Inline Thickness Measurement for
Feed-Forward
Etch Process Control
Enabling Sub-Nanometer CD Uniformity Through Pre-Etch Film Characterization
As semiconductor devices continue to scale toward atomic dimensions, the margin for process variation has shrunk dramatically. At the 5 nm technology node, allowable critical dimension (CD) variation across the wafer is less than 0.5 nm—equivalent to just two or three silicon atoms. Traditional time-based etching cannot achieve this level of precision when incoming film thickness varies across the wafer.
This application note presents how non-contact eddy current thickness measurement, integrated directly into or upstream of plasma etch equipment, enables feed-forward process control. By characterizing the true thickness profile of incoming wafers, etch systems can dynamically adjust zone-specific parameters to compensate for upstream variations before they become yield-limiting defects.
SURAGUS imaging tools provide full-wafer thickness maps with over 5,000 measurement points in under 60 seconds, delivering the high-resolution data required for advanced etch compensation algorithms. This capability is particularly critical for high-aspect-ratio (HAR) etching in 3D NAND, staircase formation, and advanced logic gate patterning where thickness uniformity directly determines device performance and yield.
Table of Contents
The Critical Role of Etching in Semiconductor Manufacturing
Plasma etching is one of the most critical processes in semiconductor fabrication, responsible for transferring lithographically defined patterns into functional device structures. Every wafer undergoes dozens of etch steps before completion, and the precision of each step directly impacts device performance, reliability, and manufacturing yield.
The fundamental goal of any etch process is to remove material uniformly across the wafer while maintaining precise control over:
- Etch depth — Removing exactly the intended amount of material
- Critical dimensions (CD) — Preserving pattern fidelity from lithography – **Profile control** — Achieving vertical sidewalls or controlled taper angles –
- Selectivity — Etching target materials while preserving masks and underlayers
- Uniformity — Achieving consistent results from center to edge
The Uniformity Challenge
Modern etch equipment manufacturers have developed sophisticated technologies to achieve uniform plasma conditions across the wafer. These include symmetric chamber designs, multi-zone temperature control, tunable gas injection, and advanced RF power delivery systems.
However, even a perfectly uniform etch process will produce non-uniform results if the incoming film is non-uniform. Variations in film thickness, density, stress, or composition from upstream deposition processes propagate directly into post-etch CD variation.
This application note addresses how pre-etch thickness measurement enables etch equipment to actively compensate for incoming variations, transforming a reactive process into a proactive, self-correcting system.
The Problem: Incoming Film Variation
Sources of Film Non Uniformity
Thin films deposited by CVD, PVD, ALD, or electrochemical deposition inherently exhibit some degree of non uniformity.
Common sources include:
| Source | Typical Pattern | Impact on Etch |
|---|---|---|
| Gas flow distribution | Center to edge gradient | Radial CD variation |
| Temperature gradients | Radial or azimuthal | Variable etch rate |
| Target erosion (PVD) | Progressive drift | Wafer to wafer variation |
| Precursor depletion | Center thick or edge thick | Non uniform clear time |
| Chamber asymmetry | Azimuthal variation | Rotational CD pattern |
| Wafer clamping effects | Edge roll off | Edge yield loss |
For a typical film with 2 to 3 percent thickness non uniformity on a 300 mm wafer, thickness variation can exceed 5 to 10 nm,
far greater than the sub nanometer CD tolerances required at advanced nodes.
Consequences of Uncompensated Variation
Under Etching
Regions with thicker than nominal film do not fully clear during the standard etch time.
Residual material causes electrical shorts, pattern defects, or blocked contacts.
These wafers require rework or become scrap.
Over Etching
Regions with thinner than nominal film clear early and continue to be exposed to the plasma.
This causes:
- Erosion of the underlying layer
- CD loss as sidewalls are attacked
- Increased line edge roughness (LER)
- Damage to sensitive device structures
Non Uniform Critical Dimensions
Even when all regions eventually clear, the variable time to clear results in differential sidewall exposure.
Areas that clear early experience more lateral etching, resulting in larger CDs than areas that clear late.
Reduced Process Window
To ensure all regions clear without excessive over etch, engineers must operate with tight margins.
This reduces throughput (conservative etch times) and limits the process window for other optimization.
Limitations of Time Based Etching
Traditional etch processes rely on fixed time recipes developed from historical data.
While endpoint detection systems can identify when etching is complete, they typically provide only a single wafer average signal and cannot resolve spatial variation across the wafer.
Time based etching assumes:
- Incoming film thickness is constant (wafer to wafer)
- Incoming film uniformity is constant (within wafer)
- Chamber conditions are stable (run to run)
In practice, none of these assumptions hold perfectly, and the cumulative variation consumes the error budget allocated to etch.
