Design, development and performance evaluation of a digital quartz crystal monitor for thin film deposition.

Abstract

Thinfilm technology has provided a variety of devices with tailormade properties suited to applications in microelectronics, integrated optics, superconductivity, corrosion, lubrication, solar energy, sensors, and lasers. The development of highquality devices with repeatability and stability requires sophisticated instrumentation that provides better control over deposition conditions. In addition to ultrahighvacuum conditions and proper gas analysis and control, depositionrate control, thickness measurement, and thickness monitoring are very important for preparing devices with precision characteristics. Quartzcrystal monitors have become extremely popular and essential for this type of control. These monitors operate on the principle of change in resonant frequency of a quartz crystal due to mass loading. The stability and linearity of the crystal and its associated circuits determine the accuracy of measurement and control, which in turn determine the quality of thinfilm devices. Several versions of crystal monitors have been designed for R&D laboratories and industrial manufacturing. Microprocessorcontrolled crystal monitors are commercially available; however, they are expensive, and microprocessors are not always required for certain applications. The present investigation deals with the design, fabrication, and performance evaluation of a quartzcrystal monitor built from easily available components, comparatively inexpensive, and possessing good sensitivity and resolution. This monitor has the following features:

Digital readout of deposition rate and total thickness Fast response time Highly stable oscillators Rate and thickness display in angstroms, with materialdensity settings Thickness range: 1 - 999,999 Å Rate range: 1 - 999 Å/s Coating termination adjustable to any predetermined value up to 999 kÅ using thumbwheel switches Operates with a single power supply Simple and costeffective

The monitor oscillator uses an IC 8284A as a clock generator. A groundedcrystal configuration avoids charge collection, a common problem in electronbeam evaporation and filmretrying systems. The reference oscillator is also designed using IC 8284A. Its output is used for mixing and for generating the sampling interval required for density settings. Due to the high stability of the reference oscillator, pulse periods are measured with excellent accuracy. The frequency mixer plays a vital role in the circuit. A Dflipflop is used to generate a beat frequency. The evaporation rate is obtained by consecutive sampling of the mixer output. Thickness is computed by sampling the mixer output over one sampling period. Thickness termination is achieved using a thicknessreset and thicknesstermination unit. Detailed timing diagrams of the gate circuitry are provided. The stability, linearity, and repeatability of each circuit and subsystem were independently tested. Performance evaluation of the crystal monitor was carried out by incorporating it into a vacuum chamber pumped by a diffusionpump + rotarypump combination, achieving an ultimate vacuum of 5 × 10 torr. Both resistive evaporation and electronbeam evaporation techniques were used to deposit metals and dielectrics for calibration. Molybdenum boats were used to evaporate metals such as copper, silver, chromium, and tin, while a 6kW bentbeam (180°) electronbeam gun with autoscan was used for materials such as magnesium fluoride and zinc sulfide. The stability and sensitivity of the present monitor were compared to an imported crystal monitor (EdwardsFTM3 model) and were found to be in very close agreement. The adaptability of this monitor to different crystals was found to be better than that of the imported monitor. The monitor was also calibrated using a multiplebeam interferometer to evaluate its suitability for preparing films of various thicknesses and for studying the frequency linearity of monitor crystals. Calibration charts prepared with this monitor showed that it can be universally used for different materials by incorporating the materialdensity factor. The stability and sensitivity of the circuits used in this monitor enable the preparation of thin films with excellent reproducible characteristics. The crystal monitor developed from locally available components is stable, sensitive, and costeffective. Calibration charts for various materials have been prepared using their respective density factors.