Explain It in 30 Seconds
A semiconductor layer can look acceptable in isolation yet be misaligned with the layer underneath it. Small errors can make the finished device fail.
Optical metrology measures relevant structures and turns their optical response into information about the process. The results can help engineers correct subsequent exposures or investigate drift.
Speed matters because a fab must learn about problems before they affect large amounts of production. Measurement accuracy matters because a fast but misleading result can trigger the wrong correction.
Where It Sits in ASML
YieldStar sits in the observation and control layer around lithography. It complements scanners, computational models and electron-beam inspection.
Pre-etch and post-etch measurement answer different questions. One can examine the result in resist before further processing; the other can examine patterns after etch has changed the physical structure. [S1]
The portfolio therefore contains different systems and applications. One model's capabilities should not be assigned indiscriminately to every YieldStar product.
The Problem It Solves
Manufacturing processes drift. Equipment, materials and upstream operations can change the way patterns land on the wafer.
Without timely measurements, a line may continue producing wafers under conditions that have moved outside the acceptable process window. Discovering the problem only at final test can waste both materials and manufacturing time.
Optical measurement offers a way to observe relevant variation earlier and with sufficient sampling speed for production use.
How It Works — Simple Version
The instrument illuminates measurement structures and analyzes the resulting optical signals. Those signals carry information about the geometry or alignment of the patterns.
Models and recipes translate the response into useful measurements. Engineers or control software then use the information to identify variation and determine an appropriate correction. [S19][S55]
The measurement is not a conventional photograph of every device. Its value depends on selecting suitable structures, recipes and sampling locations.
Before and After a Feedback Loop
Without timely patterning measurement
A problem may persist across more wafers before downstream results reveal it. Engineers have fewer observations with which to separate scanner behavior from other process effects.
With an effective measurement-and-control loop
The fab can detect relevant deviations earlier and apply validated corrections. That can protect yield or reduce time spent diagnosing the process.
More measurements are not automatically better. Poor sampling or an inaccurate model can create false confidence or unnecessary adjustments.
Who Uses It?
Lithography, metrology and process-control engineers use the results. Production managers care about their effect on yield, cycle time and excursions.
The buyer is the manufacturing organization responsible for the wafer process. A measurement tool's value is realized through the decisions it improves, not through a standalone measurement count.
Different users may need different information: a development team investigating a new stack has a different task from a production team monitoring a qualified line.
Pre-Etch and Post-Etch Roles
A pre-etch measurement observes patterns before the subsequent etch process changes them. This can help separate exposure-related behavior from downstream effects.
Post-etch measurement observes a later physical state closer to the structures that matter to the finished device. It can reveal variation that would not be fully captured by examining resist alone. [S1]
The two views are complementary. Choosing between them depends on the control question, measurement feasibility and acceptable delay.
What It Depends On
The system needs suitable targets or measurable structures, a calibrated optical model, reliable wafer positioning and recipes appropriate to the material stack.
Integration with the fab's data and control systems is also important. A correct measurement that arrives too late or cannot be connected to an action may have limited production value.
Optical methods have limits in resolution and interpretability. Some questions need higher-resolution investigation using electron-beam techniques.
What Depends on It?
A patterning-control strategy can use optical data to maintain overlay and other relevant process conditions. Computational models can also use observations to improve their representation of wafer behavior.
That dependency is a control relationship, not proof that every production decision is fully automated. Engineers still establish recipes, validate corrections and investigate unexpected behavior.
Customer Economics
Customers balance precision, repeatability, sampling density and throughput. Slower measurement may provide rich information but reduce how often the fab can sample production.
ASML reported first shipment of YieldStar 1390 in 2025 for post-etch monitoring, with higher-throughput capability and recipe-related software improvements. That is a dated product milestone, not proof that every customer has deployed it. [S1][S55]
The commercial objective is a better decision soon enough to protect output. Cost per measurement matters alongside the value of the manufacturing problem avoided.
Key Milestones
Why It Matters to ASML
Measurement extends the company's role from producing an exposure to helping maintain a controllable manufacturing process. It creates another connection between customer learning and the scanner portfolio.
The market remains competitive. ASML's scanner integration can be useful, but does not remove the customer's need to compare measurement performance and application fit with other process-control options.
What Most People Misunderstand
Optical metrology is ordinary visual inspection
It uses measured optical responses and calibrated interpretation to answer specific process questions.
Measuring a wafer is the same as correcting it
Measurement provides information. An effective correction also requires diagnosis, control logic and a tool capable of making the change.
Faster measurement always improves yield
The information must be relevant, accurate and connected to a validated action.
Common Questions
Does YieldStar replace e-beam inspection?
No. The methods offer different information and speed-resolution trade-offs.
Why measure after etch?
The physical process can change the pattern after exposure, so a later observation can answer questions that resist measurement cannot.
Is every measurement an image of a complete chip?
No. Measurement strategies use appropriate structures and sampling plans.