In wafer fabrication, Fluorine (F) contamination may cause fluorine-induced corrosion and defects on microchip Aluminum (Al) bondpads, resulting in bondpad discoloration or non-stick on pads (NSOP). Auger Electron Spectroscopy (AES) is employed for measurements of the fluorine level on the Al bondpads. From a Process control limit and a specification limit perspective, it is necessary to establish a control limit to enable process monitor reasons. Control limits are typically lower than the specification limits which are related to bondpad quality. The bondpad quality affects the die bondability. This paper proposes a simulation method to determine the specification limit of Fluorine and a Shelf Lifetime Accelerated Test (SLAT) for process monitoring. Wafers with different F levels were selected to perform SLAT with high temperature and high relative humidity tests for a fixed duration to simulate a one year wafer storage condition. The results of these simulation results agree with published values. If the F level on bondpad surfaces was less than 6.0 atomic percent (at%), then no F induced corrosion on the bond pads was observed by AES. Similarly, if the F level on bond pad surfaces was higher than 6.0 atomic per cent (at%) then AES measured F induced corrosion was observed.

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.

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.

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.

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.

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).

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.