This work characterizes three different types of sensor defects, and investigates the applicability of the Contrast Threshold Function (CTF) of the human visual system to the manufacturing test criteria for CMOS image sensors. The sensor defect types characterized are resist streaking, dye color spots, and orange-peel photodiode sensitivity noise. Algorithms are presented to objectively identify and rate the severity of each. Visual evaluations determined the subjective level of detectability and objectionability of each. The spatial frequency and modulation of the defects were measured, and compared with an appropriate CTF. The result is the minimum defect levels noticeable in test images can be almost order-of-magnitude higher than the known CTFs determined for the limits of human visual system sensitivity.
The AMAG comprised of representatives from International SEMATECH consortium member companies and the National Institute of Standards and Technology have joined to develop a new unified specification for an advanced scanning electron microscope critical dimension measurement instrument (CD-SEM). This paper describes the result of an effort to benchmark six CD-SEM instruments according to this specification.
The Advanced Metrology Advisory Group (AMAG) comprised of representatives from International SEMATECH consortium member companies and the National Institute of Standards and Technology have joined to develop a new unified specification for an advanced scanning electron microscope critical dimension measurement instrument (CD-SEM). (Allgair, et al., 1998) This paper describes the results of an effort to benchmark six CD-SEM instruments according to this specification. The consensus among the AMAG metrologists was that many critical areas of performance of CD-SEMs required improvement. Following this assessment this specification for benchmarking was developed. The advanced CD-SEM specification addresses several critical areas for improvement, each with its own a separate section. The critical areas covered are: precision, accuracy, charging and contamination, performance matching, pattern recognition and stage navigation accuracy, throughput, and instrumentation outputs. Each section of the specification contains a concise definition of the respective performance parameter, and wherever appropriate refers to ISO definitions. The test methodology is described, complete with the relevant statistical analysis. Many parameters (including precision, matching, and magnification accuracy) are numerically specified to be consistent with the International Technology Roadmap for Semiconductors (ITRS, 1999). Other parameters, such as charging and linewidth accuracy, are targeted with guidelines for improvement. The test wafers developed for determining the level of compliance with the specification are also discussed. The AMAG circulated this report among the metrology instrument suppliers and conferred with them. Certain components of the specification have already been adopted by some of the manufacturers in their newer metrology instruments. International SEMATECH fabricated the AMAG test wafers described herein. Measurements on six state-of-the-art metrology instruments using the AMAG test wafers have been carried out and the results were processed according to this specification. A review of the results is presented in this paper.
Although the subject of frequent concern, criticism, and attention in the modern semiconductor fabrication facility, human after develop inspection (ADI) does not catch the major scrap and yield events early enough, if at all. The overall success of scrap and photo redo reduction programs over past years has resulted in residual problem levels which are difficult to improve upon -- yet still very costly. Detected 'events' are few and far-between, although evidence of their prevalence is frequently seen at subsequent inspections, or finally at probe. In the ASIC fab, they put on-time delivery to customers at risk, because individual wafer lots in an ASIC facility have a designated customer. The sampled area is limited by human throughput to less than 10% of the wafers in a lot. The visual ADI process step is unpopular among manufacturing technicians. It is often a bottleneck in the photo area. Statistically, in a photo area with capacity of 5000 wafer starts per week, only a few wafers processed per day are destined for scrap. Since wafer events occur in sporadic clusters, the photo area experiences only a few significant incidents per month. The typical operator can expect to intercept such an event less than once during several months of otherwise uneventful ADI inspection haystack.' Hence the stubbornness of our residual problem. Going beyond the statistical problem, our current manual macro-inspection equipment is engineered appropriately to ancient IC generations. A collimated, oblique-oriented light was an effective darkfield illumination source, when line widths were much larger than the wavelength of light. When line width is comparable to, or smaller than, the wavelength, the collimated light source produces scintillating diffracted colors on the wafer. Thus diffraction 'noise' significantly buries the defect 'signal' in the typical bright light visual macro inspection. In addition, there is the problem of variability between human inspectors, and the impossibility of accurate classification and recording of defect types, locations, and layer of occurrence. In this paper, we discuss a pilot implementation of an automated macro inspection system at Motorola, Inc., which has enabled the early detection and containment of significant photolithography defects. We show a variety of different types of defects that have been effectively detected and identified by this system during production usage. We introduce a methodology for determining the automated tool's ability to discriminate between the defect signal and process noise. We indicate the potential for defect database analysis, and identification of maverick product. Based upon the pilot experience, we discuss the parameters of a cost/benefit analysis of full implementation. The costs involve tool cost, additional wafer dispositions, and the engineering costs of recipe management. The most tangible measurable benefit is the saved revenue of scrapped wafers. An analysis of risk also shows a major reduction due to improved detection, as well as reduced occurrence because of better containment. This reduction of risk extends both to the customer -- in terms of field failures, OTD, maverick product -- as well as to the production facility -- in terms of major scrap incidents, forced inking at probe, redo, and containment.
