This is a sponsored post, by Sheldon Wayman, Product Manager, INFICON
Perovskite solar cells have made rapid gains in efficiency, with both single-junction and tandem configurations continuing to set new performance records. As the industry moves toward commercial-scale production, thermal evaporation has emerged as a leading deposition method for many of the layers in the perovskite device stack. This includes the perovskite absorber itself (via co-evaporation of precursors such as PbI₂, CsBr, and MAI), metal contacts, hole and electron transport layers, and buffer layers.
For manufacturers accustomed to chemical vapor deposition and solution-based processes, thermal evaporation introduces a different set of process control requirements. Deposition rates in perovskite manufacturing are often very low, materials can be exotic with poorly characterized physical properties, and multi-source co-evaporation demands simultaneous rate control of each material. The technology that addresses these requirements is quartz crystal microbalance (QCM) monitoring. QCM has decades of proven use in optical coating and OLED manufacturing but is relatively new to the solar industry. Understanding how it works and what it enables is becoming essential as perovskite fabs scale up.
This is especially relevant given one of the central challenges facing perovskite solar today. Long-term cell stability remains an active area of development for the industry. Perovskite devices are sensitive to the thickness and composition of each layer in the device stack. Variations in absorber thickness, transport layer uniformity, or buffer layer coverage can accelerate degradation mechanisms that shorten cell lifetime in the field. Accurate deposition control is not just a manufacturing efficiency concern. It directly affects how long the finished solar cell will perform at its rated output.
How Quartz Crystal Microbalance Monitoring Works
A QCM sensor uses a thin quartz crystal disc as a precision mass detector. Quartz is piezoelectric, meaning that when an alternating voltage is applied across electrodes on the crystal faces, it vibrates at a characteristic resonant frequency. This frequency is determined by the crystal's thickness, density, and elastic properties. For a standard AT-cut quartz crystal used in thin film monitoring, the fundamental frequency is typically near 6 MHz.
When material is deposited onto the crystal surface during an evaporation process, the added mass decreases the crystal's resonant frequency. This relationship was first formalized by Günter Sauerbrey in 1959, where the change in frequency is directly proportional to the mass deposited per unit area. The Sauerbrey equation provided the original foundation for QCM measurement, but it assumes the deposited film behaves acoustically like the quartz crystal itself. This assumption holds only for very thin films, roughly the first 5% of the crystal's frequency shift. Beyond that point, the acoustic impedance mismatch between the deposited material and the quartz becomes significant, and the Sauerbrey equation loses accuracy.
To address this limitation, the Z-match equation was developed. The Z-match equation accounts for the acoustic impedance ratio (Z-ratio) between the film and the crystal, removing the thin-film restriction and enabling accurate thickness measurement across the full usable life of the crystal. This is particularly important in production environments where crystals are driven well past 5% frequency shift before replacement. Modern QCM controllers, including those from INFICON, use the Z-match equation as their standard calculation method. In practice, this means that as atoms or molecules from an evaporation source land on the QCM crystal, which is positioned near the substrate inside the vacuum chamber, the sensor continuously reports how much material has accumulated and at what rate.
This is a fundamentally different approach from post-process metrology. Rather than measuring film thickness after a substrate exits the chamber, QCM provides real-time feedback during deposition. For processes where thickness tolerances are measured in angstroms and rate stability must be maintained below 1 Å/s, this in-situ measurement is critical. It enables closed-loop control of evaporation sources, where the deposition controller adjusts source power in response to measured rate deviations, keeping the process within specification throughout the run.
The sensitivity of a QCM system depends on the stability and resolution of its oscillator electronics, the quality of the quartz crystal, and the thermal environment of the sensor. Film stress, temperature gradients, and material properties all affect measurement accuracy, which is why sensor design matters as much as the electronics behind it.
Why Perovskite Manufacturing Demands Robust Sensor Design
Perovskite evaporation environments present specific challenges that push QCM sensors beyond what many conventional designs can handle.
