NAT'L INST. OF STAND & TECH R.I.C. NJST

PUBLICATIONS

A111DS 3SM53S

mistUnited States Department of Commerce

Technology Administration

National Institute of Standards and Technology

/MIST Technical Note 1500-1

Materials Reliability Series

Tensile Testing of Thin Films:

Techniques and Results

D.T. Read

1 QC 1 100 I .U5753 1

NO. 1500-1 1

1997^^1

NIST Technical Note 1500-1

Materials Reliability Series

Tensile Testing of Thin Films

Techniques and Results

D.T. Read

Materials Reliability Division

Materials Science and Engineering Laboratory

National Institute of Standards and Technology

325 Broadway

Boulder, Colorado 80303-3328

December 1997

*^TE8 °*

U.S. DEPARTMENT OF COMMERCE, William M. Daley, Secretary

TECHNOLOGY ADMINISTRATION, Gary R. Bachula, Acting Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Raymond G. Kammer, Director
National Institute of Standards and Technology Technical Note

Natl. Inst. Stand. Technol., Tech. Note 1500-1, 92 pages (December 1997)

CODEN:NTNOEF

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1997

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
Contents

1. Introduction 2

2. Overviews of the Individual Technical Papers 3

3. Discussion 5

3.1 Instrumentation 5

3.2 Computer Programs 5

3.3 Plans 6

4. References 6

Appendix A. 1993: A New Method for Measuring the Strength and Ductility of Thin Films... 8

Appendix B. 1994: Mechanical Behavior of Aluminum and Copper Thin Films 17

Appendix C. 1995: Fatigue of Microlithographically-Patterned Free-Standing Aluminum

Thin Film Under Axial Stresses 27

Appendix D. 1997: Tension-Tension Fatigue of Copper Thin Films 34

Appendix E. 1997: Piezo-Actuated Microtensile Test Apparatus 56

Appendix F. Drawings 75

Appendix G. Computer programs 87

in
Foreword

The Materials Reliability Series of NIST Technical Notes are reports covering significant research

accomplishments of the Materials Reliability Division. The Division develops measurement

technologies which enable the producers and users of materials to improve the quality and

reliability of their products. Measurement technologies are developed for process control to

improve the quality and consistency of materials, nondestructive evaluation to assure quality of

finished materials and products, and for materials evaluation to assure reliable performance.

Within these broad areas of measurement technology, the Division has focused its resources on

three research themes:

• Intelligent Processing of Materials — To develop on-line sensors for measuring the

materials characteristics and/or processing conditions needed for real-time process control.

• Ultrasonic Characterization of Materials — To develop ultrasonic measurements for

characterizing internal geometries of materials, such as defects, microstructures and lattice

distortions.

• Micrometer-Scale Measurements for Materials Evaluation — To develop measurement

techniques for evaluating the mechanical, thermal and magnetic behavior of thin films and

coatings at the appropriate size scale.

This report is the first in the series. It covers research on mechanical testing of thin films, one of

the projects in our research on Micrometer- Scale Measurements.

IV
Tensile Testing of Thin Films:

Techniques and Results

D.T. Read

Materials Reliability Division

National Institute of Standards and Technology

Boulder, Colorado 80303

Five technical papers covering the development of a set of techniques for measuring the tensile

properties of thin films are gathered here. Also included are drawings of the mechanical

components of the apparatus and listings of two computer programs. Additional necessary parts

include a computer, instrumentation, two piezoelectric stacks, and an appropriate platform

equipped with a microscope. Piezoelectric stacks are used as actuators. Noncontacting eddycurrent displacement sensors measure both the tensile displacement and the force. Closed-loop

feedback control allows a variety of test programs. The maximum available displacement is about

50 ^m, and the maximum available force is about 0.3 N. The resolution of displacement is about

25 nm, and the resolution of force is about 100 \iN. Cyclic loading has been demonstrated for

cycles as short as 20 s.

Key words: Displacement, force, mechanical property, microtensile, piezoelectric, strain, strength,

stress, thin film.
1. Introduction

Five technical papers [1-5] covering the development of a set of techniques for measuring the

tensile properties of thin films are gathered here. Also included are drawings of the mechanical

parts of the current apparatus and of the specimen. Additional necessary parts include a computer, instrumentation, two piezoelectric stacks, and an appropriate platform equipped with a microscope. The National Institute of Standards and Technology has not patented this tester, but

rather placed it in the public domain. One institution has already cloned it, and another is doing

the same. The author is interested in working with other parties who are considering building and

using this microtensile tester.

"Thin film" here means metal layers produced by physical vapor deposition or electrodeposition,

with thicknesses around 1 urn. Traditional mechanical test techniques, including specimen

designs, specimen fabrication routes, and test equipment, are not suitable for these thin films. The

apparatus described here tests specimens whose gauge section is typically 0.8 mm long and 50 to

200 urn wide. Specimens with thicknesses ranging from 0.3 to 15 urn have been tested. The

silicon substrate is removed from beneath the gauge section, so that the behavior of the freestanding film is studied.

Piezoelectric stacks are used as actuators. The current microtensile tester uses noncontacting

eddy-current displacement sensors to measure both the tensile displacement and the force.

Closed-loop feedback control allows a variety of test programs. The maximum available

displacement is about 50 urn, and the maximum available force is about 0.3 N. The displacement

resolution is about 25 nm, and the force resolution is about 100 uN. Cyclic loading has been

demonstrated for cycles as short as 20 s.

The measurement techniques and results described here were developed to serve the advanced

microelectronics industry. All microchips include thin films. Thin films are beginning to appear in

advanced electronic interconnection structures that link microchips to the external world. Their

function is principally electrical. However, their mechanical properties are relevant to the

manufacturability and reliability of advanced devices, because mechanical failure of the films

themselves and at their interfaces to other materials can cause failure of the electrical function of

such devices. As the devices are miniaturized, elements which once had only an electrical

function must now also take on a mechanical function. The specimens described here can be

produced in semiconductor fabrication facilities using normal microlithographic techniques.

As results on the mechanical behavior of thin films accumulate, certain differences between their

characteristics and those of bulk specimens of similar chemical composition are becoming clear. These differences result mainly from microstructure. Effects produced by the constraint of the

substrate are treated in the literature, but are not measured by the present experimental technique.
2. Overviews of the Individual Technical Publications

1993: A New Method for Measuring the Strength and Ductility of Thin Films [1]

This paper, included as Appendix A, introduced the use of the silicon-framed tensile specimen for

testing metal thin films. It was shown that the silicon could be etched out from underneath the

gauge section of the tensile specimen, and that the silicon frame could be cut manually just before

testing, without damaging the thin film tensile specimen. The sensors described here are still used.

