In Situ Thin Film Monitoring — Real-Time Metrology Guide
In situ thin film monitoring measures a film while the coating or drying process is running — not after. The difference is the difference between adjusting a process parameter in real time and discovering a problem 200 wafers later in the quality lab. This guide covers the two in situ monitoring routes TDM offers, how they integrate with production equipment, and which configuration matches your process.
What Is In-Situ Thin Film Monitoring?
In production environments where thin films are deposited or coated, in situ monitoring provides real-time feedback on film properties — thickness, reflectance, uniformity — without stopping the process or removing samples. This section explains the principle and the two routes TDM offers.
What "in-situ" means — and what it does not
In semiconductor and PV manufacturing, "in-situ" is sometimes used loosely to mean any measurement taken during production. Strictly speaking, in situ metrology means the measurement happens at the point of processing, on the actual production sample, while the process is running. This is distinct from:
- In-line metrology — the sample is removed from the production line, measured in a nearby station, then returned. Still fast, but not real-time.
- Off-line metrology — samples are taken to a separate quality lab, measured, and results come back hours or days later.
The value of true in situ monitoring is closed-loop process control. When you can measure film thickness every second during slot-die coating, you can adjust pump speed, gap height, or substrate velocity before the batch is ruined — not after.
The two in-situ monitoring routes
TDM offers two distinct in situ measurement approaches, each suited to a different stage of thin-film production:
| Route | Stage | What it measures | Key instrument |
|---|---|---|---|
| Wet-film route | During coating (before drying) | Wet-film thickness via reflectance spectroscopy | SE31 / SE310 |
| Dried-film route | During vacuum drying / annealing | Dried-film thickness + photoluminescence | VCD |
| Wide-band optical route | Coating or post-process | Reflectance + PL across a broad spectral range | SE31-FM |
The wet-film route gives you the earliest possible feedback — adjust the coater before the film dries. The dried-film route confirms what you actually got after solvent evaporation and annealing. Some lines run both.
How the wet-film route works: SE31 and SE310
During slot-die coating, the wet film is a liquid layer whose thickness determines the final dried-film thickness after solvent evaporation. The SE31 and SE310 measure this wet-film thickness optically — by analyzing the reflectance spectrum of the film in real time.
Principle: White light is directed at the wet film. The reflected spectrum contains interference fringes whose spacing is proportional to film thickness. A spectrometer captures this spectrum continuously, and the instrument converts fringe spacing to thickness using the known refractive index of the coating solution.
Why this matters for production: If the target dried thickness is 500 nm and your solution leaves 20% solids after drying, the wet-film target is 2,500 nm. If the SE31 reads 2,200 nm at the center of the substrate, the operator adjusts pump flow immediately — not after the batch comes out of the oven wrong.
SE31 vs SE310:
- SE31: Entry-level wet-film monitor, single-point measurement, suitable for R&D and pilot lines.
- SE310: Multi-point or scanning measurement across the substrate width, designed for full production lines where coating uniformity edge-to-edge matters.
Published SE310 performance specifications
The SE310 is the production-grade instrument in the wet-film series. Published specifications (confirm with TDM for your specific coating chemistry and substrate):
| Parameter | Specification |
|---|---|
| Measurement range (wet film) | 0.5–50 μm (chemistry-dependent) |
| Spot size | ~2 mm |
| Measurement rate | Up to 10 Hz (10 readings per second) |
| Spectral range | 400–1,000 nm |
| Working distance | 15–25 mm from coating surface |
| Integration | Analog output + digital (RS-232 / Ethernet) for closed-loop coater control |
Note: Actual range and accuracy depend on the coating solution's refractive index, substrate reflectivity, and film uniformity. Always validate with your specific chemistry. Contact TDM application engineers for a feasibility test.
The VCD route: monitoring during vacuum drying
After coating, the wet film goes through vacuum drying (VCD) to remove solvent and, in some processes, initiate crystallization or phase formation. The film shrinks as solvent leaves — a 2,500 nm wet film may become a 500 nm dried film. Measuring during this transition tells you whether the drying recipe is working.
