William N. Sharpe

A new technique for measuring the mechanical properties of thin films

Accurate measurement of mechanical properties is very difficult for films that are only a few microns thick. Previously, these properties have been determined by indirect methods such as cantilever beam and diaphragm bulge tests. This paper presents a new technique to measure the Youngs modulus of thin films in a direct manner consistent with its definition. Strain is measured by a laser-based technique that enables direct and accurate recording of strain on a thin-film specimen. Load is recorded with a 1-lb load cell, and an air bearing is used to eliminate friction in the loading system. The specimen is phosphorus-doped polysilicon that has a gage cross section of 3.5 /spl mu/m thick by 600 /spl mu/m wide. All 29 uniaxial tensile tests show brittle behavior, and the average values of Youngs modulus and fracture strength are measured to be 170/spl plusmn/6.7 GPa and 1.21/spl plusmn/0.16 GPa, respectively. One fatigue test is also reported in this paper.

Measurements of Young's modulus, Poisson's ratio, and tensile strength of polysilicon

New techniques and procedures are described that enable one to measure the mechanical properties of polysilicon films that are 3.5 /spl mu/m thick. Polysilicon is deposited onto a silicon substrate which is then etched away to leave a tensile specimen in the middle of the die. The grip ends of the structure are glued to the grips of a linear air bearing attached to a piezoelectrically actuated loading system. Strain is measured directly on the specimen with laser interferometry. The specimens are fabricated at the Microelectronics Center of North Carolina with their MUMPs process. The results of 48 tests on five different sets of MUMPs specimens yield the following material properties: Youngs modulus=169/spl plusmn/6.15 GPa, Poissons ratio=0.22/spl plusmn/0.011, and tensile strength=1.20/spl plusmn/0.15 GPa These values have a reasonably low coefficient of variation which demonstrates the consistency of both the processing and the measurement techniques.

Effect of specimen size on Young's modulus and fracture strength of polysilicon

The microstructure of polysilicon specimens of varying size was examined and tensile tests were conducted to determine if the measured modulus and strength depend on the size of the specimen. All specimens were from the same MUMPs 25 run at MCNC, and the thicknesses were 1.5, 2.0, and 3.5 /spl mu/m. Microstructure was examined in specimens as narrow as 2 /spl mu/m and ranging up to 20 /spl mu/m in width. The tensile specimens tested were 6, 20, or 600 /spl mu/m wide and 250, 1000, or 4000 /spl mu/m long. Nothing in the transmission electron microscopy (TEM) observations indicates any effect of specimen size on the microstructure; the columnar grains are fine (0.2-0.5 /spl mu/m) and uniformly distributed. The widths of all specimens were found to differ from the specified mask values, and a more pronounced variation was measured for the smaller specimens. Three different approaches are used to measure Youngs modulus, and they all give a value of 158/spl plusmn/10 GPa with no evidence of substantial effects of specimen size. However, the strength does increase somewhat as the total surface area of the test section decreases-from 1.21 GPa/spl plusmn/0.08 GPa to 1.65/spl plusmn/0.28 GPa-reflecting the fact that the larger specimens have more surface flaws. Test techniques and procedures are briefly presented along with detailed analyses of the results.

Comparison of tensile and bulge tests for thin-film silicon nitride

The mechanical properties of thin-film, low-pressure chemical vapor deposited silicon nitride were measured in uniaxial tension and by a bulge test method suitable for wafer-level testing. This research compares the two approaches and presents additional data on silicon nitride. The common property from the two test methods is the Youngs modulus. Tensile tests performed at the Johns Hopkins University provided a value of 257±5 GPa. Bulge tests conducted by Exponent, Inc., an engineering and scientific consulting firm, yielded a value of 258±1 GPa. It is concluded that this bulge test is a valid wafer-level test method. These tensile results, when added to earlier results, yield the following properties for low-stress silicon nitride: Youngs modulus =255±5 GPa, Poisson ratio=0.23±0.02, and fracture strength=5.87±0.62 GPa.

An efficient needle injection technique and radiological guidance method for percutaneous procedures

This paper presents a simple radiological image guidance method and robotic system optimized for percutaneous procedures. This simplicity is achieved by combining the proven guidance procedure normally employed by practicing surgeons with a simple and cost-effective needle injection device. The needle injector exhibits an extremely low radiological profile while providing actuated needle motion. The mechanical manipulator is designed to be used stand-alone in existing operating rooms without additional computers or personnel.

