Abstract Bond-pad is an important structure of a microelectronic device because it plays the role of enabling the device to communicate with other external devices. Its integrity directly affects the performance of the microelectronic device. This paper presents our investigation on bond-pad Inter-Metal Dielectric (IMD) crack issue. Our investigation has considered the following factors: top via pattern (sea of vias/without vias) for bond-pad, top metal thickness (8 kÅ /9 kÅ /10 kÅ) and probe overdrive force (30 um/50 um/70 um). The bond-pad IMD cracks were exposed and decorated by chemicals (Aqua Regia and Hydrochloric acid), and inspected by an optical microscope. A scoring system was designed to assess the dependence of the bond-pad IMD crack severity on the above-mentioned factors. The investigation results showed that the IMD crack severity is strongly dependent on the probe overdrive force, top via pattern, and only slightly on top metal thickness.

Abstract In the authors' previous paper, we studied the defects from Fluorine-Induced Corrosion on microchip Al bondpads using SEM, EDX, TEM, AES, IC, XPS and TOF-SIMS techniques. An unknown F-Al compound was found and identified as [AlF6]3-. In this paper, we will further study the chemical mechanisms of Fluorine-Induced Corrosion on microchip Al bondpads and propose a theoretical electrochemical model to reveal the secrets of Fluorine-Induced Corrosion on Al bondpads. To support this new theoretical model, we will provide substantiating data from TOFSIMS analysis and other failure analysis techniques. According to the theoretical model of Fluorine-induced Corrosion proposed, fluorine contamination on Al bondpads will cause two types of corrosions. First, fluorine reacts with Al and forms a complex compound [AlF6]3- on the affected area, which we will refer to as Fluorine Corrosion. Once the compound of [AlF6]3- forms on Al bondpads, it will form an Anode and cause further electrochemical reactions from O2, N2 and H2O (moisture) at the Cathode. The new products of further electrochemical reactions will be [OH]- and [NH4]+ ions. The new product of [OH]- ions will react with Al and cause further Al corrosion on bondpads and form corrosive product consisting of Al(OH)3, which we will refer to as [OH]- Corrosion. The new product of [NH4]+ ions will combine with [AlF6]3- and form a corrosive complex compound (NH4)3(AlF6). This proposed corrosion mechanism results in non-stick bondpad problem.

Abstract Fault isolation is a critical step of failure analysis, which is most important for yield improvement for any new microelectronic device manufacturing. Conventionally, electrical faults are isolated by emission microscopy, liquid crystal, LIVA/TIVA and ORBIRCH etc. techniques after final test. As microelectronic devices are becoming more complicated and with multiple metal layers, failure analysis faces more challenges than before. These challenges are even tougher in wafer foundries because little device information is available. This makes yield ramp-up take longer time. Utilizing inline E-beam inspection equipment, the electrical faults can be captured at the source layer rather than after final test. E-beam inspection can be incorporated in the manufacturing line at any critical layer of front end and back-end. This paper describes the in-line E-beam inspection and presents three cases: (1) Gate-oxide issue, (2) Contact issue, and (3) Interconnect issue to demonstrate its application.

Abstract Fluorine contamination on Al bondpads will result in corrosion, affect quality of bondpads and pose problem such as non-stick on pad (NSOP) during wire bonding at assembly process. In this paper, a fluorine contamination case in wafer fabrication will be studied. Some wafers were reported to have bondpad discoloration and bonding problem at the assembly house. SEM, EDX, TEM, AES and IC techniques were employed to identify the root cause of the failure. Failure analysis results showed that fluorine contamination had caused bondpad corrosion and thicker native aluminium oxide, which had resulted in discolored bondpads and NSOP. It was concluded that fluorine contamination was not due to wafer fab process, but was due to the wafer packaging foam material. XPS/ESCA and TOF-SIMS advanced tools were used to study the chemical and physical failure mechanism of fluorine-induced defects. An unknown Al compound was found using XPS technique and identified it as [AlF6]3- using electrochemical theories and TOF-SIMS technique. This finding was very significance, as it helped developing a theoretical electrochemical model for fluorine-induced corrosion and helped understanding of the mechanism of fluorine-induced corrosion on aluminium bondpads. It was found that fluorine contamination had formed [AlF6]3-on the affected bondpads and it had caused further electrochemical reactions and formed some new products of (NH4)+ and OH-. Then [AlF6]3- and (NH4)+ ions combined and formed a corrosive complex compound, (NH4)3(AlF6), while the OH- reacted with Al and caused further corrosion.

Abstract In failure analysis of wafer fabrication, currently, three different types of chemical methods including 6:6:1 (Acetic Acid/HNO3/HF), NaOH and Choline are used in removing polysilicon (poly) layer and exposing the gate/tunnel oxide underneath. However, usage is limited due to their disadvantages. For example, 6:6:1 is a relatively fast etchant, but it is difficult to control the etch time and keep the oxide layer intact. Also, while using NaOH to remove poly and expose the silicon oxide, the solution needs to be heated. It is also difficult to etch a poly layer with a WSi x or a CoSi x silicide using NaOH. In this paper, we will discuss these 3 etchants in terms of their advantages and disadvantages. We will then introduce a new poly etchant, called HB91. HB91 is useful for removing poly to expose the gate/tunnel oxide for identification of related defects. HB91 is actually a mixture of two chemicals namely nitric acid (HNO3) and buffer oxide etchant (BOE) in a 9:1 ratio. The experimental results show that it is highly selective in poly removal with respect to the gate/tunnel oxide and is a suitable poly etchant especially for removing polysilicon with/without WSi x and CoSi x in the large capacitor structure. Application results of this poly etchant (HB91) will be presented.

Abstract In semiconductor failure analysis, there is a demand that after mechanical polishing and scanning electron microcopy (SEM) examination, the failure site needs to be analyzed by transmission electron microscope (TEM) for a detailed examination to find the root cause. In this paper, a fast and practical TEM sample preparation method for TEM examination of specific site identified by cross-section scanning electron microscope (SEM) is demonstrated for further structural analysis.

Abstract In this paper, three low yield case studies in wafer fabrication are reviewed. These issues/problems include thicker gate oxide due to contamination from the wafer fab process, QBD failures due to silicon crystalline defects caused by charging during the BN+ implant process and memory failures relating to tunnel oxide defects in EEPROM devices. Chemical deprocessing techniques, 155 Wright Etch, Scanning Electron Microscope, Transmission electron microscopy & Secondary Ion Mass Spectroscopy were used to identify the root causes. Some new chemical deprocessing techniques in exposing the tunnel window & oxide for the memory cell failures were developed. Moreover, some new failure mechanisms relating to the low yield due to thicker gate oxide, silicon crystalline defects and QBD failure were also discussed.

Abstract Process development usually has an indispensable period of yield-learning. The length of the period is highly dependent on identification of the root causes of yield-limiting defects and thus failure analysis plays an important role in the yield improvement process. This paper presents the failure analysis of yield-limiting defects in 0.15µm process development. The failure analysis involves failure mode categorization by MOSAID tester, defective contact identification by contact-level Passive Voltage Contrast (PVC) technique and subsequent Focus Ion Beam (FIB) cross-section followed by Transmission Electron Microscopy (TEM) analysis. Finally, the root causes for the yield-limiting defects were identified.