Mechanical property measurements of thin films using load-deflection of composite rectangular membranes

When an energetic ion traverses a polymer medium, it loses its energy by electronic and nuclear processes. In the past, considerable research efforts have been devoted to understand the effects of the electronic and nuclear processes in materials. There have been, however, some conflicting reports regarding the roles of electronic and nuclear stopping in producing property changes in polymeric materials, namely the magnitude of cross-linking and scission. A consensus derived from the work conducted at ORNL indicates that electronic stopping is largely responsible for cross-linking and nuclear stopping for scission, although both processes can cause cross-linking as well as scission. The most important parameter for cross-linking is found to be the energy deposited per unit ion path length or linear energy transfer (LET). The mechanisms involved with property changes are discussed by clarifying the concepts of nuclear and electronic stopping, LET, tracks, and spurs. Experimental evidence to support the views are presented. Also addressed are specific property changes induced by ion-beams, which may be of use for industrial applications.

Ion-beam modification of polymeric materials – fundamental principles and applications

Two microfabricated structures for the in situ measurement of mechanical properties of thin films, a suspended membrane, and an asymmetric ‘‘released structure,’’ are reported. For a polyimide film on silicon dioxide, the membrane measurements yield a residual tensile stress of 30 MPa and a Young’s modulus of 3 GPa. The released structures measure the ratio of residual stress to Young’s modulus, and yield 0.011 at strains comparable to the suspended membranes, and 0.015 at larger strains. The ultimate strain as measured by both structures is approximately 4%.

Microfabricated structures for the in situ measurement of residual stress, Young’s modulus, and ultimate strain of thin films

Freestanding flexible microstructures fabricated from deposited thin films become mechanically unstable when internal stresses exceed critical values. A series of structures with varying geometries is used to determine the critical geometry at which buckling occurs. Observation with an optical microscope quickly reveals qualitative and quantitative information about the internal strain in the film. Strain values between +or-1.5% can be measured for a 2.0 mu m thick film using doubly-supported beams for compressive strain fields, and ring and beam structures for tensile strain fields. Parametric formulae are developed for diagnostic structure response with selected verification by finite element computations.

http://iopscience.iop.org/article/10.1088/0960-1317/2/2/004/pdf

Diagnostic microstructures for the measurement of intrinsic strain in thin films

Silicon nitride corrugated diaphragms of 2 mm/spl times/2 mm/spl times/1 /spl mu/m have been fabricated with 8 circular corrugations, having depths of 4, 10, or 14 /spl mu/m. The diaphragms with 4-/spl mu/m-deep corrugations show a measured mechanical sensitivity (increase in the deflection over the increase in the applied pressure) which is 25 times larger than the mechanical sensitivity of flat diaphragms of equal size and thickness. Since this gain in sensitivity is due to reduction of the initial stress, the sensitivity can only increase in the case of diaphragms with initial stress. A simple analytical model has been proposed that takes the influence of initial tensile stress into account. The model predicts that the presence of corrugations increases the sensitivity of the diaphragms, because the initial diaphragm stress is reduced. The model also predicts that for corrugations with a larger depth the sensitivity decreases, because the bending stiffness of the corrugations then becomes dominant. These predictions have been confirmed by experiments. The application of corrugated diaphragms offers the possibility to control the sensitivity of thin diaphragms by geometrical parameters, thus eliminating the effect of variations in the initial stress, due to variations in the diaphragm deposition process and/or the influence of temperature changes and packaging stress. >

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The design, fabrication, and testing of corrugated silicon nitride diaphragms

Intrinsic stress, elastic bulk modulus, and yield strength of thin films has been determined by measuring the deformation versus pressure of circular membranes of the materials. Low pressure chemical vapor deposited (LPCVD) silicon‐rich silicon nitride (SiNx) has been extensively characterized and found to have an intrinsic stress of ∼1×109 dyn/cm2 and a bulk modulus of ∼1.9×1012 dyn/cm2. A SiNx membrane 1.0 cm in diameter and 1.0 μm thick has been found to survive a differential pressure of 470 Torr with no measurable plastic deformation. Experimental data for several different types of silicon nitride membranes is given. It is shown that the stress measurement technique can be extended to measure accurately the intrinsic stress of thin films deposited onto SiNx membranes.

A technique for the determination of stress in thin films

High resolution masked ion beam lithography (MIBL) is demonstrated at large mask‐to‐sample gaps using two new types of membrane stencil masks. Single layer Si‐rich silicon nitride (SiN) membranes and Si3N4‐SiO2‐Si3N4 (N‐O‐N) sandwich structure membranes are deposited by processes which allow the stress in the films to be adjusted. Transmission holes are reactive‐ion etched entirely through the membranes. This type of stencil mask virtually eliminates mask‐induced scattering. Lines and spaces of 160 nm have been exposed in 0.5‐μm polymethylmethacrylate (PMMA) at gaps as large as 275 μm, using 100‐keV protons. Some of the stencil mask limitations are overcome by multiple exposures. The results suggest that MIBL can be an extremely high resolution proximity printing technique.

High resolution ion beam lithography at large gaps using stencil masks

A new type of high resolution mask for ion beam lithography is described. The mask employs a thin membrane with regions of two different thicknesses to provide contrast for proton exposure of resist. A conventional ion implantation system is used to generate a collimated beam of protons for flood exposure of the sample. Beam energy is chosen so the the protons pass through the thin areas of the mask and emerge with sufficient energy to expose the resist, but are stopped in the thick areas. Experiments using polyimide membrane masks have yielded promising results. The ability to reactive ion etch structures with nearly vertical sidewalls in polyimide allows the fabrication of masks having both high contrast and high resolution. The stopping power of polyimide was determined by measuring the transmission of protons through membranes of known thickness, as a function of incident beam energy. In the energy range from 50–200 keV, the stopping power of polyimide for protons was measured to be ∠100 keV/μm. Mask fabrication procedures and exposure experiments to determine the resolution of this technique are reported. Patterns with linewidths of 〈0.1μm have been successfully replicated.

Summary Abstract: High resolution ion beam lithography

Masked ion beam lithography using silicon nitride stencil masks at a 25 μm mask‐to‐sample gap has been used to replicate 80 nm lines and spaces in PMMA. An improved reactive ion etching technique for the silicon‐rich silicon nitride (SiNx) mask material using CHF3 at a 500 V self‐bias potential is reported. A grid support mask is proposed as a means of exposing arbitrary patterns with a stencil mask. The principle of this technique is demonstrated in the special case of a grating.

Silicon nitride stencil masks for high resolution ion lithography proximity printing

Submicrometer resolution ion beam resist exposure using a new type of stencil mask is described. The mask uses a grid support structure in transmission areas to remove restrictions on pattern geometry and improve stability. The image of the grid is eliminated by rocking the incident angle of the ion beam during exposure, or by special processing of the resist. We describe the mask fabrication sequence and show examples of masks that employ a 320 nm period grid structure. Patterns with a minimum feature size of less than one‐half micrometer have been replicated using this type of mask with both single and multilayer resists. In addition, experiments are described which used low energy ion beams to test the ultimate resolution of stencil masks without grid support. 40 nm wide lines were printed in PMMA at a mask‐to‐wafer gap of 25 μm. The applicability of such masks in high resolution proximity printing systems is discussed.

D. C. Flanders

Papers

A technique for the determination of stress in thin films

High resolution ion beam lithography at large gaps using stencil masks

Summary Abstract: High resolution ion beam lithography

Silicon nitride stencil masks for high resolution ion lithography proximity printing

Masked ion beam resist exposure using grid support stencil masks