This version of the tester was actuated by a manual differential micrometer, and the load was

measured by a flat meander spring. Both of these two characteristics of the tester have been

changed in later versions, described below.

Electron-beam evaporated aluminum was used as the specimen material. The specimens consisted

of up to four parallel tensile strips, so that the measured values of strength and elongation

reflected the average behavior. The reason for including four narrow strips instead of one wide

one was that previous attempts had shown that wider strips tore themselves during preparation,

because the stresses in the film caused it to curl at the edges. Narrower strips curled less severely.

This version of the technique produced evidence for the two major differences between thin film

and bulk behavior: the films have higher strengths, because of their small grain sizes; and the films

have very low tensile elongation to failure, for reasons still not completely understood.

1994: Mechanical Behavior of Aluminum and Copper Thin Films [2]

The previous work provided convincing evidence that the tester could be redesigned with a much

smaller stroke capacity, because elongation to failure of the films was quite low. The shorter

stroke opened up some advantageous possibilities: more restraint of the specimen motion could

be provided, to make it easier to cut the specimen without damage; the tension could be actuated

by thermal expansion of aluminum columns, so that the load could be ramped smoothly and

reproducibly; and the moving grip could be cantilevered, so that friction could be eliminated. A

second eddy-current sensor for specimen displacement was added, and the two sensors were mounted in the same horizontal plane as the specimen, to avoid possible spurious indications of

displacement.

This paper clarified and confirmed the differences indicated above between thin-film and bulk

behavior for electron-beam-evaporated aluminum and copper films. The widths of the individual

tensile strips in the specimens were varied to look for size effects. The limited data were

scattered, but no effects of specimen width on measured properties were found. This paper did

not address the issue of specimen thickness. A decrease in elongation to failure with decreasing

thickness in the range of 1 to 10 urn of thickness is suspected.

1995: Fatigue of Microlithographically-Patterned Free-Standing Aluminum Thin Film

under Axial Stresses [3]

This paper used the original tester [1] to study the effect of repeated loading on electron-beam-
evaporated aluminum films. The paper includes details of specimen preparation not previously

shown. The reproducibility of the fatigue life among the specimens was surprisingly good. The

stress-stress fatigue resistance, normalized against ultimate tensile strength, seemed poorer than

that of bulk sheet-metal specimens as described in the literature. Geometrical defects produced in

specimen fabrication and metallurgical effects related to the high strength of the films, relative to

that of typical pure, bulk polycrystalline aluminum, were the suspected causes of the observed

behavior.

1997: Tension-Tension Fatigue of Copper Thin Films [4]

This paper introduced a significant redesign of the tester to include piezoelectric stacks as actuators to replace the heated columns and the application of closed-loop feedback control. The

sensitivity and reproducibility of the tester were good enough that specimens with only one tensile

strip, instead of four, could be used routinely. A dental drill was used to cut the silicon specimen

frame, instead of simply chipping it away with a sharp stylus.

On the previous version of the tester [2], the force signal drifted whenever the tester was actuated

by heating the columns. This was attributed to twisting and bending of the reference and forcesensing beams caused by temperature variations. The temperature could not be kept constant

because heat was needed to produce the tensile displacement. The force could be measured only

by interpolating the zero-force signal between the beginning of loading and the failure of the

specimen, and taking the difference between the estimated zero-force signal and the actual signal.

This was a laborious and error-prone process. In addition, while the extension rate could be

controlled by the electrical current in the heaters, the cooling rate was much harder to control.

Cooling fluid could not be applied to the columns with acceptable control.

The replacement of the heated columns by piezoelectric stacks was a great improvement. The

drift of the force signal with extension of the tester was reduced to a small fraction of its previous

level. The moving grip could be extended and retracted smoothly at controlled rates.

More specimens were used in this study than had been used in the previous study of fatigue in

aluminum films, and the apparent scatter in the data was higher. However, relative to its ultimate

tensile strength, the fatigue resistance of the copper was superior to that of the aluminum

previously tested. The fatigue resistance of the copper appeared to reach normal levels for typical

bulk materials. Fatigue lives of over 100 000 cycles were demonstrated, as opposed to the

maximum of 78 reached in the previous study.

An attempt was made to study the mechanism of the fatigue process using optical and

transmission electron microscopy. Small cracks that developed in the titanium passivation layer on the copper films were shown to be arrested at the copper surface. Few individual dislocations

and no sign of dislocation cells were found.
1997: Piezo-Actuated Microtensile Test Apparatus [5]

This paper, to be published in the ASTM Journal of Testing and Evaluation and reprinted in

Appendix E, documents the tester in its present state. Detailed mechanical drawings are given in Appendix F.

3. Discussion

3.1 Instrumentation

The drawings in Appendix F include all the mechanical parts of the tester itself. Several additional

components are needed in order to conduct tensile tests. These are obtainable commercially as standard products. They include mainly a probing station to serve as a base for the tester, the

sensors, a computer equipped with an analog-to-digital converter card, the piezoelectric stacks,

and a power supply to drive them. A dental drill to cut the silicon frame on the specimen chip is also needed. The traveling microscope built in to the probing station used as a platform for the

microtensile tester has proved very useful. The three displacement sensors used have a full-scale

range of 50 urn, and a corresponding output voltage that ranges from -1 to 1 V . These eddycurrent sensors apply negligible force on the moving grip of the tester. Alternative noncontacting

sensor types could be used. For instance, capacitive and optical displacement sensors with similar

sensitivities and ranges are commercially available.

The piezoelectric stacks used have a stroke of about 50 urn. This was sufficient for most

specimens, but one set of very ductile electrodeposited copper films challenged the extension

capacity of the tester. Therefore, actuators with this stroke or higher are recommended.

Alternatives such as magnetostrictive actuators could be considered; however, the perturbations

and instabilities introduced by the dissipation of energy associated with the necessary electric

current would have to be considered carefully. A power supply is needed to drive the piezos, and

it must be controllable by the computer to enable closed-loop feedback.

3.2 Computer Programs

Source listings of the computer programs used with this tester are included on floppy disk as Appendix G. There are two programs: test and data analysis. PC-compatible computers are used

for both testing and data analysis. These programs operate satisfactorily with the present tester and its data. However, they have not been tested on other computers or by other users. Therefore, caution is in order regarding the use of these programs and any calculated results.

The test program is menu-driven and provides displacement, load, and piezo-voltage-controlled

ramps, and sinusoidal displacements and loads. It controls the motion of the moving grip of the

tester in closed-loop feedback. It also stores raw instrument readings five times per second. This

program is written for a commercially available Basic compiler.
The data-analysis program accepts a raw data file and a parameter file as input, and displays the

force-displacement or stress-strain curve in metric units. The parameter file includes specimen

dimensions and instrument calibration factors that can vary from test to test. It is usually

generated by modifying the parameter file for a previous specimen, though it could be created

from scratch. The yield and ultimate strengths can be calculated, and a line can be fitted to the

loading slope. Results can be stored as text files. This program is written for a commercially

available C++ compiler.

