Kent H. Nakagawa

Impact of 14-nm photomask uncertainties on computational lithography solutions

Abstract. Computational lithography solutions rely upon accurate process models to faithfully represent the imaging system output for a defined set of process and design inputs. These models rely upon the accurate representation of multiple parameters associated with the scanner and the photomask. Many input variables for simulation are based upon designed or recipe-requested values or independent measurements. It is known, however, that certain measurement methodologies, while precise, can have significant inaccuracies. Additionally, there are known errors associated with the representation of certain system parameters. With shrinking total critical dimension (CD) control budgets, appropriate accounting for all sources of error becomes more important, and the cumulative consequence of input errors to the computational lithography model can become significant. In this work, we examine via simulation the impact of errors in the representation of photomask properties including CD bias, corner rounding, refractive index, thickness, and sidewall angle. The factors that are most critical to be accurately represented in the model are cataloged. CD bias values are based on state-of-the-art mask manufacturing data, and other variable changes are speculated, highlighting the need for improved metrology and communication between mask and optical proxmity correction model experts. The simulations are done by ignoring the wafer photoresist model and show the sensitivity of predictions to various model inputs associated with the mask. It is shown that the wafer simulations are very dependent upon the one-dimensional/two-dimensional representation of the mask, and for three-dimensional, the mask sidewall angle is a very sensitive factor influencing simulated wafer CD results.

The impact of 14-nm photomask uncertainties on computational lithography solutions

Computational lithography solutions rely upon accurate process models to faithfully represent the imaging system output for a defined set of process and design inputs. These models, which must balance accuracy demands with simulation runtime boundary conditions, rely upon the accurate representation of multiple parameters associated with the scanner and the photomask. While certain system input variables, such as scanner numerical aperture, can be empirically tuned to wafer CD data over a small range around the presumed set point, it can be dangerous to do so since CD errors can alias across multiple input variables. Therefore, many input variables for simulation are based upon designed or recipe-requested values or independent measurements. It is known, however, that certain measurement methodologies, while precise, can have significant inaccuracies. Additionally, there are known errors associated with the representation of certain system parameters. With shrinking total CD control budgets, appropriate accounting for all sources of error becomes more important, and the cumulative consequence of input errors to the computational lithography model can become significant. In this work, we examine with a simulation sensitivity study, the impact of errors in the representation of photomask properties including CD bias, corner rounding, refractive index, thickness, and sidewall angle. The factors that are most critical to be accurately represented in the model are cataloged. CD Bias values are based on state of the art mask manufacturing data and other variables changes are speculated, highlighting the need for improved metrology and awareness.

OASIS vs. GDSII stream format efficiency

The OASIS format was designed to be a replacement for the GDSII stream format. Previous papers have reported that OASIS files can be 5-20X smaller than comparable GDSII files. This paper examines the storage capabilities of OASIS, as well as other benefits, in more detail. The primary focus of this study is on OASIS integers, deltas, point-lists, and its explicit support for rectangles & squares. We also show how the two OASIS integer types and four delta types can be implemented using a single core procedure.

OASIS: progress on implementing the new stream format for containing data size explosion

The data volumes of individual files used in the manufacture of modern integrated circuits have become unmanageable using existing data formats specifications. The ITRS roadmap indicates that single layer MEBES files in 2004 exceed 200 GB threshold, worst case. OASIS, the new stream format developed under the sponsorship of SEMI, has been approved in the industry-wide voting in June 2003. The new format that on average will reduce the file size by an order of magnitude, enables to streamline data flows and provides increased efficiency in data exchange. The work to implement the new format into software tools is in progress. This paper gives an overview on the new format, reports results on data volume reduction and is a report on the status and benefits the new format can deliver. A data flow relying on OASIS as the input and transfer format is discussed.

14-nm photomask simulation sensitivity

This study quantifies the impact of systematic mask errors on OPC model accuracy and proposes a methodology to reconcile the largest errors via calibration to the mask error signature in wafer data. First, we examine through simulation, the impact of uncertainties in the representation of photomask properties including CD bias, corner rounding, refractive index, thickness, and sidewall angle. The factors that are most critical to be accurately represented in the model are cataloged. CD bias values are based on state of the art mask manufacturing data while other variable values are speculated, highlighting the need for improved metrology and communication between mask and OPC model experts. It is shown that the wafer simulations are highly dependent upon the 1D/2D representation of the mask, in addition to the mask sidewall for 3D mask models. In addition, this paper demonstrates substantial accuracy improvements in the 3D mask model using physical perturbations of the input mask geometry when using Domain Decomposition Method (DDM) techniques. Results from four test cases demonstrate that small, direct modifications in the input mask stack slope and edge location can result in model calibration and verification accuracy benefit of up to 30%. We highlight the benefits of a more accurate description of the 3D EMF near field with crosstalk in model calibration and impact as a function of mask dimensions. The result is a useful technique to align DDM mask model accuracy with physical mask dimensions and scattering via model calibration.

