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Temperature Compensated Quartz Crystal Monitor for Accurate Deposition of Extremely Thin Films

IP.com Disclosure Number: IPCOM000106813D
Original Publication Date: 1993-Dec-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 2 page(s) / 87K

Publishing Venue

IBM

Related People

Locquet, JP: AUTHOR [+2]

Abstract

Any thin film technology requires the controlled deposition of material with a well defined thickness. The instrument most commonly used in this respect is the quartz crystal monitor. It consists of a vibrating quartz crystal whose fundamental frequency is determined by the total mass of the vibrating body. When material is added to or removed from the crystal, a change of the resonant frequency is observed. This change can be easily measured. It allows a determination of the amount of material added to or removed from the crystal. The quartz is usually placed near the substrate in the deposition system to monitor the deposition process.

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Temperature Compensated Quartz Crystal Monitor for Accurate Deposition of Extremely Thin Films

      Any thin film technology requires the controlled deposition of
material with a well defined thickness.  The instrument most commonly
used in this respect is the quartz crystal monitor.  It consists of a
vibrating quartz crystal whose fundamental frequency is determined by
the total mass of the vibrating body.  When material is added to or
removed from the crystal, a change of the resonant frequency is
observed.  This change can be easily measured.  It allows a
determination of the amount of material added to or removed from the
crystal.  The quartz is usually placed near the substrate in the
deposition system to monitor the deposition process.

      High quality commercial instruments allow a very precise
determination of the resonant frequency - typically 6MHz, determined
within 0.01 Hz - which leads to a thickness resolution of about
0.01A.  In practice, however, a film cannot be deposited
with that accuracy due to a strong temperature dependency of resonant
frequency of the crystal.

      Temperature Dependence of the Vibrating Crystal - The resonant
frequency of a quartz crystal depends strongly on the actual
temperature.  Typically, a change of 1ºC gives a difference in
frequency  corresponding to a false thickness reading around
10A.  The temperature change can result from radiation
heating, the heat of condensation, the heat of transformation, or the
losses in the kinetic energy of the deposited material.  To diminish
the temperature effect, the crystal is usually enclosed in a water
cooled holder.

      In evaporation processes, the main contribution of heat comes
from radiation.  For example, opening the shutter of a Dysprosium
filled effusion cell operating at 900ºC and at a distance of
35 cm from the quartz oscillator will induce a heating effect
equivalent to the ablation of a layer of 20A  of a material
with an assumed density of 1.00.  Closing the shutter has the
opposite effect and the cooling will induce a frequency change
equivalent to the deposition of a 20A  layer.

The proposed method consists of a calibration step and a deposition
step.

      During the calibration step, the substrate is shielded from...