David T. Read

A new method for measuring the strength and ductility of thin films

A new method of measuring the mechanical strength of thin films is described. We prepare miniature arrays of four tensile specimens, each 0.25 mm wide, 1 mm long, and 2.2 mm thick, using deposition, patterning, and etching processes common to the semiconductor industry. Each array of four specimens is carried on and protected by a rectangular silicon frame. Thirty-six such specimens are produced on a single wafer. After a specimen frame is mounted, its vertical sides are severed without damaging the specimens. The load is applied by micrometers through a special tension spring. Tensile properties of a 2.2 mm thick Ti‐ Al ‐Ti film were determined.

Electron beam moiré

A method of writing very high frequency line and dot pattems, in excess of 10,000 lines/mm, is described. This method uses a very small diameter, 10 to 20 nm, beam of electrons to sensitize a 100-nm thick layer of electron resist. The line and dot patterns are produced by etching the sensitized resist. Moiré fringe patterns occur when the line arrays are observed in the scanning electron microscope. Moiré fringes with excellent contrast have been produced at magnifications as high as 1900x. This capability permits e-beam moiré to be employed in micromechanics. Examples of line arrays, dot arrays and moiré fringe patterns on a brass disk and on a tensile specimen fabricated from glass-fiber-reinforced plastic are demonstrated to introduce the possibilities for micromechanics applications.

Elastic moduli, strength, and fracture initiation at sharp notches in etched single crystal silicon microstructures

We designed and fabricated a series of micromechanical test structures for microtensile testing by anisotropically etching epitaxial silicon. Specimens were fabricated to study Young’s moduli, the uniaxial tensile strength, and the strength of T-structures which are tensile bars with an abrupt reduced cross section that have a 90° corner at the point of reduction. They are a generic mimic of actual transitions that occur in micromechanical structures due to anisotropic etching. The test structures were loaded in uniaxial tension in a piezoactuated microtensile test apparatus. The applied force and crosshead displacement were recorded and displacements in the specimen gage section were directly measured using a speckle interferometry technique. During tensile loading of the T-structures, fracture always initiates at the sharp 90° corners. This results in an interesting apparent strength scaling where the nominal strength of the structures increases as their width decreases. In order to understand the fract...

Resistivity dominated by surface scattering in sub-50 nm Cu wires

Electron scattering mechanisms in copper lines were investigated to understand the extendibility of copper interconnects when linewidth or thickness is less than the mean free path. Electron-beam lithography and a dual hard mask were used to produce interconnects with linewidths between 25 and 45 nm. Electron backscatter diffraction characterized grain structure. Temperature dependence of the line resistance determined resistivity, which was consistent with existing models for completely diffused surface scattering and line-edge roughness, with little contribution from grain boundary scattering. A simple analytical model was developed that describes resistivity from diffuse surface scattering and line-edge roughness.

Young's modulus of thin films by speckle interferometry

Speckle interferometry has been applied to the measurement of Youngs modulus in thin films. The present study has two novel aspects: the specimens used were approximately 1 m thick and less than 1 mm long, so the speckle images were obtained by photomicroscopy; and the digital images were analysed by quantitative treatment at intense speckles, rather than by the more standard techniques. Displacements and strains within the gauge length were obtained with low statistical uncertainties. Youngs modulus values for three varieties of copper thin films were obtained. All were lower than the polycrystalline bulk average value and the electrodeposited films modulus was lower than those of both the vapour-deposited films.

Theory of Electron Beam Moire

When a specimen surface carrying a high-frequency line grating is examined under a scanning electron microscope (SEM), moiré fringes are observed at several different magnifications. The fringes are characterized by their spatial frequency, orientation, and contrast. These features of the moiré pattern depend on the spatial frequency mismatch between the specimen grating and the raster scan lines, the diameter of the electron beam, and the detailed topography of the lines on the specimen. A mathematical model of e-beam moiré is developed that expresses the spatial dependence of the SEM image brightness as a product of the local intensity of the scanning beam and the local scattering function from the specimen grating. Equations are derived that give the spatial frequency of the moiré fringes as functions of the microscope settings and the spatial frequency of the specimen grating. The model also describes the contrast of several different types of moiré fringes that are observed at different magnifications. We analyze the formation of these different fringe patterns, and divide them into different categories including natural fringes, fringes of multiplication, fringes of division, and fringes of rotation.

Scanning moiré at high magnification using optical methods

Methods of employing scanning moiré at high magnification are developed and demonstrated. Modern lithographic techniques for producing custom moiré gratings with a frequency up to 250l/mm are described. On a probing station equipped with a video system, pseudo-color moiré fringes are produced using the scannning lines of the color charge-coupled-device (CCD) camera. Fringe multiplication from 1 to 5 is possible with correct combinations of magnification and grating pitch. An analysis is given to show that strain sensitivity depends only on the number of scanning lines used to record the image. The grating pitch and the magnification are important because they reduce the gage length of the strain measurement. The high-magnification scanning moiré was used to study plastic- strain fields in an aluminum tensile specimen. Local disturbances in the strain field were observed at 2 to 2.5 percent applied strain. These discontinuities became more significant at higher levels of applied strain.

Tensile testing low density multilayers: Aluminum/titanium

Yield stresses, ultimate tensile strengths, and specific strengths of aluminum/titanium multilayer thin films are determined from the results of uniaxial tensile tests. The plasticity in the stress-strain curves, the nature of the fracture surfaces, and the relationship of the yield stress and the bilayer thickness are discussed. Properties are compared with those of other multilayer materials published in the literature.

Fracture and strength properties of selected austenitic stainless steels at cryogenic temperatures

Abstract Austenitic stainless steels have an excellent combination of mechanical and physical properties for load-bearing structures of large superconducting magnets for plasma containment in magnetic fusion experiments. To assess their relative suitability fracture toughness, fatigue crack growth, and tensile properties data for five austenitic steels at 295, 76, and 4 K have been obtained. The steels were AISI 304, 316, 304LN, and 316LN, and an Fe-21cr-12Ni-5Mn alloy with a higher nitrogen content than the other four grades. The two principal findings were the systematic variation of yield strength with nitrogen content and a systematic inverse correlation between fracture toughness and yield strength. Data from previous studies are reviewed which confirm the trends of the present data.

Orthorhombic elastic constants of an NbTi/Cu composite superconductor

Elastic properties of a niobium‐titanium‐filament copper‐matrix composite superconductor were studied experimentally at room temperature. Ultrasonic pulse and resonance measurements showed the material has orthorhombic symmetry and, therefore, nine independent elastic constants. With respect to copper, C11, C22, and C33 are about 7% lower; C44, C55, and C66 are about 15% lower; the off‐diagonal elastic constants are the same; and the bulk modulus is about 5% lower. Deviations from isotropic elastic behavior are small.