3.3 Plans

The present tester works well for specimens from 0.3 to 15 urn thick, in widths down to about 20

um for specimens 1 urn thick and greater. The very small specimens introduce problems in two

areas: accuracy of the force measurement, and damage to the specimen in the wet etch that removes the silicon from beneath the tensile gauge section. The very small specimens also seem

to be more susceptible to damage during the cutting of the silicon, from fragments and exhaust air

from the dental drill, for example.

The thinking at present is that the tester described in this technical note is a valuable intermediate

between full-scale conventional testers and a future, truly submicrometer-scale, test technique.

The submicrometer scale is clearly important, because metal lines on present-day integrated circuit

chips have widths well within the submicron range. The author is not currently pursuing

incremental steps to modify the present tester to test smaller specimens, primarily because the key

step in the present procedure for specimen preparation, that is, etching all the way through the

silicon wafer, seems out of scale with submicrometer-width specimens. Instead, alternative

concepts for measuring tensile properties measurements on specimens with widths of 1 um and

less are under consideration.

4. References

[1] Read, D.T.; Dally, J.W.; A new method for measuring the strength and ductility of thin

films. Journal of Materials Research 8(7): 1542-1549; 1993.

[2] Read, D.T.; Dally, J.W.; Mechanical Behavior of Aluminum and Copper Thin Films.

Mechanics andMaterialsfor Electronic Packaging: Volume 2 Thermal andMechanical

Behavior andModeling, edited by M. A. Schen, H. Abe, and E. Suhir, American Society

of Mechanical Engineers, pp. 41-49; 1994.

[3] Read, D.T.; Dally, J.W.; Fatigue of Microlithographically-Patterned Free-Standing

Aluminum Thin Film Under Axial Stresses. Journal ofElectronic Packaging,

Transactions ofthe ASME 117(1): 1-6; 1995.

[4] Read, D.T.; Tension-Tension Fatigue of Copper Thin Films. International Journal of

Fatigue, to be published.
[5] Read, D.T.; Piezo-Actuated Microtensile Tester. ASTM Journal of Testing and

Evaluation, to be published.
Appendix A. 1993: A New Method for Measuring the Strength

and Ductility of Thin Films
A new method for measuring the strength and ductility

of thin films

David T. Read

Materials Reliability Division, National Institute of Standards and Technology, Boulder,

Colorado 80303-3328

James W. Dally

Mechanical Engineering Department, University of Maryland, College Park, Maryland 20742

(Received 1 June 1992; accepted 10 February 1993)

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 /nm 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 yum thick Ti-Al-Ti film were determined.

I. INTRODUCTION

A. Background

In spite of the widespread application of thin films, we know very little about their constitutive properties.

Considering the vapor deposition process used in prepar- ing thin films, we anticipate that some of the mechanical

properties of the thin films may differ significantly from

those of bulk materials of the same chemical composition. Measurements of the constitutive properties of thin

films, including elastic modulus, yield strength, ultimate

strength, ductility, strain-hardening coefficient, creep and

stress relaxation constants, fatigue strength, and crack

initiation toughness, are necessary to develop models

that predict the behavior of films when subjected to mechanical and thermal stresses. The experimental chal- lenges encountered in measuring the constitutive properties are due to difficulties in fabricating the thin film

tensile specimens, in handling these thin and fragile spec- imens with cross-sectional areas of the order of 10~3 mm2

, and in measuring small forces and displacements.

Experimental techniques applied to thin-film test- ing1-9 can be divided into four different categories. In the first approach, 1 a thin film is deposited on a suitable

substrate, and then the specimen array is patterned in the

film. The specimens are then stripped from the substrate and manually mounted in the grips of a miniature tensile testing device. The second approach is identical to the

first, except that the grips are attached to the film specimen before it is stripped from the substrate.

2-4 Mounting the grips on the specimen before stripping

facilitates handling and improves alignment in the tensile loading device.

A third group of investigators5 ' 6 uses nanoindentation to measure yield strength and elastic modulus. In nano-indentation, the force on the indenter is determined as a function of the penetration depth, with

the film bonded to the substrate. The fourth approach7- 8 involves tensile testing of

small thin-film specimens, where the substrate is re- moved from under the specimen by etching. The advantage of separating the film and the substrate by etching

is the relief of the residual stress produced in the vapor

deposition process.

B. Features of the new method

In this paper, we describe a new method to measure the mechanical strength of thin films. The method

involves: (1) The production of damage-free thin-film

tensile specimens with controlled geometry, using microlithography and substrate etching. (2) The use of a miniature rectangular silicon frame suspension for the specimens; the frame maintains alignment and protects the specimens. The frame also provides a test element

large enough to handle efficiently. (3) The production of

many (36) identical frame-supported specimen arrays on

a single 51 mm diameter wafer of silicon. (4) Mounting

a specimen frame in the microtensile tester and severing

the vertical members of the frame to free the specimen,

while maintaining the alignment required for testing.

(5) The use of a microloading system with an optical

microscope, video recording system, and an x-y table to permit the controlled application of axial tensile loads up

to 1 N (100 gram-force) with the capability to observe

the specimens at magnifications up to 2500 X. (6) The

use of an instrumentation system to measure and record

the applied loads and the resulting specimen elongation.

Many elements of this method are borrowed from

previous investigations, as detailed above. The most

1542 J. Mater. Res., Vol. 8, No. 7, Jul 1993 9
D.T. Read and J.W. Dally: A new method for measuring the strength and ductility of thin films

novel aspect is the ability to produce a freely suspended

thin-film specimen that has never been handled or deformed. The process of stripping a thin film from a

substrate and handling it during mounting admits the

possibility of mechanical damage to the specimen. Use

of water-soluble substrates such as sodium chloride is an

option, but sodium chloride is a deleterious contaminant

in semiconductor fabrication, while the silicon substrate used here is compatible with semiconductor fabrication processes. An additional novel benefit of the present

method is that it lends itself conveniently to replicate

testing. Because several specimens are fabricated with

the same geometry, results of multiple tests can be

treated statistically.

II. SPECIMEN FABRICATION METHODS

The tensile specimens are produced by employing

photolithographic techniques common to the semiconductor industry. The process begins with a 51 mm (2 in.) diameter by 0.25 mm (0.010 in.) thick silicon wafer.

