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Film thickness monitor
the connections are tightened to a specified torque. In the ultrasonic wrench (Fig 4) a frequency converter converts line power to ~&HZ, timed to provide 3s output pulses. The wrench assembly has a PZT transducer, which delivers from 70-85% of the electrical power into an acoustic load, and has a dial, which is calibrated in in-lb of torque. In Fig 5, the use of the wrench is shown. The operator first tightens the coupling nut to the specified torque, shown on the dial; then he presses the thumb switch to excite the transducer to the preset power level. During the fixed 3s pulse, ultrasonic energy reduces friction and there is additional tightening of the nut as the operator maintains the desired torque.
Torque indicator
-Coupling
nut
Transducer assembly
/
~
I Switch
Marshall Space Flight Center, Huntsville, Alabama 35812, USA
NEW EQUIPMENT FILM THICKNESS MONITOR An oscillating quartz crystal inside coating chamber is used in the measurement of the thickness and rate of deposition of thin films. The measurements are recorded on separate meters with reference to a crystal outside the coating chamber. As deposition proceeds, the frequency of the crystal in the coating chamber is changed by evaporant deposited on it, while the frequency of the external crystal is constant The crystal inside oscillates with a natural frequency of 6.OMHz. The external reference crystal oscillates with a natural frequency of 6.5MHz. A voltage is derived from the difference between the frequencies of the crystals. This derived voltage is amplified and fed into a main unit where a final difference frequency of between 1OOHzand 50kHz is produced. The mass of the deposited material reduces the natural resonant frequency of the monitor crystal to increase the final difference frequency. This change is converted to a dc signal which actuates both the frequency shift meter and the rate-of-change meter. In this way, both the thickness of the film and its rate of deposition on the crystal face are displayed on conventional meters. For extreme accuracy, the frequency shift should be calibrated against film thickness by an independent method such as optical interferometry. In the applications shown at The Physics Exhibition, the film thickness monitor was used to measure the fatty acid concentration held in solution to the order of a few parts per million and the oil vapour backstreaming rate from a working diffusion pump with a diameter of 5 in. Edwards High Vacuum Limited, Manor Royal, Crawley, Sussex, England (shown at The Physics Exhibition 1968)
CADMIUM SULPHIDE CRYSTALS Cadmium sulphide crystals are produced within the specification below and are suitable for acousto-electric, acousticoptic applications. The crystals are grown by AEI Central Research Laboratory from the vapour phase. Careful control of the composition can produce either semi-insulating . or semiconducting crystals of cadmium sulphide. Typical data Boule size: lcm diameter by 2. 5cm in length (Fig 1) Crystal aixs: The c-axis is usually inclined to the axis of the boule. 6mm oriented cubes can be cut from these boules
Transparency:
The crystals are generally free from scattering centres and are transparent in the range 0.52-15pm Dislocation density: About 104/cm2 on cleaved surfaces Electronic properties Semi-insulating
material
Dark resistivity: Greater than 10752cm(after 5min) Photoconductivity: Free carrier lifetime is 10e2s. Resistivity can be reduced to 102Qcm under strong illumination (5 X 1031m/ft2) Electron drift mobility at 300°K: Greater than 200cma/V-s as inferred from the knee in the current-voltage curve at a resistivity of lOaStcm, that is, under illuminated conditions. Acousto-electric interaction is very pronounced. Semiconducting
Resistivity:
material
About l&m
AEI Central Research Laboratory, Rugby, England (shown at The Physics
Exhibition
1968)
X-BAND DELAY LINE
The delay medium is a high purity sapphire rod, 1.7cm long, lcm diameter, with its C-axis parallel to the rod axis. Opposite ends are polished flat to one tenth of a wavelength of sodium light and parallel to 2-4s of arc. Onto one end, a thin layer of gold is sputtered with a thickness of one-seventh of a wavelength at mid-band. A layer of cadmium sulphide-_half-wave thickness of 8. SGHz-is evaporated onto the layer of gold. The cadmium sulphide film is evaporated from separate cadmium and sulphur sources onto the heated rod substrate with a baffle between to prevent direct molecular beam transmission. The CdS film has a dark resistivity of 107Gcm, and is polycrystalline with a preferred orientation of the C-axis normal to the substrate. In this form it acts as a longitudinal acoustic wave transducer, one film being used as transmitter and receiver. At room temperature, longitudinal acoustic wave attenuation in sapphire changes as the square of the frequency, so that a ULTRASONICS July 1968
