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Computationally Effient Method for Determining Sub-Resolution Assist Feature Rules

IP.com Disclosure Number: IPCOM000202390D
Publication Date: 2010-Dec-15
Document File: 5 page(s) / 138K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for efficiently and accurately determining Sub Resolution Assist Feature (SARF) rules. The method involves incremental, iterative simulations on possible geometries using optical model for determining rules that would support most geometries for a given process. The proposed method, by iteratively picking features for simulation and careful selection of a search algorithm and grid, reduces the simulation resources/ time needed and eliminates any need for interpolation in rules which are most significant constraints/ disadvantages of the existing methods.

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Computationally Effient Method for Determining Sub -Resolution Assist Feature Rules

Disclosed is a method for efficiently and accurately determining Sub Resolution Assist Feature (SARF) rules.

Depth of Focus (DOF) has always been of great concern in Lithography. Insertion of SRAF (as shown in Fig. 1) or Scatter Bars is a common technique used to equalize DOF through the variety of geometries in a design. SRAFs are much smaller than the resolution capability of the process and hence do not print but at the same time they emulate a dense layout enhancing the DOF of features at larger pitch.

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Figure 1

The insertion of an additional SRAF between the main features is termed transition. The SRAFs are normally placed at a specified distance from a main feature but when the space cannot afford to have two SRAFs a single one is placed at the center of the two features or closer to a more critical edge/ feature. The transitions start with one SRAF in the center of two features with increasing number of SRAFs per edge as the space between the features increases (as shown in Fig. 2).

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Figure 2

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There are multiple ways of placing SRAFs in a design. One practical method has been placing the SRAFs based on a simple set of rules predetermined by experimental means or simulations. The method disclosed herein aims at a more efficient and effective method of determining the SRAF rules by through simulations than the existing methods.

The method involves use of a search algorithm to find out various stages of SRAF transitions (as shown in Fig 4). Following are the parameters to be determined

Maximum feature size to get SRAFs


SRAF width for each transition
SRAF transition points(Pitches) for each transition SRAF offset from main features for each transition SRAF spacing for each transition

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Figure 4

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In order to determine maximum feature size to get SRAFs, a search algorithm is used to search between a range of minimum CD (Critical Dimension) to much larger CD and simulate isolated features (features pitch -> larger of 2x of optical diameter or 10x of CD) in this range. The CD where the process variation becomes unacceptable is taken as maximum feature size for which SRAFs need to be inserted (Fig 5(a)).

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Figure 5

In order t...