Instrumentation and Metrology as Key Drivers for all the NNI Grand Challenges

Michael T. Postek, National Institute of Standards and Technology, Gaithersburg, MD, 20899 ()

Instrumentation and Metrology (measurement science) are key components to the success of all the nine grand challenges of the National Nanotechnology Initiative. Metrology is pervasive and it is unifying. Instrumentation provides the necessary data upon which scientific conclusions can be based and correct metrology provides the ability to properly and accurately interpret those data.

The unifying effect of metrology is most epitomized for one of the more universal tools used in nanotechonology today, the scanning electron microscope (SEM). The SEM was initially evolved from an X-ray microanalytical tool into an imaging tool. The biologists were some of the first adopters of this technology and a great deal of fundamental biological research was done with this instrument. In the mid-1980’s, visionaries in the semiconductor industry saw the need of the application of SEMs in the inspection and metrology in semiconductor production since SEMs were already being used for research applications. Device and integrated circuit feature sizes were shrinking below the optical microscope capabilities achievable at that time. Low accelerating voltage operation was chosen for its potential for non-destructive imaging and the fact that conductive coating was unnecessary to minimize sample charging [1,2,3,4]. Low accelerating voltage operation also provided limited beam penetration into the sample and a more precise “look” at the surface structure. Imaging and information content was greatly different between high and low accelerating voltage operation and a great deal of new knowledge about the samples resulted. The higher brightness lanthanum hexaboride electron guns were being pushed to their limits, digital frame buffering was introduced and eventually field emission instrumentation was introduced and became superior, proving their value of increased electron source brightness and higher resolution. All this took a tremendous amount of investment in research and development. Fortunately, the semiconductor industry was willing to pay for the development of special fully automated SEMs capable of meeting the needs of semiconductor production to view and measure nanometer-sized gate structures used on the modern semiconductor chips. In the meantime, this development benefited all other SEM applications for nanotechnology. This is especially true for the biologists who have greatly benefited in improved resolution and imaging capabilities for their nanometer-sized structures. This is a perfect example of cross-discipline needs and the interdigitation of technological solutions needed for the success for commercialization of nanotechnology. This also points out that there is an immense cost for instrument research and development that must be born by some segment of the industry. In the case of the SEM, the semiconductor industry was already established and could bear that burden. Is there an emerging nanotechnology industry sector that can do the same for nanomanufacturing?

Measurements generally go hand in hand with ability to image. The need to measure nanometer-sized semiconductor structures is a similar need to the biologist or environmental nanometrologist who would like the size measurement of particles. Automated analysis for these purposes has yet to be fully developed. Precise metrology in the production environment is routine to the 1-5 nanometer level for semiconductor metrology, but accuracy remains lacking. To provide accurate measurements of any structure in the SEM an integrated model of the electron beam/sample/ instrument interactions must be developed. Such models have been developed for application to semiconductor manufacturing and employment of them in the measurement process has already improved the precision by a factor of 3x. [5]. However, for lesser known samples especially those of biological nature new measurement techniques need to be employed in order to obtain the necessary input data for the model in order for it to be reliable.

The SEM has another problem to overcome which is charging. Another crossover technique which has been used in the biological and food research but has just recently been adapted to semiconductor metrology and inspection that minimizes, if not eliminates the sample charging is environmental or high pressure scanning electron microscopy. It offers the advantage and possible application of higher landing energies or accelerating voltages, different signal forming and contrast mechanisms and charge neutralization. This method employs a gaseous environment to help neutralize the charge build-up that occurs under irradiation with the electron beam. Although very desirable for the charge neutralization, for various technical reasons, this methodology has not been in common use in nanometrology for semiconductor inspection or metrology until just recently [6]. This is a relatively new application of this technology to this area and a good deal still needs to be learned. This technology shows great promise in the inspection, imaging and metrology of nanometer-sized structures on optical photomasks in a charge-free operational mode. In addition, this methodology affords a path that minimizes, if not potentially eliminates, the need for charge modeling which is needed for higher accuracy measurements. As stated earlier, the modeling of charging is exceptionally difficult since each sample, instrument and operating mode can respond to charging in different ways. Therefore, this methodology, which effectively eliminates the charging, shows great potential if the optimal balance can be achieved in a reproducible manner. Further research needs to be undertaken to understand the ways to optimize the operating conditions for these instruments for nanotechnology research and metrology. .

References:

[1] Postek, M. T. 1984. Low Accelerating Voltage Inspection and Linewidth Measurement in the Scanning Electron Microscope. SEM/1984/III, SEM, Inc. 10651074.

[2] Postek, M. T. 1994. Critical Issues in Scanning Electron Microscope Metrology. NIST J. Res. 99(5): 641-671.[#83]

[3] Postek, M. T. 1997. The Scanning Electron Microscope. Handbook of Charged Particle Optics. (ed. Jon Orloff) CRC Press, Inc., New York. 363-399.

[4] Postek, M. T. and Joy, D. C. 1987. Sub-micrometer microelectronics dimensional metrology: scanning electron microscopy. NBS J. Res. 92(3): 205-228.

[5] Villarrubia, J. S. Vladar, A. E., Lowney, J. R. and Postek, M. T. 2002. A scanning electron microscope analog of scatterometry. SPIE 4689 304-312.

[6] Postek, M. T., Vladár, A. E. and Bennett M. H. 2002. Photomask Metrology: Has anything really changed? SPIE 22nd BACUS Symposium on Photomask Technology 4489:293-308.