137
Torque indicator
-Coupling
nut
Transducer assembly
/
~
I Switch
Marshall Space Flight Center, Huntsville, Alabama 35812, USA
NEW EQUIPMENT FILM THICKNESS MONITOR An oscillating quartz crystal inside coating chamber is used in the measurement of the thickness and rate of deposition of thin films. The measurements are recorded on separate meters with reference to a crystal outside the coating chamber. As deposition proceeds, the frequency of the crystal in the coating chamber is changed by evaporant deposited on it, while the frequency of the external crystal is constant The crystal inside oscillates with a natural frequency of 6.OMHz. The external reference crystal oscillates with a natural frequency of 6.5MHz. A voltage is derived from the difference between the frequencies of the crystals. This derived voltage is amplified and fed into a main unit where a final difference frequency of between 1OOHzand 50kHz is produced. The mass of the deposited material reduces the natural resonant frequency of the monitor crystal to increase the final difference frequency. This change is converted to a dc signal which actuates both the frequency shift meter and the rate-of-change meter. In this way, both the thickness of the film and its rate of deposition on the crystal face are displayed on conventional meters. For extreme accuracy, the frequency shift should be calibrated against film thickness by an independent method such as optical interferometry. In the applications shown at The Physics Exhibition, the film thickness monitor was used to measure the fatty acid concentration held in solution to the order of a few parts per million and the oil vapour backstreaming rate from a working diffusion pump with a diameter of 5 in. Edwards High Vacuum Limited, Manor Royal, Crawley, Sussex, England (shown at The Physics Exhibition 1968)
CADMIUM SULPHIDE CRYSTALS Cadmium sulphide crystals are produced within the specification below and are suitable for acousto-electric, acousticoptic applications. The crystals are grown by AEI Central Research Laboratory from the vapour phase. Careful control of the composition can produce either semi-insulating . or semiconducting crystals of cadmium sulphide. Typical data Boule size: lcm diameter by 2. 5cm in length (Fig 1) Crystal aixs: The c-axis is usually inclined to the axis of the boule. 6mm oriented cubes can be cut from these boules
Transparency:
The crystals are generally free from scattering centres and are transparent in the range 0.52-15pm Dislocation density: About 104/cm2 on cleaved surfaces Electronic properties Semi-insulating
material
Dark resistivity: Greater than 10752cm(after 5min) Photoconductivity: Free carrier lifetime is 10e2s. Resistivity can be reduced to 102Qcm under strong illumination (5 X 1031m/ft2) Electron drift mobility at 300°K: Greater than 200cma/V-s as inferred from the knee in the current-voltage curve at a resistivity of lOaStcm, that is, under illuminated conditions. Acousto-electric interaction is very pronounced. Semiconducting
Resistivity:
material
About l&m
AEI Central Research Laboratory, Rugby, England (shown at The Physics
Exhibition
1968)
X-BAND DELAY LINE
The delay medium is a high purity sapphire rod, 1.7cm long, lcm diameter, with its C-axis parallel to the rod axis. Opposite ends are polished flat to one tenth of a wavelength of sodium light and parallel to 2-4s of arc. Onto one end, a thin layer of gold is sputtered with a thickness of one-seventh of a wavelength at mid-band. A layer of cadmium sulphide-_half-wave thickness of 8. SGHz-is evaporated onto the layer of gold. The cadmium sulphide film is evaporated from separate cadmium and sulphur sources onto the heated rod substrate with a baffle between to prevent direct molecular beam transmission. The CdS film has a dark resistivity of 107Gcm, and is polycrystalline with a preferred orientation of the C-axis normal to the substrate. In this form it acts as a longitudinal acoustic wave transducer, one film being used as transmitter and receiver. At room temperature, longitudinal acoustic wave attenuation in sapphire changes as the square of the frequency, so that a ULTRASONICS July 1968
137