The Solution: Feed Forward Etch Control
Concept of Feed Forward Control
Feed forward control is a process strategy where upstream measurements are used to adjust downstream process parameters before the process begins.
Unlike feedback control, which reacts to post process measurements, feed forward control prevents errors from occurring in the first place.
For etch applications, feed forward control uses pre etch thickness measurements to:
- Characterize the incoming film thickness profile across the entire wafer
- Calculate the required compensation for each zone or region
- Adjust etch parameters (time, temperature, power) on a zone by zone basis
- Execute a customized etch recipe that produces uniform results regardless of incoming variation
This approach effectively creates a mirror image correction, regions with thicker film receive more aggressive etching, while regions with thinner film receive gentler treatment.
Requirements for Effective Feed Forward Control
Successful implementation of feed forward etch control requires thickness measurement that provides:
| Requirement | Rationale |
|---|---|
| Full wafer coverage | Variations occur everywhere, including edges |
| High spatial resolution | Must resolve local features, not just radial trends |
| Non contact measurement | Cannot damage or contaminate production wafers |
| High throughput | Must not become a bottleneck in production flow |
| Production compatibility | Must operate in fab environment with minimal footprint |
| Data integration | Must interface with etch equipment control systems |
Traditional metrology approaches such as ellipsometry or four point probe provide only sparse sampling (typically 9 to 49 points) and often cannot measure the critical edge region.
These limitations make them unsuitable for advanced feed forward applications.
SURAGUS Eddy Current Technology
Measurement Principle
SURAGUS thickness measurement systems utilize high frequency eddy current technology to characterize conductive thin films non destructively.
When an alternating electromagnetic field is applied near a conductive layer, eddy currents are induced in the material.
The magnitude and phase of these currents depend on the film’s sheet resistance, which is directly related to thickness for films of known resistivity.
Key advantages of eddy current measurement include:
- Non contact operation, no mechanical contact with the wafer surface
- No consumables, no probe tips to replace or calibrate
- High speed, measurements in milliseconds per point
- Robust operation, tolerant of wafer bow, warpage, and surface conditions
- Full wafer capability, including edge regions inaccessible to contact probes
Measurement Modes
SURAGUS sensors operate in two primary configurations:
Transmission Mode (Double Sided)
Sensors positioned above and below the wafer measure the combined response through the entire wafer stack.
This mode offers:
- Large sensor to wafer gaps (up to 100 mm)
- High tolerance to wafer position variation
- Ideal for robot handling and tool integration
- Suitable for thin and thick wafers without setup changes
Reflection Mode (Single Sided)
A sensor positioned on one side of the wafer measures the surface response.
This mode provides:
- Smaller spot sizes (down to 1 mm)
- Surface sensitive measurements
- Depth profiling capability for thick substrates
Product Portfolio for Etch Integration
EddyCus® TF Inline Sensorline
Designed for integration into process equipment, the inline sensor series provides:
- Measurement speeds up to 50 readings per second
- Fixed or traversing sensor installations
- Integration of 1 to 99 monitoring lanes per system
- Direct interface to process control systems
- Operation in atmosphere or vacuum
EddyCus® Sensor Integration Kits
For OEM equipment manufacturers, SURAGUS provides sensor integration kits that include:
- Compact sensor heads optimized for tool integration
- Electronics modules for signal processing
- Software APIs for data communication
- Application engineering support
Measurement Performance
| Parameter | Specification |
|---|---|
| Measurement speed | Up to 50 points per second |
| Spatial resolution | Down to 0.1 mm pitch |
| Data points per wafer | 5,000+ (200 mm) to 30,000+ (300 mm) |
| Full wafer scan time | Less than 60 seconds |
| Thickness range | Sub nm to hundreds of µm |
| Repeatability | Less than 0.1% (1σ) |
| Edge exclusion | As low as 1 mm |
Integration Architectures
In Tool Integration
The most powerful implementation integrates SURAGUS sensors directly into the etch equipment, measuring wafers immediately before they enter the process chamber.
┌─────────────────────────────────────────────────────────────┐ │ ETCH EQUIPMENT │ │ ┌──────────┐ ┌──────────────┐ ┌──────────────────┐ │ │ │ Loadport │───│ SURAGUS │───
│ SURAGUS │───Advantages:
- Zero additional handling or transport
- Minimal cycle time impact
- Tightest possible correlation between measurement and process
- Single equipment footprint
Standalone Pre Etch Measurement
For fabs seeking flexibility or phased implementation, SURAGUS offers standalone measurement systems that operate upstream of etch equipment.
