The appearance of smaller photomask feature sizes, high density patterns, and optical enhancements such as phase shifters and OPC features, and the increasing importance of subresolution mask characteristics, can render traditional mask metrology techniques inadequate. Such high resolution techniques as SEMs and scanning probe microscopes can present serious problems in modeling, throughput, and accuracy. Accordingly, photomask metrology must be examined in the larger context of mask design, specification, manufacture, and application. Critical mask features may not always be measured directly as in the past. Instead, their optical effects can be measured and compared with specifications. Discrepancies indicate needed corrections in mask fabrication process control. In the early days of integrated circuit lithography, an era which might be termed paleolithography, the pattern on the photomask was transferred conformally to the wafer. Mask CDs were measured directly in a metrology microscope and represented their corresponding wafer CDs. The optical transform from mask to wafer was simply the scalar exposure system reduction ratio. In the present era of neolithography, with wafer CDs near the exposure wavelength, this is no longer the case. The wafer exposure system performs a nonlinear optical transform on the mask pattern to produce an aerial image in the resist. OPC and mask phase shifters are manifestations of the inverse of this transform. The image in the developed resist depends on the mask pattern and the exposure and development parameters in nonlinear ways. These parameters can be optimized in software, but their effects are equally important in both mask design and mask metrology. A conceptualization of the photomask life cycle will be presented showing the role of mask metrology in context. This approach can lead to practical solutions of the mask metrology problems of neolithography.
Proceedings Title: Proceedings of SPIE, 17th Annual BACUS Symposium on Photomask Technology and Management, James A. Reynolds, Brian J. Grenon, Editors
Conference Dates: September 17, 1997
Conference Location: Redwood City, CA
Conference Title: Bacus '97 Special Focus Program: Understanding And Meeting Cd Specifications For Advanced Reticles
Pub Type: Conferences
aerial image, exposure emulation, metrology, microlithography, neolithography, photomask