The requirements on the use of scanning electron microscopy (SEM) as a measurement technique for the process control of dimensional parameters are most challenging in the production of masks and wafers in the semiconductor industry Subtle details of SEM signal variations have to be thoroughly understood to be able to monitor and trace small process variations to changes in dimensional parameters of the features to be controlled This paper reviews the fundamentals, special performance features and applications of existing SEM image contrast simulation packages based on Monte Carlo methods

https://iopscience.iop.org/article/10.1088/0022-3727/42/18/183001/pdf

Model-based SEM for dimensional metrology tasks in semiconductor and mask industry

In the IC industry the mask cost and cycle time have increased dramatically since the chip design has become more complex and the required mask specification, tighter The lithography technology has been driven to 65-nm node and 90-nm product will be manufacturing in 2004, according to ITRS's roadmap However, the optical exposure tools do not extend to a shorter wavelength as the critical dimension (CD) shrinks In such sub-wavelength technology generation,
the mask error factor (MEF) is normally higher Higher MEF means that tighter mask specification is required to sustain the lithography performance The tighter mask specification will impact both mask processing complexity and cost The mask is no longer a low-cost process In addition, the number of wafers printed from each mask set is trending down, resulting in a huge investment to
tape out a new circuit Higher cost discourages circuit shrinking, thus, prohibits commercialization of new technology nodes

Mask cost and cycle time reduction

Mask defects are of increasing concern for future lithography generations. The improved resolution capabilities of immersion and EUV systems increase also the sensitivity of these systems with respect to small imperfections of the mask. Advanced mask technologies such as alternating phase shift masks (AltPSM), chromeless phase shift lithography (CPL), or "thick" absorbers on EUV masks introduce new defect types. The paper presents an application of rigorous electromagnetic field modeling for the study of typical defect printing mechanisms in ArF immersion lithography and in EUV lithography. For standard imaging and mask technologies, such as binary masks or attenuated phase shift masks, small defects usually print as linewidth or critical dimension (CD) errors with the largest effect at best focus. For AltPSM, CPL masks, and EUV masks this is not always the case. Several unusual printing scenarios were observed: placement errors due to defects can become more critical than CD-errors, defects may print more critical at defocus positions different from the center of the process window, the defect printing may become asymmetric through focus, and the risk of defect printing depends on the polarization of the used light source. Several simulation examples will demonstrate these effects. Rigorous EMF simulations in combination with vector imaging simulations are very useful to understand the origins of the observed defect printing mechanisms.

Mask defect printing mechanisms for future lithography generations

CD-SEM is the metrology tool of choice for patterning process development and production process control. We can
make these applications more efficient by extracting more information from each CD-SEM image. This enables direct
monitors of key process parameters, such as lithography dose and focus, or predicting the outcome of processing, such as
etched dimensions or electrical parameters. Automating CD-SEM recipes at the early stages of process development can
accelerate technology characterization, segmentation of variance and process improvements. This leverages the
engineering effort, reduces development costs and helps to manage the risks inherent in new technology. Automating
CD-SEM for manufacturing enables efficient operations. Novel SEM Alarm Time Indicator (SATI) makes this task
manageable. SATI pulls together data mining, trend charting of the key recipe and Operations (OPS) indicators, Pareto
of OPS losses and inputs for root cause analysis. This approach proved natural to our FAB personnel. After minimal
initial training, we applied new methods in 65nm FLASH manufacture. This resulted in significant lasting improvements
of CD-SEM recipe robustness, portability and automation, increased CD-SEM capacity and MT productivity.

Automated CD-SEM metrology for efficient TD and HVM

A systematic attempt has been undertaken to investigate the printability of mask defects for 100nm lithography using 193nm wavelength. The main purpose is the study of soft defects (particles), which are mimic-ed by programmed resist dots. We report on the impact of defects additive to the Cr line, within the size range of 60nm to 260nm, at reticle level. Printability of different phase and transmission defects is first assessed by simulation, using PROLITH v7.0. Also the influence of the defect area, its location, and its shape is investigated. Printing experiments are performed using QUASARTM and annular illumination, the preferred settings in combination with a binary reticle. We demonstrate that aerial image simulations and AIMS measurements can predict the qualitative trends in defect printability. A thorough quantitative correlation between printing, simulation and AIMS evaluation is presented.