First, operating temperatures are high. Substrate and chamber temperatures during perovskite evaporation can reach well above 200°C, and radiant heat from evaporation sources adds further thermal load to anything inside the chamber. This matters because quartz crystals are sensitive to temperature. Every AT-cut quartz crystal has a frequency versus temperature (FT) characteristic curve, and the crystal delivers stable, accurate readings only within a narrow temperature window around its designed operating point. A standard QCM crystal optimized for 70°C, for example, provides stable frequency output in a range of roughly 59 to 81°C. Outside that window, temperature-induced frequency shifts introduce measurement error that the deposition controller cannot distinguish from actual mass changes on the crystal surface.
For processes that operate at elevated temperatures, specialized high-temperature crystals are available with FT curves optimized for higher operating points such as 120°C, 240°C, or 285°C. These crystals maintain frequency stability at temperatures where a standard crystal would exhibit drift, mode hopping, and eventual failure. However, the stable operating range narrows as the optimization temperature increases, which makes thermal management of the sensor environment even more critical.
Thermal shock is another consideration in evaporation processes. When a source or sensor shutter opens, the crystal is suddenly exposed to a large increase in radiant heat. This causes a rapid jump in crystal temperature and film stress, which produces a spike in the measured rate and thickness values. In processes with very thin target layers, this spike can cause incorrect thickness termination or control loop instability. The effect is most pronounced on the first layer deposited onto a fresh crystal, when the thermal environment changes most dramatically. Sensor designs that provide effective water-cooled thermal shielding help mitigate these effects by maintaining a more stable crystal temperature throughout the process.
Second, perovskite deposition runs can be long. Co-evaporation of multi-layer stacks, or thick absorber layers, can consume crystal life quickly. When a crystal reaches end of life mid-run, the process must either be interrupted, or a new crystal must be introduced without breaking vacuum. Both scenarios have a direct impact on yield and throughput.
Third, some perovskite precursor materials, particularly lead-containing compounds like PbI₂, present material compatibility considerations. The sensor housing and crystal exchange mechanism need to withstand these process chemistries over extended service.
INFICON RSH-800: Continuous Monitoring for Demanding Evaporation Processes
The INFICON RSH-800 rotary sensor head was designed to address exactly these challenges. It houses six quartz crystals in a stainless-steel body with water-cooled thermal shielding, enabling continuous operation at chamber temperatures up to 300°C while maintaining crystal thermal stability.
The six-crystal carousel means that when one crystal reaches end of life, the sensor automatically indexes to the next crystal without interrupting the process. Crystal switching is pneumatic. A one-second air pulse advances the carousel, so there is no motor inside the vacuum, no electromagnetic interference, and no additional heat introduced to the measurement environment. Crystal position feedback is provided through individual switch closures or via BNC signal connection compatible with INFICON Zevision IMC300 for direct system integration.
When all six crystals have been used, crystal exchange is a simple routine maintenance task, not a rebuild. Crystals are replaced using INFICON's crystal flip tray packaging and the sensor goes back into service.
The RSH-800 is available in five in-vacuum lengths from 200 mm to 650 mm, with flathead or angle-head configurations in stainless steel or copper, and with CF63 or round flange mounting options. Adjustable flange accessories allow precise positioning of the sensor within the chamber, which is particularly important for co-evaporation geometries where the sensor must be located near the substrate plane to accurately represent the deposition conditions at the substrate surface.
Scaling with Confidence
As perovskite solar cell manufacturers transition from pilot lines to volume production, the metrology infrastructure inside the vacuum chamber becomes as important as the deposition sources themselves. QCM-based monitoring provides the real-time rate and thickness data that closed-loop control depends on, and the sensor must be reliable enough to keep pace with production demands.
For more information on the RSH-800 and INFICON's thin film deposition monitoring solutions, visit inficon.com/en/products/thin-film-technology/rsh-800-rotary-sensor.