New stream format: progress report on containing data size explosion

The data volumes of individual files used in the manufacture of modern integrated circuits have become unmanageable using existing data formats specifications. The ITRS roadmap indicates that single layer MEBES files in 2002 reached the 50 GB range, worst case. Under the sponsorship of SEMI, a working group was formed to create a new format for use in describing integrated circuit layouts in a more efficient and extendible manner. This paper is a report on the status and potential benefits the new format can deliver.

The impact of 14nm photomask variability and uncertainty on computational lithography solutions

Computational lithography solutions rely upon accurate process models to faithfully represent the imaging system output for a defined set of process and design inputs. These models rely upon the accurate representation of multiple parameters associated with the scanner and the photomask. Many input variables for simulation are based upon designed or recipe-requested values or independent measurements. It is known, however, that certain measurement methodologies, while precise, can have significant inaccuracies. Additionally, there are known errors associated with the representation of certain system parameters. With shrinking total CD control budgets, appropriate accounting for all sources of error becomes more important, and the cumulative consequence of input errors to the computational lithography model can become significant. In this work, we examine via simulation, the impact of errors in the representation of photomask properties including CD bias, corner rounding, refractive index, thickness, and sidewall angle. The factors that are most critical to be accurately represented in the model are cataloged. CD bias values are based on state of the art mask manufacturing data and other variables changes are speculated, highlighting the need for improved metrology and communication between mask and OPC model experts. The simulations are done by ignoring the wafer photoresist model, and show the sensitivity of predictions to various model inputs associated with the mask. It is shown that the wafer simulations are very dependent upon the 1D/2D representation of the mask and for 3D, that the mask sidewall angle is a very sensitive factor influencing simulated wafer CD results.

A novel mask-based approach to improve low-k1 corner and angle definition in alternating-aperture phase-shift mask structures

A novel approach to improve the imaging of the critical magnetic pole structure in the disk drive read head is introduced. A 90-degree sub-resolution opening is added to an alternating aperture phase shift mask to reduce a strong proximity effect in the non-Manhattan tapered section, while maintaining the enhanced printability of the linear segment of the pole region.. Simulation indicates that this opening provides a method to correct the observed distortion in the printed edge without reducing the effectiveness of the altPSM character of the pole itself. We have designed test patterns with this concept and built photomasks to evaluate mask manufacturability and to empirically test the impact of the 90-degree window on final pattern fidelity on wafer. Preliminary results indicate positive correction effects, as well as some potential issues which may be resolved using additional, established correction approaches.

Through pitch intensity balancing and phase error analysis of 193-nm alternating phase shift masks

An investigation of the predominant industry approaches to transmission balance and phase error through pitch of Alternating Aperture Phase-Shifting Mask manufacturing approaches has been conducted. Previous theoretical studies have shown both clear pattern bias and phase error changes through pitch. These variations are significant for the Low K1 applications. Several approaches have been proposed and discussed in previous papers, including undercut, asymmetric pattern biasing, mask phase-only, dual trench, SCAA, and others. Although much of the discussion has focused on lithographic process performance, some of the constraints in the mask making infrastructure may differentiate between processes of similar performance. Two manufacturable approaches, wet etch undercut and asymmetric pattern biasing, have been studied by electromagnetic field simulation to explore the across pitch performance at 193nm. This has been compared to experimental measurement of photomasks measured with a 193 Zeiss AIMS (Aerial Image Microscope System). Both mask fabrication approaches are compared to the simulations. The performance of both mask approaches to pattern bias and phase error was evaluated, and the feasibility of through pitch correction and its impact on design and manufacturability of the photomask is discussed.

Study of OPC for AAPSM reticles using various mask fabrication techniques

AAPSM masks require OPC correction through pitch in order to print a linear dark line response vs the design CDs. The masks also require correction for the clear intensity imbalance caused by the phased etched Qz wall edge. The clear intensity can be balanced by two approaches;(or a combination of the two) data biasing or wet undercut etching of the Qz etched opening. IC manufacturers would like to use one OPC model that will work for any mask fabrication approach. This paper shows that there is no OPC difference observed in either the aerial image or the printed image of several OPC learning patterns. The study includes CD through pitch for dense (1:1) L/S Patterns and Isolated Line CD vs line-space ratio. The images were analyzed for the dark line linearity, the clear CD balance though pitch, and the clear CD balance with focus (phase error effects -PES).