Defect printability for 100-nm design rule using 193-nm lithography

The paper presents a revolutionary technology to inspect advanced contact layers. Instead of finding defects based on a size-dependent defect specification, defects are found according to their impact at the wafer CD result. The inspection methodology used is aerial imaging. The main advantage of this method is that only defects, which actually affect the wafer result, will be detected and classified. The paper presents first inspection results on contact layers designed for the 130nm and 90 nm technology node.

Aerial-image-based inspection of binary (OPC) and embedded-attenuated PSM

The industry roadmap for IC manufacturing at design rules of 90nm and below foresees low k1-factor optical lithography at 193nm exposure wavelength. Aggressive model-based OPC are being used more and more frequently in order to achieve the extremely tight mask CD specifications required by 90nm technology node. State-of-the-art mask inspection is challenged to detect CD defects close to metrology resolution. Inspection of OPC is critical; OPC feature dimensions are usually near or below the resolution limits of mask exposure. In addition, chrome defects can be semitransparent and change the intensity of light on the wafer. In this paper aerial-image based mask inspection is investigated and presented. The concept inspects a given mask based on its aerial image with selected wafer exposure conditions, thus “finds only defect which will print”. This paradigm shift in mask inspection philosophy provides the unique opportunities of verifying and controlling the entire aerial image generated by the inspected mask. As reticle enhancement techniques like OPC are designed to enhance the aerial image of a mask, this concept offers a comprehensive way of inspecting these techniques. The inspection is shifted from detecting every single minor change on mask to detecting what on mask could possibly impact the printing image quality on the wafer. In this paper an advanced application of aerial-image based mask inspection is discussed in more detail. As a standard, the Aera193 uses the best-focus aerial image for defect detection. From HNA mask inspection it is a well-known fact, that shifting the inspection off-focus, can provide a more sensitive detection. In the csase of aerial-image based inspection, going off-focus can be compared with lithography exposure out of focus. In other words, the lithography process window will be taken into account for defect detection. This methodology provides additional important information
·Understand process window printability of defects detected at best-focus
·Detect additional defects, which may print at the borders of the process window.
This information is of extreme value for wafer lithography and may help by decisions about lithography process and mask usage.
Focus of the paper is to analyze the application of aerial image-based off-focus inspection. Advanced OPC test plates are used to analyze detection at best and off-focus. The inspection results are compared to actual wafer results. Wafer lithography benefit is discussed.

Aerial-image-based off-focus inspection: lithography process window analysis during mask inspection

Manufacturing of reticles, which combine both OPC and PSM, is becoming more and more challenge. Materials cost is high, several accurate writing processes are needed and repair is almost impossible. This makes inspection a critical and very complicated process. This study describes an inspection of a test vehicle consisting of 55 cells targeted for sub- wavelength design rule technology. This study describes an inspection of the 55 cells test plate targeted for 0.17 micrometer design rule technology. The plate is written on a MoSi layer with 18% transmission for 248 nm lithography. The MoSi has higher transmittance in I-line and G-line that reduces the contrast between the MoSi and the glass (relative to the usual contrast in binary plates). The technique for inspection by Applied Materials RT8000ES 436nm die-to-database is described. The technique is based on expansion of the reduced dynamic range of gray level that results from the lower contracts, re-gaining the inspection capability. This paper reviews the results of G-line versus I-line inspection of high transmission PSM and describes the method of the sensitivity verification including CD defects analysis.

High-transmission PSM inspection sensitivity

Inspection of aggressive OPC represents one of the major challenges for today's mask inspection methodologies. Systems are phased with high-density layouts, containing OPC features far below the resolution limit of conventional inspection systems. This causes large amounts of false and nuisance defects, especially on production applications. The paper presents the use of Aera193, a new inspection system using aerial imaging as inspection methodology.

Inspection of aggressive OPC using aerial image-based mask inspection

Inspection is one of the major challenges in mask making, as it is one of its most performance crucial steps in the entire mask making process. Especially contact patterns show difficulties in die-to-database inspection as the CAD data asks for square corners. The paper presents the impact of Laser Proximity Correction (LPC) on the inspectability of contact and line patterns. LPC is a mask enhancement technique improving image quality and CD linearity for laser pattern generators. The use of the linewidth bias monitor tool in order to characterize CD uniformity over the entire plate is demonstrated.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Reuven Falah

Papers

Aerial-image-based inspection of binary (OPC) and embedded-attenuated PSM

Aerial-image-based off-focus inspection: lithography process window analysis during mask inspection

High-transmission PSM inspection sensitivity

Inspection of aggressive OPC using aerial image-based mask inspection

Improvements to mask inspectability by use of pattern proximity correction