The color filter array (CFA) for an image-producing semiconductor device is composed of patterned red-, and green- and blue-colored photoresist structures. CFA photolithography is rather different from that of most semiconductor process levels.
The stringent critical dimension control requirements in cutting edge device facilities have placed significant demands on metrologists and upon the tools they use. We are developing a unified, advanced critical dimension scanning electron microscope specification in the interests of providing a unified criterion of performance and testing. The specification is grounded on standard definitions and strong principles of metrology. The current revision is to be published as a SEMATECH document. A new revision, now in progress, will embody the consensus of a vendor/user conference.
High temperature metal deposition produces large grain size and a highly visible surface morphology due to grain boundaries. When an interconnect layer photoresist pattern is aligned, grainy metal results in noisy signals from optical metrology equipment. The overlay metrology tool hardware and software configuration and target design must be optimized to obtain the best possible signal-to-noise. A powerful metric is developed herein to single out the noise component due to the overlay target image distortions. This methodology is suitable to a production environment. A variety of techniques based upon the target noise metric, including designed experiments, are employed to optimize the overlay measurements configuration.
A joint Motorola/IBM experiment was performed in mix-and-match lithography across widely separated locations. A simple pattern placement metrology data set was created, and x-ray masks were manufactured according to this data. The same data was converted into a 5x reticle and optically stepped on wafers. The x-ray mask was designed to print upon two optical fields with one x-ray exposure. The x-ray mask was aligned to the wafers to produce box-in- box images for overlay metrology. The main overlay problems encountered were systematic offsets between x-ray and optical images, and average magnification error of approximately 8 ppm. The magnification error is substantial because of the 3 degree(s)C temperature difference between the optical stepper stage and the x-ray mask-writer. In an actual device run, the magnification differences will be removed by compensation in the e-beam writing of the x-ray mask. Offsets will be removed by use of a send-ahead wafer to determine the correct offset alignment in the x-ray stepper.
It is widely recognized that the 1:1 x-ray mask is the most technically challenging aspect of proximity x-ray lithography, since high resolution and precise pattern placement must be achieved completely free of defects. SEM investigation is an excellent tool for x-ray mask inspection. However, it is sometimes assumed that only the SEM has sufficient resolution to perform meaningful defect detection on x-ray masks. An electrically probed test chip pattern for x-ray mask defectivity measurement and improvement has been designed and implemented. This pattern is printed with an optical stepper on silicon wafers with plating base. These are then processed like an x-ray mask through x-ray absorber definition. Since the absorber is a high-conductivity metal and the substrate is insulating, electrical shorts and opens correspond to extra and missing absorber. This paper describes a series of defect types revealed by these two rather different methodologies: SEM inspection of completed masks using the KLA SEMSpec, as well as the electrically probed test chip pattern. The two methods are compared and contrasted. The main nuisance and genuine defects in our x-ray mask process are catalogued.
An accelerated test method and resulting metrology data are presented to show the effects of x- ray radiation on various x-ray mask membrane materials. A focused x-ray beam effectively reduces the radiation time to 1/5 of that required by normal exposure beam flux. Absolute image displacement results determined by this method indicate imperceptible movement for boron-doped silicon and silicon carbide membranes at a total incident dose of 500 KJ/cm2, while image displacement for diamond is 50 nm at 150 KJ/cm2 and silicon nitride is 70 nm at 36 KJ/cm2. Studies of temperature rise during the radiation test and effects of the high flux radiation, i.e., reciprocity tests, demonstrate the validity of this test method.