New test structures and techniques for measurement of mechanical properties of MEMS materials

This paper presents techniques and procedures for addressing the three major problems of mechanical testing of the thin films used in surface micromachined microelectromechanical systems--specimen handling, friction, and strain measurement. The polysilicon tensile specimens are fabricated with two supporting side strips on silicon wafers at the Microelectronic Center of North Carolina. The tensile specimen is released by etching away the wafer, and the two support strips are cut after the specimen is glued in the test machine. Friction is reduced by a linear air bearing in the load train, and strain is measured with a noncontacting technique based on laser interferometry between two gold lines on the tensile specimen. The Youngs modulus of polysilicon is 170 +/- 7 GPa and the strength is 1.21 +/- 0.16 GPa from a series of 29 tests. preliminary measurements have been made of Poissons ratio and the fatigue behavior, and an attempt is underway to measure the fracture toughness.

Mechanical properties of LIGA-deposited nickel for MEMS transducers

This paper presents a new technique for measuring the stress-strain curve of nickel produced by the LIGA method. The results of nine tests on 200 /spl mu/m thick specimens produced on the 3rd LIGAMUMPS run show a Youngs modulus similar to that of pure nickel, but strengths that are considerably higher.

Mechanical properties of epitaxial 3C silicon carbide thin films

Microscale tensile specimens of epitaxial 3C silicon carbide (3C-SiC) thin films were fabricated on Si substrates and tested to provide measurements of strength and elastic modulus. Samples were fabricated using both micromolding and reactive ion etching (RIE) processes to pattern the 3C-SiC films. All specimens were on the (100) plane with a <110> tensile direction. Testing was performed on a microsample tensile testing machine previously used on other materials. The samples had a thickness of 0.5 to 2 /spl mu/m, a gauge length of 4 mm, and a minimum width of 600 /spl mu/m. Testing results show an average strength of 1.19 /spl plusmn/ 0.53 GPa and 1.65 /spl plusmn/ 0.39 GPa for micromolded and RIE patterned specimens, respectively. The elastic modulus was measured to be 424 /spl plusmn/ 44 GPa, which was consistent with but slightly lower than the elastic modulus calculated with single crystal elastic constants found by ab initio calculations.

Tensile properties of LIGA nickel

This paper presents the results and conclusions from tensile tests performed on four lots of microtensile specimens produced by the LIGA method. The specimens tested are 3 millimeters long, 0.2 millimeters wide and 0.2 millimeters thick. The specimen is held in specially designed grips, and friction is eliminated by the use of a linear air bearing in the load train. Strain is measured by laser interferometry between two Vickers microhardness indentations placed 0.2 millimeters apart in the center of the specimen. Two sets of pure nickel specimens from two different Microelectronics Center of North Carolina (MCNC) LIGA Multi-User MEMS Process (LIGAMUMPs) runs were tested along with one set of pure nickel specimens from the HI-MEMS Alliance. In addition, four tests were conducted on a 0.03 millimeter thick nickel-iron alloy with a target composition of 78% Ni, 22% Fe. One set of LIGAMUMPs specimens had an average Youngs modulus of 181 GPa, which was almost exactly the same as measured on the HI-MEMS material. However, the other set showed a much lower modulus of 158 GPa. Modulus measurements of the nickel-iron alloy were inconclusive. The two LIGAMUMPs materials had yield and ultimate strengths on the order of 330 and 530 MPa respectively, while the HI-MEMS material was stronger at 420 MPa and 600 MPa. The nickel-iron alloy was considerably stronger, with a yield strength of 1.8 GPa and an ultimate strength of 2.4 GPa.

Fracture tests of polysilicon film

A new test approach is presented to measure the fracture toughness of thin films. The polysilicon specimen is a center-cracked panel that is 3.5 {micro}m thick and 3 mm wide with a 100 {micro}m long slot in the center. It is subjected to tensile loading, and the crack-opening displacement is measured by interferometry. The average toughness is 1.4 {+-} 0.65 MPa-m{sup 1/2}.