The silicon is n-type, and the wafer is cut in the (100)

orientation. The wafers are procured with both sides

polished. The first step in specimen fabrication is to heat the wafers in a steam environment to obtain an

oxide layer about 0.5 yum thick. The wafers are patterned

by photolithography and etched to remove the oxide

layer from the locations of the gage sections of the

tensile specimens. Next, the metal films are deposited over the entire surface of the wafer using electron beam

evaporation. In this investigation, the metallization was

not sintered. A multilayer film was deposited, consisting

of titanium-aluminum-titanium with thicknesses of 0.1,

1.98, and 0.1 /um. Other materials and other thicknesses can be deposited as well. The metallized side of the wafer was then coated

with negative photoresist, and the pattern of the spec- imen illustrated in Fig. 1 was contact printed in the

photoresist. The glass master used in this printing operation had 36 specimen patterns geometrically arranged to

fit on the 51 mm diameter wafer. The specimen geometry was produced subtractively using a series of etching

treatments. The titanium layers are etched with a dilute mixture of hydrofluoric and nitric acids buffered with

ammonium nitrate. The aluminum layer is etched in a phosphoric-acetic acid mixture at 40 °C. After the specimen geometries were produced at each site on the wafer surface, both sides of the wafer were coated with a positive photoresist. Two additional

glass masters were then used to contact print the lattice representing the rectangular frames: one on the front side

of the wafer and the other on the back. Both masters were aligned to the specimen pattern in a manual mask

aligner. The pattern on the back side of the wafer was

aligned by viewing the wafer and master with infrared

0.25 mm

1.00 mm

FIG. 1. Array of four thin-film tensile specimens.

light transmitted through the wafer. After developing

the photoresist, the wafers were etched in buffered

hydrofluoric acid to remove the SiC>2 layer over areas not incorporated in the frame lattice. The SiC>2 remaining

served as a mask for the silicon etch. A water solution of hydrazine9* 10 at 110 °C was

employed to etch the silicon wafer in the areas not protected by the Si02 patterns. The hydrazine etches

the silicon at a relatively rapid rate, about 2 yum/min.

The etch is highly anisotropic in that it etches the (111)

faces much more slowly than the others. The hydrazine

etchant also does not attack either Al or Ti, although

it may attack the grain boundaries to some degree. It has several disadvantages. Most important, it is quite

hazardous and requires effective precautions in venting

and disposal during and after use. It also causes the metallic films to peel if they are not well bonded to the substrate during vapor deposition.

This method of specimen fabrication produces a very

large number of specimens with uniform properties. For

example, the 51 mm wafer shown in Fig. 2 has a total of

36 specimens. Each specimen is supported by a rectan- gular frame 6 by 8 mm, which is essentially a die. Scribe

lines are etched in the wafer to facilitate separation of

the dies by controlled fracture. A photograph showing

several frames is presented in Fig. 3(a). An enlargement

of the specimen array is shown in Fig. 3(b).

J. Mater. Res., Vol. 8, No. 7, Jul 1993 10

1543
D.T. Read and J.W. Dally: A new method for measuring the strength and ductility of thin films

FIG. 2. Picture frame supported tensile specimens are produced on a single 50-mm-diameter wafer.

III. MICROTENSILE LOADING SYSTEM

A microtensile loading system was developed to accommodate the thin-film specimens with the frame

suspension. The system is capable of applying uniaxial

static loads in excess of 1 N (100 gf). Both the load

and the resulting elongations are measured using eddy

current sensors. The mechanical features of the loading system,

illustrated in Figs. 4 and 5, include a fixed and a slid- ing platen with a pair of specimen-mounting plates, a

calibrated spring, and a series connected pair of micrometers. The mounting plates are fixed to the two platens

with small screws, so that they can be removed and

recycled. The specimen frame, after alignment, is bonded

to the mounting plates with a quick-setting methyl-2-

cyanoacrylate adhesive. The load is applied by displacing one end of the spring with one or the other of the two

micrometers. One micrometer is a differential type with

graduations of 1 fim, and the other is a conventional

type with graduations of 10 /im. The other end of the spring is fastened to the sliding platen. As the spring

is extended, the spring force is transmitted through the

sliding platen to the specimen array. With the spring

stiffness of 0.667 N/mm, a minimum load increment of

0.667 mN can be applied with the differential micrometer. Since the loads to failure usually range from to 0.49 N (50 gf ), the minimum increment is sufficiently

small for controlled application of the load. The primary disadvantage of this loading technique

is the frictional constraint developed on the sliding

platen. The frictional effects are minimized by applying

the axial load in small increments (3 to 6 mN) and

then tapping the support structure of the loading system

with a light striker. The resulting vibrations unlock the

(a)

(b)

FIG. 3. (a) Picture frame supported array of specimens, (b) Enlarge- ment showing the specimen array.

asperities and relieve the frictional forces that impede

the motion of the sliding platen.

The instruments used to measure the load and speci- men elongation employ noncontacting eddy current sensors. The load is measured by sensing the displacement

of the spring. This measurement is made by mounting

the coil side of a conventional eddy current displacement

transducer in the support block for the spring. The

transducer senses the position of an aluminum flag that is attached to the sliding platen. The operating principles

of the eddy current sensor are illustrated in Fig. 6(a), where the flag on the sliding platen is represented by the "target" that is being tracked. The voltage output from this sensor is linear over a range of spring extension from to 1 mm. A calibrated

output is possible for spring extensions as large as 5 mm,

but the voltage output is nonlinear. The sensitivity of the sensor in the linear range is 0.4 V/mm with the gain

1544 J. Mater. Res., Vol. 8, No. 7, Jul 1993 11
D. T. Read and J. W. Dally: A new method for measuring the strength and ductility of thin films

Flat Spring Specimen R

. Frame /Raton

-»- Micrometer

Driven Platen

FIG. 4. Illustration of the microtensile loading system with sensors

for measuring load and elongation.

provided by the amplifiers in the signal conditioning

circuits shown in Fig. 6(a). The output voltage is observed on a liquid crystal display during loading. It is also recorded on the y-axis of an x-y recorder that is used to provide an on-line load-elongation record during

the tensile test. The output is also digitized and stored on

a hard disk. The compliance of the measuring spring was

repeatable to within 1%. The point-to-point scatter in the load reading is less than 0.2%. Each point on the plots

shown below represents the mean of 20 such readings.

The uncertainty in the force measurements is ±1%.

The specimen elongation is also sensed with an eddy current transducer, which is mounted in the support

block for the fixed platen. Again, the transducer senses

the position of the flag attached to the sliding platen.

Because the displacement of the flag due to specimen

elongation is so small (about one wavelength of light at the yield stress in aluminum), it is necessary to employ

a sensor with enhanced sensitivity. This improvement

in sensitivity is accomplished in the signal conditioning

circuit shown in Fig. 6(b). Note that the log amplifier

has been removed. This modification produces a much

higher voltage output near the zero position of the sensor, but severely limits the range over which the output is linear (25 yu,m). The sensitivity of the instrument system

is 40 mV//im, which corresponds to a voltage output of

400 mV for a strain of 1% imposed on the specimens

with a gage length of 1 mm. The resolution of the system

is specified as 0.0025 /nm, which is equivalent to the displacement due to a strain of 0.00025%, acting on a

tensile specimen 1 mm long. However, the governing

uncertainty turned out to be larger, as noted below.

