Visualization of Dynamic Near-Surface Processes
- PDF / 2,277,218 Bytes
- 6 Pages / 576 x 792 pts Page_size
- 84 Downloads / 184 Views
Visualization of Dynamic Near-Surface Processes Ray D. Twesten, J. Murray Gibson, and Frances M. Ross Introduction Electron microscopy has played an undeniably important role in the development and understanding of new materials. One particular method, plan-view transmission electron microscopy (TEM), has been used to reveal many aspects of materials and to help answer questions that are not accessible by other methods. In planview TEM, materials are studied either by the image contrast caused by diffraction or by direct diffraction analysis. The studies are often at a somewhat lower resolution than is possible with some other electron-based microscopies or, in the case of diffraction, must be interpreted in terms of an average effect. Nevertheless, plan-view TEM is very useful because properties at these intermediate length scales typically determine important materials characteristics. It is plan-view microscopy that gives us the big picture of what is happening in a material. Until recently, TEM studies of materials have entailed comparing samples taken at various stages of processing and trying to observe and understand the mechanisms that determine the material behavior. With the advent of controllable specimen environments, materials processing has been taken into the TEM, allowing direct observation of the microstructural development. Here we will focus particularly on surface and near-surface processes, such as oxidation. Equipment Developing a controlled environment in a TEM is hampered by the design considerations necessary to achieve high resolution and mechanical stability, in addition to the logistics of controlling a typically
38
200 keV electron beam. For example, the sample needs to be rigidly held within the narrow gap of the objective lens pole piece to achieve high resolution, but this will hamper attempts to control the specimen environment. One solution is to modify an existing microscope to achieve ultrahigh vacuum (UHV) conditions by keeping most of the microscope as is but modifying the specimen chamber. This involves increasing the pumping speed, modifying existing static and dynamic seals for UHV,
200 kV E Gun EB earn Condensor Lenses UHV Pumps
Tilt
Coils '
Auxiliary
Liquid Helium Gate Transfer Valvf System
~10'7T valve ^-Differential Pumping i , J n , i
UHVSamp e Chamber _10-9x
J ^Gatt Valve
Pump
j
Ion Gauge ( f i ) Sample Prep chamber
Chamber
Sample Airlock
Mirror
' „ / Pyr o m e t e r ^ ^ Window
i
Cryoshield^A /-•-Evaporator Sample—^_Gas Capillary
10°»T-1200°K
UHV
"^-Differential Pumping Residual Gas Analyzer
~10-7T
Imaging
Lenses
Viewing ) 1
System
I ransrer 1
Chamber
i
Figure 1 Block diagram of JEOL 200CX UHV electron microscope conversion. Broken lines indicate possible additional modification (from Reference 1).
MRS BULLETIN/JUNE 1994
Visualization of Dynamic Near-Surface Processes
Ewald Sphere
Bulk Reflections
Surface Reflections
Figure 2. The effect of beam tilt on diffracted intensities. The distribution of scattering power in reciprocal space
Data Loading...
