Applying Calibrated Models For Wafer Print Simulation Using Mask Optical Images
Publication Date: 2002-Nov-12
The IP.com Prior Art Database
The application of calibrated models to mask inspection images for wafer print simulation is described. In particular, two approaches for determining the appropriate thresholds are described. Also, the handling of isolated defects is considered. Use of these approaches can facilitate more automated and accurate mask inspection processes. In particular, it is possible to automatically compute defect severity scores using this approach without the need for human intervention to calibrate the model. Also, the same model can be used from the design stage (e.g. OPC) through to manufacturing.
1. Descriptive Title
Applying Calibrated Models for Wafer Print Simulation using Mask Optical Images
This disclosure presents a method to apply a localized variable threshold model, and in general any aerial image intensity derived localized calibrated models, to software that works on mask, or wafer, images of defect regions from inspection/review systems such as the Virtual Stepper® System from Numerical Technologies, Inc. This will eliminate the need for manual threshold selection, and make it possible to use a single model from design to fabrication. The calculation will be very fast because only the defect area needs to be simulated, which takes the advantage of localized calibrated model - only the aerial image intensity in the defect surrounding area is required in the simulation. In addition, it will significantly simply the way of using such software products. Using the localized calibrated model, the user can apply the model they calibrated in design directly without any additional steps.
The basic ideas are 1) to detect the edge location based on mask image, 2) use the reference image as the perfect image to determine the edge location, but 3) use aerial image intensity from defect image when applying calibrated model.
Effects: 1) Present a new application area for localized calibrated model. 2). Disclose a general method to apply localized calibrated model to defect printability simulation. 3) In particular, disclosed a general method to apply any localized calibrated model to mask defect printability simulation using gray scale mask images captured from inspection/review systems.
Background about calibrated models:
A calibrated model is a type of intensity-derived model. It consists of two components: the first component is the optical model (this also includes the kernel perturbation or convolution based diffusion model) to generate an aerial image intensity profile around each selected location, e.g. evaluation point; the second component is to apply the photo-resist/etching (PR) formula parameterized with a set of empirically tuned parameters P, on selected intensity points, to obtain the predicted wafer image. The intensity-derived calibrated model includes a fixed- threshold and a threshold-less deviation model, and the popular variable threshold model.
In the calibration procedure, 1). a test reticle with all desired patterns (line/space, contact, line end, phase-shifted edges, dense and isolated features, etc.) are used in wafer print under particular settings for real production (Figure 1). 2). The CDs on the wafer print were then measured, and the deviation d = wafer CD - target CD, were calculated. 3). For each sample point, intensities I at multiple surrounding locations based on some selection criteria are obtained from the aerial image simulated from the design layout. 4). Intensities I are obtained for all
sample points, which are used to fit into the PR formula f=f(I...