered inside almost all holes. EDX results on the nucleus show high Cu concentration comparing to that on normal area. It is concluded that galvanic corrosion was due to DI water problem during wafer sawing at assembly. In this paper, the mechanism of galvanic corrosion will be further discussed.