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Photomask Metrology in the Era of Neolithography

IP.com Disclosure Number: IPCOM000125640D
Original Publication Date: 1997-Sep-01
Included in the Prior Art Database: 2005-Jun-09
Document File: 9 page(s) / 129K

Publishing Venue

National Institute of Standards and Technology

Related People

James Potzick: INVENTOR

Abstract

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 severely challenge traditional mask metrology techniques. Such high resolution imaging techniques as SEMs and scanning probe microscopes can present serious metrology 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 in neolithography.

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Photomask Metrology in the Era of Neolithography

         James Potzick National Institute of Standards and Technology Gaithersburg, Md. 20899

The appearance of smaller photomask feature sizes, high density patterns, and optical enhance- ments such as phase shifters and OPC features, and the increasing importance of subresolution mask characteristics, can severely challenge traditional mask metrology techniques. Such high resolution imaging techniques as SEMs and scanning probe microscopes can present serious me- trology 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 opti- mized 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 me- trology in context. This approach can lead to practical solutions of the mask metrology problems in neolithography.

KEY WORDS photomask, metrology, aerial image, microlithography, emulation, simulation, paleolithography, neolithography

Introduction The global economic engine that is the integrated circuit electronics industry requires a continuing increase in the density of electronic functions on silicon as its fuel. This has been ac- complished largely by reducing the part sizes in electronic circuits. Photomask metrology presents interesting new challenges as these wafer feature sizes and consequent mask feature sizes and toler- ances become ever smaller.

Photomask linewidth measurements are needed for vendor/buyer communication, for developing specifications and ensuring that products meet specifications, and sometimes for compliance with

legal requirements. These measurements are also needed for process monitoring for either optimiza- tion or stability, and for process modeling. A large par...