Oliver C. Wells

Low‐Loss Image for Surface Scanning Electron Microscope

Images have been obtained from the surface scanning electron microscope (SEM) by collecting backscattered electrons that have suffered a small energy loss in the specimen. This method can be applied to smooth specimens when viewed at oblique incidence. The modulation depth in the electron channelling pattern can be as great as 75%, as compared with 2–5% for the secondary electron signal or 40% for the backscattered electron signal. In surface microscopy, the image is obtained from a surface layer of thickness about 100 A, so that the effects of electron penetration are greatly reduced. A point‐to‐point resolution of 170 A has been obtained.

Coating, Mechanical Constraints, and Pressure Effects on Electromigration

The effect of the mechanical constraints exerted by coatings upon electromigration in thin films is evaluated on the basis of known pressure effects upon diffusion.

Low-energy scanning transmission electron microscope

Low-energy scanning transmission electron microscopy is achieved by using a sharply pointed electrode as a source of electrons having energies less than 10 eV and scanning the electron emitting pointed source across the surface of a self-supported thin film of material to be investigated at an essentially constant distance on the order of nanometers. The electrons transmitted through the specimen are sensed by a suitable detector and the output signal of the detector is used to control a display unit, such as a CRT display or a plotter. A scanning signal generating means simultaneously controls both the scanning of the electron emitting point source and the display unit while a separation control unit holds the distance between the point source and surface at a constant value. The electron emitting point source and associated mechanical drives as well as the specimen film and electron detector are all positioned in a vacuum chamber and isolated from vibration by a damped suspension apparatus.

REVERSIBLE HILLOCK GROWTH IN THIN FILMS

Thin films of lead and aluminum on substrates of lower thermal expansion coefficient were examined using a hot stage in a scanning electron microscope. Hillocks were seen to grow as the temperature was increased and then to shrink as the temperature slowly fell. This effect is thought to be caused by a change from compressive into tensile stress in the film.

NEW CONTRAST MECHANISM FOR SCANNING ELECTRON MICROSCOPE

With normal electron incidence, the resolution of the backscattered electron image in the scanning electron microscope (SEM) is approximately equal to the classical electron pentration depth. With oblique electron incidence, a significant number of plurally scattered electrons leave the specimen in an apparently specular direction after penetrating for a distance that is an order of magnitude smaller than this. Thus with 15‐keV electrons incident onto Al at 45°, a significant number of backscattered electrons leave the specimen after penetrating to less than 500‐a.u. depth. These electrons can be collected over an angle that is close to the plane of the specimen surface. Other electrons leave the specimen more nearly at right angles to the surface, and these have been scattered from a greater depth. The image in the SEM can change completely if the position of the collector is changed.

Method for examining solid specimens with improved resolution in the scanning electron microscope (SEM)

We have obtained a low‐loss image from a solid specimen in the high‐field region of the condenser‐objective lens in a high‐resolution SEM. These experiments have demonstrated an edge sharpness of 15 A and a point‐to‐point resolution of better than 50 A between gold dots on a latex ball on a solid substrate. In theory, it should be possible to obtain micrographs from solid specimens with a resolution of 10 A by this method.

Imaging and analysis of subsurface Cu interconnects by detecting backscattered electrons in the scanning electron microscope

Cu–SiO2–SiNx interconnects that were located 0.65–2.7-μm below the surface of silicon-integrated circuits were imaged in a scanning electron microscope and a transmission electron microscope with a scanning attachment by detecting backscattered electrons (BSEs) with an incident electron-beam energy (Eo) in the range of 30–400keV. BSE images could be used to detect voids in subsurface Cu interconnects, even in regions covered with upper level Cu lines or vias. As Eo was increased from 30to400keV, structures could be seen as a result of atomic number (Z) contrast farther below the surface while structures closer to the surface had reduced Z contrast. The subsurface beam diameter was measured from BSE images as a function of Eo and depth below the surface. For all Eo, the subsurface beam diameter initially rapidly increased with SiO2 overlayer thickness but, for 150keV, a leveling off in the beam spread was seen for depths >1.7μm. Beam broadening affected whether the TaN∕Ta liners that surrounded the Cu cond...

Magnetically filtered low loss scanning electron microscopy

An electron microscope which includes a detector which is located in the magnetic field used to focus the primary electron beam onto the sample. The focusing magnetic field is used to energy-filter and/or energy analyze the scattered electrons without the need for additional equipment, such as a retarding-field energy filter. The magnetic field of the condenser-objective lens (or of any other type of magnetic lens) of the microscope provides the filtering and/or analyzing action, and the detector can be located so as to collect only low-loss electrons.

Low‐loss electron images of uncoated photoresist in the scanning electron microscope

Low voltage scanning electron microscopy is an important part of microelectronic inspection technique. This makes it possible to examine devices without changing the electrical properties, and to examine nonconducting samples such as photoresist without the use of a surface metal layer. The secondary electron imaging method suffers, however, from the difficulty that the image can be spoiled by slight charging of the specimen by the incident electron beam. This problem can be solved by the use of the low‐loss electron image.

Magnetically filtered low‐loss scanning electron microscopy

The resolution of the scanning electron microscope can be improved by mounting the sample in the high‐field region of a condenser‐objective lens. Low‐loss electrons (LLEs) are scattered from the sample with an energy loss of less than a few percent of the incident energy. In the past, LLEs have been collected with a retarding‐field energy filter. A way has been found to collect LLEs using a detector located within the magnetic field of the condenser‐objective lens which provides the required energy‐filtering action. This greatly simplifies the apparatus and makes it possible to obtain LLE images with less tilt of the specimen and with a higher beam energy than before.