┌──────────────┐ ┌──────────────┐ ┌──────────────┐ │ Deposition │────────
Advantages:
- Can serve multiple etch tools
- No modification to existing etch equipment
- Enables lot based or wafer based recipe selection
- Provides data for deposition process monitoring
Hybrid Approaches
Some implementations combine both approaches:
- Standalone mapping for comprehensive characterization
- In tool sensors for real time verification and fine adjustment
Application Examples
3D NAND High Aspect Ratio Channel Hole Etch
The Challenge
3D NAND fabrication requires etching channel holes through stacks of 200+ alternating oxide and nitride layers, reaching depths of 10 µm or more with aspect ratios exceeding 100:1.
More than one trillion holes must be etched simultaneously and uniformly on every wafer.
The multilayer stack is built through repeated deposition cycles, and thickness variations accumulate through the stack.
A 1 percent non uniformity in each deposited layer compounds to significant total stack height variation.
Impact of Thickness Variation
- Incomplete etch: thicker regions may not reach the bottom, creating open circuits
- Over etch: thinner regions punch through to the substrate, damaging the device
- CD variation: variable stack height causes variable hole diameter at the bottom
- Profile distortion: bowing and twisting affect device geometry
Feed Forward Solution
Pre etch measurement of total stack thickness enables:
- Adjustment of etch time by zone to ensure uniform breakthrough
- Modification of ion energy profiles to compensate for depth variation
- Optimization of cryogenic temperature distribution for uniform passivation
Results
Fabs implementing feed forward control for HAR etch have demonstrated:
- Reduction of CD variation by 30 to 50 percent
- Improved edge yield by 0.5 to 2 percent per wafer
- Reduced over etch margin requirements, improving throughput
3D NAND Staircase Etch
The Challenge
The staircase structure in 3D NAND provides contact pads for each wordline layer.
Formation requires repeated cycles of lithography trimming and vertical etching, with each step removing one layer pair.
Thickness errors accumulate through dozens of cycles.
Impact of Thickness Variation
- Step height variation: inconsistent stair dimensions affect contact formation
- Cumulative error: small per step errors multiply through 100+ steps
- Overlay challenges: variable topography complicates subsequent lithography
Feed Forward Solution
Measuring the incoming stack thickness before staircase formation enables:
- Adjustment of per step etch depth to achieve uniform stair heights
- Compensation for layer to layer thickness variation
- Prediction and prevention of cumulative error buildup
Advanced Logic Gate Etch
The Challenge
Gate patterning at the 7 nm node and below requires CD uniformity approaching atomic dimensions.
High NA EUV lithography uses thinner photoresists (less than 30 nm), demanding extreme selectivity from the etch process.
Impact of Thickness Variation
- Gate length variation: directly affects transistor performance
- Threshold voltage spread: non uniform gates cause Vt variation
- Timing uncertainty: variable gate dimensions impact circuit timing
Feed Forward Solution
Pre etch measurement of hard mask thickness enables:
- Zone specific etch rate adjustment for uniform CD
- Compensation for lithography non uniformity
- Optimization of selectivity vs profile tradeoffs by region
DRAM Capacitor Etch
The Challenge
DRAM capacitors require extremely high aspect ratio structures to maximize capacitance within limited footprint.
As DRAM scales, capacitor height increases while diameter shrinks, pushing aspect ratios beyond 50:1.
Impact of Thickness Variation
- Capacitance variation: non uniform height causes memory cell mismatch
- Structural integrity: variable etch can weaken tall structures
- Yield loss: incomplete etch creates shorts, over etch causes opens
Feed Forward Solution
Measuring dielectric stack thickness before capacitor etch enables:
- Uniform etch depth across the memory array
- Consistent capacitor height for matched electrical properties
- Optimized process margins for yield improvement
Hard Mask Open and Pattern Transfer
The Challenge
Hard masks (SiO2, SiN, TiN, SiOC) are used when photoresist alone cannot withstand the main etch process.
The hard mask open step must precisely transfer the resist pattern without distortion.
Impact of Thickness Variation
- Pattern distortion: variable hard mask thickness causes variable etch time
- CD bias: non uniform clearing affects final dimensions
- Mask consumption: over etch erodes mask, degrading selectivity
Feed Forward Solution
Pre etch measurement of hard mask thickness enables:
- Optimized etch time to minimize over etch while ensuring clearing
- Zone specific adjustment for uniform pattern transfer
- Extended effective mask lifetime
Implementation Benefits
Yield Improvement
Feed forward etch control directly improves die yield through:
| Mechanism | Typical Improvement |
|---|---|
| Reduced CD variation | 30 to 50 percent reduction in within wafer variation |
| Edge yield recovery | 0.5 to 2 percent more good die per wafer |
| Reduced defectivity | Fewer under etch and over etch defects |
| Tighter distributions | More die within specification limits |
For a fab processing 50,000 wafers per month, a 1 percent yield improvement translates to 500 additional good wafers, representing millions of dollars in additional revenue.