IV. TESTING PROCEDURES

The specimens are mounted in the microtensile

loading device by adhesive bonding. The platens are positioned axially so that both the load and elongation sensors are at the beginning of their linear ranges. Speci- men alignment is achieved by positioning the rectangular

frame specimen assembly squarely on the platens. Then

the specimen is bonded to the mounting plates with an

adhesive that polymerizes almost instantly. The speci- mens are ready to be tested except that the silicon frame

which served as a carrier must be removed.

The vertical sides of the frame are effectively re- moved by fracturing, thus freeing the specimen. The

horizontal members of the frame are retained and serve

as miniature platens, assuring that the alignment of the specimen array is maintained. The difficulty in fracturing

the vertical members is preventing any motion of the

sliding platen, which would damage the specimens dur- ing the fracturing process. Any slight movement imposed

on the sliding platen as the frames are fractured produces

large stresses in the specimens and causes premature

failure. To avoid damage to the specimen during fracturing

requires that the sliding platen be locked to the fixed

platen. A special clamp was fabricated, as shown in Fig. 7, to lock the sliding platen to the fixed platen.

This clamp, which bridges the sliding platen, is fastened

to the fixed platen with two end positioned screws. The clamping action is applied with other screws that engage the flexure plate and drive it downward onto the sliding platen. The flexure plate limits motion of the

/jim/»/yi»(im|[m/M»mi|ifflpi|»ip|iii[jtt,

;!

.

:

, . ;pi|iiii|!iii[»ii|iiii|iiii|ii»jii:i|iiiijiai|iiii|iiii|iin|iii

iliiilililiiil^liiiliiiliiJihli hlihJiiiliiiMlhliiJiiiliiiliiiliiilniliiJ

FIG. 5. Photograph of the microtensile loading system.

J. Mater. Res.. Vol. 8. No. 7. Jul 1993 12

1545
D. T. Read and J. W. Dally: A new method for measuring the strength and ductility of thin films

INDUCTIVE BRIDGE CIRCUIT

tl

fyy\ » imru

SYNCHRONOUS

DEMODULATOR

LOG AMP

CNVTR

COARSE LINEARITY

Hh '-.- CABLE CAPACITANCE ZERO ADJUST

REF COIL ACTIVE COIL

OUTPUT

AMP.

tl

GAIN ADJUST

ANALOG

VOLTAGE OUT

(a)

Target Moves 'Normal*

to sensor' s face

Sensor

Gain —

Adjustment

Target

Active

Coil Oscillator

Synchronous

Demodulator

Output

Amplifier Analog

Output

Offset (Zero) Adjustment

(b)

FIG. 6. (a) Block diagram of electronic components in a conventional eddy current transducer, (b) Block diagram of electronic components

in an enhanced sensitivity eddy current transducer.

sliding platen as the clamp is engaged and prevents any

movement of the picture frame.

When the clamp is tight and the sliding platen is fixed, the vertical sides of the frame are fractured. We

initially fractured the vertical sides by promoting cracks

that propagated across the width of each side. This was

not reliable because in many instances the cracks were

arrested by the thin surface layer of Si02 , and the relief of the constraint was incomplete. A more effective proce- dure was to use a fine scribe to break away small pieces

of the silicon until a band of material was removed from

each of the vertical members. When the breakage was

completed, the flexure plate was released and the clamp

removed. A photograph showing the specimen and the fractured members of the frames is given in Fig. 8. The specimen array and frame were examined under

high magnification at the conclusion of this process to ensure that no damage had been induced and that the specimens were in perfect alignment. Load was then

applied in small increments (about 2% of ultimate load)

by extending the spring. The frictional constraints at the sliding platen were relieved by tapping with a light striker after each load increment.

The voltages from the eddy current sensors monitoring load and elongation were recorded on an x-y

recorder that could be observed during the test. The

1546 J. Mater. Res., Vol. 8, No. 7, Jul 1993 13
D.T. Read and J.W. Dally: A new method for measuring the strength and ductility of thin films

jjfjwmuyimjDtt C JESS.

Ill|!llf|llli|lltl|llll|fffl|llll|llll|llll|ll!!|ll!t|llll|lin|i

1 1 1 1

1 xi !

1 1 1 1 1 1 1 1

1 lu j ijJxJLUjjjJLixijJuJUi

FIG. 7. Clamp, incorporating a flex plate, for locking the sliding

platen.

output voltages were also digitized and stored on a hard

disk at a rate of four readings per second.

Because of problems with an accurate measurement

of the elastic response of these films, the yield strength was determined by using a 0.2% offset with an assumed

elastic modulus of 70 GPa, Young's modulus of bulk

aluminum.

The in-plane specimen dimensions were measured

using the displacement readouts of the optical microscope. The widths of the miniature coupons, nominally 250 /xm, were measured to within approximately

±2 /xm.

The thickness was measured on untested portions

of the film using a commercially supplied instrument

that records the vertical displacement of a diamond

stylus as it is displaced across the edge of the thin

film. These measurements were made on the whole film and, after etching away the components one by one, are believed accurate to 5% based on their reproducibility.

This uncertainty controls the measurement uncertainty

for the stress which is ±5%.

Because all the specimens in Table I came from

the same wafer, all have the same film thickness. The

wafer was mounted 50 cm from the crucible and rotated during deposition, so variations of ±5% in the film thick- ness on this wafer can be excluded. While the average

stress value is uncertain by ±5%, this uncertainty is not the primary cause for the scatter in the measured

strength.

An uncertainty in the effective gage length re- sults from the persistence of projections of the silicon substrate under the ends of the specimens. These are not exactly the same from specimen-to-specimen and

assembly-to-assembly and add an uncertainty of up to -20%, or ±10%, to the strain results.

V. RESULTS AND DISCUSSION

Yield and ultimate strengths determined for seven specimens are listed in Table I. The scatter in these

results is larger than the measurement uncertainty of 5%

and is attributed to variability in the specimens and the specimen material, to be discussed below.

The test conditions enforce equal displacement on

the four individual specimens, while the total force is recorded. So the load-displacement record gives the aver- age force (over the four individual coupons in the array)

at the indicated displacement. However, because the

TABLE I. Mechanical properties of thin Ti-Al-Ti films.

Specimen Yield strength Ultimate Strain at failure identity (MPa) strength (MPa) (%)

a 172 a A 121 199 1.18 B 115 186 1.15 C 96.4 191 1.81 D 71.5 156 1.28 E

b 120 144 0.54 F 91.0 146 0.85

FIG. 8. Photograph of the specimen array and a broken vertical member on the frame.

a Not measured.

b Single specimen rather than an array of four specimens.

