Next Generation Crystal Microbalances for Thin Film Deposition Monitoring and Control
Scott Grimshaw
Tangidyne Corp.
Abstract
The "Achilles heel" of thin film deposition process monitoring and control is the quartz crystal microbalance (QCM). Since its advent in the 1960's, QCM's have been an integral part of most commercial film coating systems. Unfortunately, the limitations of QCM's for processes such as AR coating, ion beam sputtering, and low pressure CVD have not been adequately addressed, until now. A new class of QCM sensor, designed for elevated temperatures, high stress dielectrics and extremely thick coatings is now available. This new sensor embodies an advanced crystallographic cut, "smart" sensor housings and novel electrode coatings to allow precise control of advanced coating processes.
Introduction
Quartz crystal microbalances operate in a relatively simple fashion. The QCM consists of a disc of quartz cut at a specific angle and shape from a bar of synthetically grown quartz. This quartz disc is then coupled into an electrical circuit and caused to vibrate at its natural resonance frequency. The resonance changes (decreases in frequency) whenever a thin film coating collects on the crystal surface. If the density of the film material is known, an algorithm can be used to compute the film thickness.
Performance Limitations
The use of QCM's for thickness monitoring and rate control is not without difficulties. Users find that quartz crystals are ideal for the deposition of thin metal films since they cause a reproducible, stable change in the crystal resonance frequency. But when coatings of dielectric materials such as silicon dioxide or magnesium fluoride are monitored, the crystal can become very erratic or even abruptly fail. This is the result of strong tension or compression generated in these films. As quartz is stress sensitive, the QCM responds to these forces and often becomes unstable.
The heat of optical thin film depositions also adversely affects QCM's. Quartz is heat sensitive and must be water-cooled in order to minimize any changes of resonance frequency with temperature. But even with cooling, the QCM will respond to radiant heat from a deposition source and will be prone to large thickness errors when monitoring very thin films (<100 A).
The ability to correlate QCM thickness readings with optical film thickness measurements can be very difficult as well. This becomes a primary concern in the manufacture of thin film interference filters where the QCM provides deposition rate information (which effects film properties such as refractive index) while reflection or transmission monitors provide optical properties. The physical placement of these two separate monitors leads to significant film measurement errors.
Performance Improvements
The shortcomings of QCM's have been addressed by three new technologies: 1) an advanced crystallographic cut of quartz, called the RCTM or radiation compensated crystal, insensitive to radiation heat from deposition sources, and as a result, extremely accurate for very thin film depositions; 2) a temperature regulated QCM sensor housing called the ThermaHeadTM that precisely controls the crystal temperature during depositions and, by operating at an elevated temperature (~90 C), minimizes thin film stress and extends crystal life dramatically; and 3) an optically transparent QCM, called OptoCrystalTM, which utilizes transparent electrodes and a see-through sensor housing, allowing optical and mass measurements to be done on the same sensor, eliminating correlation errors.
Future Developments
The advancement of the QCM will be further enhanced by several new products in development: 1) Multi-resonator quartz crystals consisting of several discrete vibrating areas on a single piece of quartz, offering redundant, "failure proof" crystal sensors, 2) QCM's which incorporate thin film stress measurement functions into the sensor body, allowing real-time monitoring of compression and tension in the coating as it deposits, 3) Pre-packaged crystal sensors, eliminating the need for handling the QCM and mounting it in the sensor housing (a major source of crystal problems) and 4) Wafer sliced, miniature crystal sensors, which will drastically reduce the price of QCM's used in thin film monitoring.