Process Window Enhancement
By compensating for incoming variation, feed forward control effectively widens the process window:
- More tolerance for upstream variation, etch compensates rather than propagates errors
- Reduced need for over etch margin, precise thickness knowledge enables precise timing
- Greater flexibility in recipe optimization, can optimize for other parameters when uniformity is assured
Throughput Improvement
Adding a measurement step can improve overall throughput:
- Reduced rework, fewer wafers require re processing
- Shorter etch times, reduced over etch margin enables faster processing
- Fewer test wafers, production wafer data replaces monitor wafers
- Faster process development, high resolution data accelerates recipe optimization
Cost Reduction
| Category | Cost Impact |
|---|---|
| Scrap reduction | Direct material savings |
| Rework elimination | Labor and equipment time savings |
| Test wafer reduction | Consumable cost savings |
| Extended chamber life | Reduced over etch means less chamber wear |
| Faster qualification | Reduced time to production for new processes |
Process Intelligence
Beyond real time control, thickness data provides valuable process intelligence:
- Deposition monitoring, pre etch data reveals upstream process drift
- Correlation analysis, link incoming variation to final device performance
- Predictive maintenance, track deposition tool health through film uniformity trends
- Root cause analysis, distinguish etch induced vs deposition induced variation
Data Integration and Communication
Equipment Interfaces
SURAGUS measurement systems support multiple interface options for integration with etch equipment and factory systems:
| Interface | Application |
|---|---|
| SECS/GEM | Standard semiconductor equipment communication |
| OPC UA | Modern industrial automation protocol |
| TCP/IP | Custom socket communication |
| File based | CSV, XML, or custom formats for offline analysis |
| Analog outputs | 4 to 20 mA or 0 to 10 V for simple integrations |
Data Formats
Thickness maps can be provided in formats optimized for different use cases:
- Full wafer maps, complete thickness at every measured point
- Zone averages, mean thickness for equipment control zones
- Uniformity metrics, statistical summaries (range, σ, percent NU)
- Compensation tables, pre calculated adjustment values for equipment recipes
Real Time vs Batch Processing
Real Time Integration
- Thickness measured immediately before etch
- Data transmitted directly to equipment controller
- Recipe adjusted on the fly for each wafer
Batch Processing
- Thickness measured at upstream station
- Data stored in factory MES or database
- Recipe selected or adjusted when wafer arrives at etch
Getting Started
Feasibility Assessment
SURAGUS offers feasibility studies to evaluate the potential benefit for specific applications:
- Sample measurement: provide representative wafers for thickness mapping
- Data analysis: quantify existing uniformity and identify patterns
- Benefit estimation: model expected improvement from feed forward control
- Integration planning: define optimal implementation architecture
Pilot Implementation
A phased approach minimizes risk while demonstrating value:
Phase 1: Offline Characterization
- Standalone measurement of incoming wafers
- Correlation of thickness data with etch results
- Validation of measurement accuracy and repeatability
Phase 2: Recipe Optimization
- Use thickness data to develop improved recipes
- Implement wafer binning or lot based recipe selection
- Quantify yield improvement from classification approach
Phase 3: Full Feed Forward Integration
- Integrate sensors into etch equipment
- Implement real time recipe adjustment
- Realize full benefit of zone by zone compensation
Support and Partnership
SURAGUS provides comprehensive support for implementation:
- Application engineering: process specific measurement optimization
- Integration support: hardware and software integration assistance
- Training: operator and engineer training programs
- Ongoing service: calibration, maintenance, and upgrades
Conclusion
As semiconductor manufacturing pushes toward atomic scale precision, process control strategies must evolve beyond traditional time based approaches. Feed forward etch control, enabled by high resolution pre etch thickness measurement, represents a fundamental advancement in achieving the uniformity required for advanced devices.
SURAGUS eddy current technology provides the non contact, full wafer, high throughput measurement capability essential for implementing feed forward control in production environments. By characterizing incoming film conditions before etching begins, fabs can:
- Compensate for upstream variation rather than propagating it
- Achieve sub nanometer CD uniformity across the wafer
- Improve yield, throughput, and process margins simultaneously
- Gain process intelligence for continuous improvement
Whether integrated directly into etch equipment or deployed as standalone measurement stations, SURAGUS thickness measurement systems enable the transition from reactive to proactive process control, a critical capability for success at advanced technology nodes.