J. Mater. Res., Vol. 8, No. 7, Jul 1993 14

1547
D.T. Read and J.W. Dally: A new method for measuring the strength and ductility of thin films

loading spring is soft, the test conditions are essentially force-controlled. When one individual specimen of the array fails, the remaining three are unable to sustain

the applied force, so the whole array fails. That means

the reported strength values correspond to the lowest

failure load among the four individual specimens, at the displacement indicated. The force-displacement and

stress-strain curves of Fig. 9 indicate average force and

stress values at the indicated displacement and strain. The present technique has not produced acceptable

values of Young's modulus because of problems with

the accuracy of the displacement measurement. Slight

rotations of the sensing flag (Fig. 4) contribute spurious

displacements of the order of 1 /tm to the measured

displacements. Attempts to measure the spurious dis- placement and calibrate it out produced load-elongation

and resulting stress-strain curves, as shown in Figs. 9(a)

40.8

306

20.4

10.2

-0.1 2 4 6 8 10 12

Displacement, micrometers

102

(a)

-0.002 0.002 0.004 0.006 008 0.01 012 0.014

Strain

(b)

FIG. 9. (a) A typical force-elongation record forTi-Al-Ti (0.1-2.0-

1.0 ftm) films, (b) A typical stress-strain curve for Ti-Al-Ti

(0.1-2.0-0.1 ^m) films.

1548

and 9(b). These stress-strain curves gave varying values

of the initial slope, so we did not attempt to derive elastic modulus values. However, the qualitative features of the curves shown in Fig. 9 were reproduced in all the tests. We

have listed the total elongation to failure of each

specimen in Table I. These values, ranging from 0.54%

to 1.81%, are much lower than the value of 15%

or so that characterizes commercial aluminum alloys.

This discrepancy is far too large to be accounted

for by the measurement uncertainty of ±10% of the

strain to failure. Furthermore, the behavior of the specimens during elongation to failure, as observed in the microscope, was quite different from normal bulk

tensile specimens, including specimens of the thin sheet. The behavior appeared quite brittle. The specimens

fractured suddenly with no apparent stretching, thinning, or narrowing (Fig. 10). The brittle fracture strength of a specimen is strongly

dependent on its flaws or cracks. Brittle fracture strength

is usually measured on different materials than ductile fracture strength, so the variabilities cannot be compared

directly. But, usually, the brittle fracture strength, when

it is controlled by small flaws or cracks, is more vari- able than the ductile fracture strength, as measured on

replicate specimens of a ductile material. We conclude that the present thin-film specimens

exhibit behavior that is considerably less ductile than

bulk commercial aluminum alloys. The low ductility is evident in the low strains to failure and is reflected in the scatter of the strength properties. Effects of misalignment can be important in tensile testing. The present specimens are too small for the standard alignment check, which requires mounting

multiple strain gages around a specimen and checking for equal strains. However, the multiple specimens

FIG. 10. Brittle failure of the thin films near the ends.

J. Mater. Res., Vol. 8, No. 7, Jul 1993 15
D.T. Read and J.W. Dally: A new method for measuring the strength and ductility of thin films

in the specimen arrays can be observed through the microscope. Their appearance changes noticeably as they

are initially loaded, because they become straighter and,

thus, reflect more of the normally incident light back up

to the eyepiece of the microscope. Visual observations

indicated that all specimens straightened out together,

with no systematic side-to-side progress or asymmetry.

No systematic order of specimen failure was noted. The

interior two specimens showed cracking before fracture

to the same degree as the outer two, and the left and

right specimens seemed to fail similarly. In addition, the loading spring is soft relative to bending about its tensile axis. In the presence of small misalignments, only

a small difference in tensile force across the specimen

array would be produced. Therefore, the scatter indicated

in Table I is not attributable to misalignment.

VI. CONCLUSIONS

The use of photolithography for fabricating the specimens permits a large number of specimens to be

produced with uniform characteristics. With relatively

rapid testing, it is possible to gather a significant amount

of data and to provide properties with statistical signif- icance. For instance, in the seven measurements of the ultimate tensile strength of the Ti-Al-Ti films, we can

report a mean strength of 170.6 MPa and an estimated

standard deviation of 22.3 MPa.

The ductility of these films is very low (0.54 to 1.81%) when compared to the ductility of either bulk

aluminum or titanium, which are both an order of

magnitude higher. The low ductility is due to one or

all three of the following factors: (1) The edges of the specimens were not straight,

but contained strain concentrations which produced pre- mature failure. (2) Flat tensile specimens with a very high ratio of

width to thickness, 114 in this case, are incapable of

reaching the levels of plastic tensile strain that are seen

in specimens with aspect ratios around one.

(3) These metallic films produced by electron beam

evaporation are inherently brittle because of their microstructure, which is much different from that of normally

processed bulk metals.

The most significant problems in the use of this microtensile loading system are the friction which in- hibits the free motion of the sliding platen and the sensor

flag rotation which limits the accuracies in measuring

elongations at small loads. The most significant problems

with specimen fabrication and inspection are an accurate determination of the cross-sectional area and the gage

length, and the necessity of the Ti protective layers.

ACKNOWLEDGMENTS

A variety of facilities and apparatus are required for fabricating tensile specimens on silicon. Use of facilities offered by R. E. Harris, leader of the Cryoelectronics

Group of the Electromagnetic Technology Division of

N.I.S.T., and by William May of the Electrical Engineering Department of the University of Colorado at Boulder is gratefully acknowledged. Paul Glairon is acknowledged for a helpful discussion on etching silicon. REFERENCES

1. J. E. Steinwall and H. H. Johnson, in Thin Films: Stresses and

Mechanical Properties II, edited by M. F. Doerner, W. C. Oliver, G. M. Pharr, and F. R. Brotzen (Mater. Res. Soc. Symp. Proc. 188,

Pittsburgh, PA, 1990), pp. 177-182.

2. R. W. Hoffman, in Thin Films: Stresses and Mechanical Properties, edited by J. C. Bravman, W. D. Nix, D. M. Barnett, and D. A.

Smith (Mater. Res. Soc. Symp. Proc. 130, Pittsburgh, PA, 1989),

pp, 295-306.

3. C. T. Rosenmayer, F. R. Brotzen, and R. J. Gale, in Thin Films:

Stresses and Mechanical Properties, edited by J. C. Bravman,

W.D. Nix, D.M. Barnett, and D.A. Smith (Mater. Res. Soc. Symp. Proc. 130, Pittsburgh, PA, 1989), pp. 77-86.

