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OBS 40 on DN40 (O.D. 2.75") CF-flange

• O₂ dissociation up to more than 80% depending on operation conditions
• Atomic oxygen flux density up to 1*10¹⁵/(cm² s)
• No high-energy particles and ions
• Low power consumption
• Integrated water cooling, low thermal load on other experimental equipment
• Customized aperture and integrated shutter optional

The Oxygen Atom Beam Source OBS is a thermal gas cracker which produces an ion-free oxygen gas beam to avoid ion induced damage of the substrate. It exhibits a very compact design and is easy to install and to operate.
In contrast to oxygen plasma sources, based on HF or HV ionization of gas molecules, or gas sources heated by HV electron bombardment, the OBS is heated by a DC operated filament, which surrounds a long cracking tube.
The OBS has been developed in collaboration with Dr. Karl Georg Tschersich at the Institute of Bio- and Nanosystems (formerly Institute of Thin Films and Interfaces), Research Centre Jülich.

The Oxygen Atom Beam source is extensively tested and well characterized. The intensity of the source can be controlled by the flow rate of oxygen and the heating power. The heating power determines the temperature of the cracking tube. Operation temperatures up to 1900°C provide an excellent thermal cracking of O₂ molecules within the tube.
The high purity cracking tube is the only part of the OBS with direct contact to the oxygen gas and forms a narrow angle distributed gas beam ejected towards the specimen. The narrow beam of the OBS provides high atomic oxygen flux rates at the sample position, while keeping the O₂ background pressure of the chamber limited as compared to plasma sources. In addition, the high cracking efficiency of up to 80%, which is available for low flux applications, reduces the chamber background pressure significantly as compared to ozone generators where the ozone concentration is limited to about 10-15%.
Despite the high temperatures used for thermal gas cracking, the thermal load of the chamber is low due to the integrated water cooling and the use of a small diameter aperture.

Grade of dissociation as a function of the gas flow for typical cracking tube temperatures of he OBS (values are calculated for thermo-dynamical equilibrium condition)

The following options are available:

-  Usually the gas flow is regulated by a UHV leak valve or a mass flow controller which allow switching on and off the atom flux within several seconds. Short exposures can be terminated by a shutter which attenuates the oxygen atom flux down to the detection level.

-  Oxygen atoms passing the sample and hitting the chamber walls can induce reactions detrimental to the experiment. A customized aperture is available which limits the solid angle of oxygen atom emission. Outside this angle no beam atoms could be detected by the QMA. Although tested so far with oxygen only, the source can be used as well as a radical beam source by decomposing other molecules at temperatures up to 2200 K.

Total flux of O-atoms leaving the cell orifice for different cell operation parameters (gas flow and tube temperature). A flux of about 1*10¹⁷ atoms/s corresponds to a typical deposition rate of 1 monolayer/s at a typical substrate distance of about 200mm

Application

The use of the long, slender cracking tube results in a narrow angle distribution of the atomic oxygen beam (typical FWHM: 10-30°) which makes the OBS ideally suited for medium and low gas flux applications (up to 0.1 sccm) and smaller sample sizes or long substrate distances.
Due to its high efficient oxygen cracking mechanism and the resulting low gas load on the vacuum system, the OBS is well suited for use in standard UHV, MBE and ALD systems, where it proves to be a reliable, highly efficient and low cost source for atomic oxygen.

Typical applications for the OBS are:

Oxide layer deposition
The OBS allows precise layer deposition of oxide layers, due to the fast and easy control of the gas flow by shutter or UHV leak-valve.

Surface cleaning procedures
Atomic oxygen is used for removal of surface carbon contaminations.

Surface oxidation
In surface analysis applications as well as in ALD technology atomic oxygen is used for the preparation of well defined oxide surface layers.

Atomic spectroscopy of single atoms
The particle density of about 10¹¹ low energy O-atoms per cm³ near the cell exit allows a wide range of fundamental spectroscopic experiments on single O-atoms.



References

1. K.G. Tschersich, J. Appl. Phys. 87, 2565 (2000).


2. K.G. Tschersich and V. von Bonin, J. Appl. Phys. 84, 4065 (1998).

Gas Injection System

For optimized performance we provide a complete Gas Injection System for atomic oxygen. The Gas Injection System is completely mounted and only an O₂ gas bottle is needed to start operation. A simple turbo molecular pump can be used to evacuate the O₂ gas line. The O₂ gas line and the all metal leak valve can be baked up to 180°C.

• fully UHV compatible
• O₂ gas cracking cell
• all metal UHV leak valve
• all metal valve for gas feed evacuation
• O₂ gas purifier
• flexible UHV tube 1m max. length
• bakeable system (180°C)
• O₂ bottle and stainless steel gas
  pressure reducing regulator - not included !!

Alternatively, the gas injection system can be equipped with a mass flow controller (MFC) instead of an all metal UHV leak valve. A mass flow controller in the gas line is preferred when the OBS is mainly used for high intensity long-term runs where mass flow controllers due to their automated operation require less attentiveness in comparison to the aforementioned gas feed setup.
There are many different mass flow controllers on the market. It is therefore recommended that the customer provides the MFC on his own.

• fully UHV compatible
• O₂ gas cracking cell
• all metal valve for shut-off during MFC maintenance
• mass flow controller (MFC)
• all metal valve for gas feed evacuation
• O₂ gas purifier
• flexible UHV tube 1m max. length
• bakeable system (depends on MFC type)
• O₂ bottle and stainless steel gas
  pressure reducing regulator - not included !!

Technical Data

Dimensions

Specific Data

 LAST UPDATE: JULY, 2006

© 2003 Dr. Eberl MBE-Komponenten GmbH