The VCD instrument monitors:
- Dried-film thickness — via reflectance spectroscopy, same principle as SE31 but calibrated for solid films.
- Photoluminescence (PL) — laser excitation of the film during or after drying reveals material quality (defect density, bandgap uniformity, phase purity). This is the same PL principle used in TDM's offline PV inspection instruments, now applied in situ.
Measuring both thickness and PL during drying catches problems that a thickness-only measurement misses: a film may hit the right thickness but have poor optoelectronic quality because the drying rate was wrong or the atmosphere had trace contaminants.
The wide-band optical route: SE31-FM
The SE31-FM extends the optical approach to a broader spectral range (UV through NIR), combining reflectance and photoluminescence in a single measurement head. This is designed for processes where film properties change during coating or post-processing and multiple optical signatures are needed to track them:
- Reflectance → thickness (same principle as SE31/SE310)
- PL intensity and spectral shape → material quality, defect density, composition
- Transmittance (with a detector on the opposite side) → absorption edge, bandgap
The SE31-FM is typically deployed in R&D and pilot lines where the process is still being developed and engineers need more data to correlate optical signatures with final device performance.
How to choose an in situ configuration
| Your situation | Recommended route | Instrument |
|---|---|---|
| Slot-die coating, need real-time thickness to control the coater | Wet-film | SE31 (R&D) or SE310 (production) |
| Need to verify dried-film thickness and material quality after drying | Dried-film | VCD |
| Developing a new process, need reflectance + PL + transmittance | Wide-band optical | SE31-FM |
| Full production line, want closed-loop control end-to-end | Wet-film + dried-film | SE310 at coater + VCD after dryer |
| Perovskite or OPV research, need bandgap and quality data during processing | Wide-band | SE31-FM |
Frequently asked questions
How is in situ monitoring different from taking a sample to the lab?
In situ gives you data every second during the process. Lab measurement gives you one data point per sample, after the fact. If your coating process drifts, in situ monitoring catches it immediately; lab measurement catches it after the batch is done. For a production line running 1,000 substrates per day, a 30-minute delay between coating and lab results means up to 500 substrates could be coated with the wrong thickness before anyone knows.
Can the SE31 measure any coating chemistry?
The SE31 works with any coating that forms a transparent or semi-transparent wet film on a reflective or partially reflective substrate. Opaque coatings (carbon-based, metal pastes) do not produce usable interference fringes. Always run a feasibility test with your specific chemistry — TDM application engineers can do this on your samples.
What is the accuracy of in situ thickness measurement?
Published accuracy for the SE310 is typically ±2% of reading or ±20 nm (whichever is larger), but this assumes the refractive index of the coating solution is well-characterized and stable during the measurement. If your solution's refractive index changes with temperature or solvent evaporation rate, accuracy degrades. The VCD route on dried films is generally more accurate (±1% or ±5 nm) because the optical constants of solid films are more stable.
Do I need both wet-film and dried-film monitoring?
If you only monitor wet-film thickness, you know what went into the dryer but not what came out. If you only monitor dried-film thickness, you know the result but cannot adjust the coater in time to fix it. Lines that need tight thickness control run both: SE310 at the coater for real-time adjustment, VCD at the dryer exit for quality verification.
Can the VCD measure PL during vacuum drying, or only after?
The VCD is designed to measure PL during the drying process — this is its key advantage over taking a sample to a separate PL instrument. The measurement head is integrated into the vacuum chamber and can collect PL spectra at programmed intervals as the film dries.
Which instruments are available for integration into an existing coater or dryer?
SE31, SE310, VCD, and SE31-FM are all designed for integration into production equipment. They provide analog and digital outputs for connection to coater/dryer control systems. Physical integration (mounting brackets, viewport requirements, working distance) should be discussed with TDM's application engineering team during the specification phase.
Related guides and products
- How to Measure Thin Film Thickness Guide
- Thin Film Thickness Measurement Guide
- Transmittance and Reflectance Measurement Guide
- EL vs PL Inspection Guide
- PV Inspection products
- SE310 Slot Die Wet Film Metrology Module
- Request a consultation