4. I. Kim and R. Weil, Testing of Metallic and Inorganic Coatings,

edited by W. B. Harding and G. A. DiBari (American Society for Testing Materials, STP 947, Philadelphia, PA, 1990),

pp. 177-182.

5. T.P. Weihs, S. Hong, J.C. Bravman, and W.D. Nix, in Thin Films: Stresses and Mechanical Properties, edited by J. C. Brav- man, W. D. Nix, D. M. Barnett, and D. A. Smith (Mater. Res. Soc. Symp. Proc. 130, Pittsburgh, PA, 1989), pp. 87-98.

6. W.D. Nix, Metall. Trans. 20A, 2217-2245 (1989).

7. L. S. Fan, R. T. Howe, and R. S. Muller, Sensors and Actuators A21-A23, 872-874 (1990).

8. M. Mehregany, R.T. Howe, and S.D. Senturia, J. Appl. Phys.

62, 3579-3584 (1987).

9. D. B. Lee, J. Appl. Phys. 40, 4569-4574 (1969).

10. M. Mehregany and S.D. Senturia, Sensors and Actuators 13, 375-390 (1988).

J. Mater. Res., Vol. 8, No. 7, Jul 1993 16

1549
Appendix B. 1994: Mechanical Behavior of Aluminum and

Copper Thin Films

17
AMD-Vol. 187, Mechanics and Materials for Electronic Packaging:

Volume 2 - Thermal and Mechanical Behavior and Modeling

ASME 1994

MECHANICAL BEHAVIOR OF ALUMINUM

AND COPPER THIN FILMS

David T. Read

Materials Reliability Division

Boulder Laboratories

National Institute of Standards and Technology

Boulder, Colorado

James W. Dally

Department of Mechanical Engineering

University of Maryland

College Park, Maryland

ABSTRACT

The mechanical behavior of thin films produced by electron- beam evaporation of Al and Cu and deposited on silicon substrates has been studied experimentally. Thin layers of Ti, similar to adhesion-promoter and diffusion-barrier layers used in microelectronics fabrication, were usually included on the specimen films. Tensile specimens that consisted of four parallel coupons were patterned by microlithography. The gage sections of the specimens were freed by etching away the substrate, leaving the tensile coupons supported by a die-sized silicon frame.. These specimens were tested in a specially designed thin film testing machine mounted under an optical microscope.

Improvements in the apparatus have eliminated a sliding—

friction error, greatly reduced spurious contributions to the measured displacement, stiffened the load sensor so that behavior

of the specimen past maximum load can be observed, and added

closed-loop control of imposed displacement or load.

INTRODUCTION

Motivation: Reliability of thin films in electronic

packaging

This study is motivated by the need to improve the general

levels of understanding and predictability of the reliability of

electronic devices and electronic packaging and interconnect

structures. Aluminum thin films are widely used in the manufacture of integrated circuits, to form electrical connections among circuit elements and bonding pads. Aluminum and copper

films are used in the manufacture of MCM-D (multi-chip-module-

-deposited) interconnect structures, and are finding new uses in the MCM-L (multi-chip-module—laminated) structures.

The treatment of the critical issues of manufacturability and

reliability of integrated circuits and of packaging and interconnect

structures is reaching a new level of sophistication. It has been

reported that savings of cost and time-to-market have been

achieved through the use of finite element analysis of mechanical

behavior, to streamline and focus procedures in design, inspection,

and test (Dunn, 1993). This use of finite element analysis has been proved effective in the design and evaluation of highperformance structures such as nuclear pressure vessels, aircraft, and a variety of aerospace structures. Several reports of advanced

analyses of the mechanical behavior of electronic packaging and

interconnect structures can be found in the literature. A few examples are: Pao at al, 1991; Guess and Burchett, 1992; and Lau

and Rice, 1992. A key advantage of the use of advanced mechanical

engineering analyses, particularly finite element analysis, is the quantitative link achieved between material properties and

structural behavior. Macro-world failure analyses are considered complete only when the predicted failure conditions, based on the material properties, structural geometry, and failure criterion, agree quantitatively with the actual loading at failure. To date

this quantitative link has been absent in the analysis of electronic interconnect structures because of the lack of material properties

data. In fact, material property values are often taken as variable parameters in analyses of electronic packaging structures. The

present study attempts to address this lack of data.

Background: Mechanical behavior of thin films

Many studies of the mechanical behavior of thin films have been reported in the literature. These are reviewed by Hoffman

(1966) and Hardwick (1987). Often the principal subject has been

the fundamental nature of thin films, or a new testing technique. This literature can be briefly summarized in a few general

18
statements, although these are clearly oversimplifications of a complex subject:

1. It is necessary to be specific about the meaning of the term "thin film." This study treats films produced by physical

evaporation and vapor deposition, with thicknesses in the range from 0.2 to 2.5 micrometers thick. The behavior of metal thin films alone, independent of substrate, is considered here. Direct

effects of the substrate on mechanical behavior are excluded by

removing the film from the substrate before testing. Indirect

effects, such as an influence of the substrate on the microstructure

or chemical composition of the deposited thin film, are of interest.

2. Mechanical behavior of metal thin films can be studied experimentally using a variety of special techniques; test procedures designed for bulk materials cannot be used directly

(Nix, 1989, Read, 1992).

3. It seems clear that evaporated metal films in the thickness range considered here have the same crystal structure, lattice spacing, and Young's modulus as their bulk metal counterparts. During the course of a series of studies on compositionally modulated films (Baral el al., 198S), it was shown that these three physical properties are the same for pure-metal films as for bulk

material. The latest reports (Davis et al., 1992), indicate that compositionally modulated films also have elastic properties

nearly identical to those of bulk mixtures with the same average

composition.

4. Thin films seem to have low elongation to failure compared to bulk specimens. Reported elongations to failure for

thin films are often around 1 percent. Hardwick (1987) indicates

that low measured values of ductility may be a consequence of

the difficulty of testing suspended thin films. The present study

will conclude the opposite: that thin films do in fact have low

tensile elongation to failure, and this behavior is not an artifact of

measurement.

5. The strength levels reported for thin films are usually

higher than for pure bulk specimens of the same chemical

element. This is considered to result from some combination of

the very small grain sizes often found in thin films and the effect of impurities introduced during the vapor deposition process.

SPECIMEN PREPARATION

The tensile test techniques used here include a specimen

preparation procedure that is compatible with the equipment and

procedures used in the fabrication of electronic devices. Direct handling of the thin metal film is avoided, uniaxial tensile force- displacement records are produced, and the specimen size can be

scaled downward through microlithography. The specimen

preparation procedure has been described in detail elsewhere (Read and Dally, 1993) Here a brief description emphasizing

recent improvements is given.

Figure 1 shows the Si, Si02

, and metal layers in the specimen assembly when it is ready for testing. Substrate

preparation begins with oxidized silicon wafers, polished on both

sides. We have used wafers that are 50 mm in diameter by 0.3 mm thick, with a [100] orientation. The Si02 layer is about 5000

A thick. The Si02 on both sides of the wafer is patterned with

photoresist to mask die-sized specimen frames 8 mm square, with

a window that is 1 mm by 2 mm in the center, figure 2.. The

UJJ.UJ .WW

[IJlllliilllii

Metql t9rglte tpedmen

1000

I llll I Imposed displacement

Dimensions In Mm

FIGURE 1

.

SCHEMATIC CROSS-SECTION OF THE

SPECIMEN ASSEMBLY AFTER

FABRICATION IS COMPLETE.

111111 —h

-:::( : •:::::•::-:-:: A

- IfIII ||S

BSj SUt

111111 Hi i n 1

1

111111 i! Jill .:.:.;J|l|

J_—._

; :x:;

: ::

: ::

:

:

: :: ::

:

:

:

: :

"':" : *v: : *'*;:': : x\[:'\:': : :;';:

: :;:;:

:

:

: Ivx^vSSv T

: '

1 1 FIGURE 2. PHOTOMASK LAYOUT OF THE

SPECIMEN ASSEMBLY. THE GRID

SPACING IS 1 mm. THE LIGHTER

GRAY IS THE Si02 LAYER, AND THE

DARKER GRAY IS THE SPECIMEN

METAL.

photoresist pattern on the back side of the wafer is aligned to the developed photoresist pattern on the front side, using infra-red

illumination. Etching in BOE (buffered oxide etchant) removes

the unmasked Si02

, leaving a pattern of frames with windows in the remaining Si02 on both sides of the wafer.

Final plasma-cleaning of the wafer and deposition of the metal film are carried out in a single specially-equipped vacuum

chamber. Argon gas at a pressure of 10 to 50 milliTorr forms a plasma. The plasma is driven with 100 watts of electrical power

for ten minutes. This cleaning procedure has been found

necessary, because poorly-adhering films will be destroyed during

the Si etch step. Commercially supplied aluminum and copper of

19
5 nines purity have been used as specimen materials. The metal

is heated in a crucible by an electron beam until the deposition

rate reaches the selected level, 3 to 10 A per second. Evaporation

is continued until the desired metal thickness is reached. At 10 A per second a 2 u,m film can be deposited in about half an hour. This paper reports data for specimens with thin titanium layers on

top and bottom and for some pure metal specimens. Ti layers 0.05 and 0.1 um thick have been used. Grain sizes of thin films are much smaller than those of

bulk metals, except for epitaxial films. The grain sizes of the

films tested here have been measured to be in the range of 0.5 to

1 um. Transmission electron microscope (TEM) examination of

as-deposited films in the 0.2 to 0.5 um thickness range reveals

typical polycrystalline metal, with clearly defined grain

boundaries, a significant population of twins in Cu, and few builtin dislocations outside the grain boundaries. Next, tensile coupons (strips) are patterned in the metal film, with widths from 14 to 250 um. Each specimen consists of

multiple parallel strips mounted to the same frame. The strips on

a frame will all be loaded at the same time; the measured force

will be the sum of the forces on each of the individual strips. The 250-um-wide coupons have gage lengths of about 1 mm.

The narrower coupons have proportionally smaller gage lengths, with the ratio of length to width maintained between 3 and 10. The tensile strips are formed using a typical wet-etch lithographic

process. Figure 2 shows the design, with the metal layer

superposed on the oxide layer. The pattern of the tensile strips is realized on a photomask. Negative photoresist on the specimen

wafer is exposed to ultraviolet light through the photomask, after

it has been aligned to the frames. The resist is developed and

baked, and then the specimens are etched. The aluminum etchant

is the usual phosphoric acid solution, used at 40 °C. The same

solution is used 35 °C to etch copper. The Ti layers are etched

with a dilute solution of H2 2 and HF in water. If the solution is old and the H2 2 has dissipated, the photoresist can be attacked, causing damage to the specimens. The wet-etch produces edges

which have no tears. At high magnification in the SEM,

deviations from straightness of a fraction of a um appear all along

the specimen edges.

The wafers are introduced into an aqueous hydrazine solution

in a reflux condenser (Mehregany and Senturia, 1988), and etched

for 4 hours at 100 °C. This treatment removes the unprotected

silicon in the windows under the gage sections of the tensile coupons, and leaves an array of frames containing suspended

tensile coupons. These usually have the appearance of being

slightly curled out-of-plane in the transverse direction. They do

not appear taut, and are probably slightly buckled. APPARATUS

The testing machine used in the previously reported study

(Read and Dally, 1993) has been replaced. The new testing

machine consists of a fixed grip and a movable grip, connected by

columns that can be expanded, figure 3. The expandable columns

are made with heaters to produce thermal expansion, or are made

of piezoelectric stacks. The moveable grip is connected to a

flexible steel beam that bends in proportion to the applied force. This grip is cantilevered just above the base of the device, so

Displacement

sensors

Moveable grip Specimen

Fixed grip Expandable

columns

Embedded

heater

FIGURE 3. SCHEMATIC DIAGRAM OF THE

MECHANICAL TESTING DEVICE. THE

DEVICE MEASURES ABOUT 14 BY 12

CM.

sliding friction, which was present in our previous apparatus, is eliminated. The testing machine is actuated by current to theheaters or by voltage applied to piezoelectric actuators. The

thermal device used aluminum columns with cartridge heaters mounted in holes. The piezoelectric device has two piezoelectric stacks 60 mm tall. Displacement is imposed at a constant rate, by using a closed loop control control system to apply current to the heaters or voltage to the piezoelectric stacks. The control loop is driven by

a computer which accepts input from the sensors and updates the drive three times per second. Displacement is measured at the

grips, using noncontacting eddy current sensors, figure 3. The

grips are massive and rigid compared to the thin film specimens,

so the extraneous displacements are negligible. Two sensors are used, mounted in the plane of the thin film at either side of the

grip. These sensors produce 1 mV of signal for each 25 nm of

displacement.

Tension on the specimen bends the beam that carries the moveable grip. This bending displacement is detected by a third eddy-current sensor. Several beams have been used. Best results have been obtained with steel beams 0.38 to 0.5 mm thick. These

have produced displacements ranging from 5 to 10 micrometers

for an applied force of 0.34 N, which is obtained by using a 35- gram test mass. The calibration factors are 1 .7 N/V or more, so

a 1-mV signal corresponds to 0.0017 N. This load sensor has a

stiffness of about 0.063 N/um. This is about one-fourth of the

elastic stiffness of the largest Cu specimen tested; it allows measurement of force-displacement behavior well beyond

maximum load for most specimens.

20
PROCEDURE

The test procedure begins with the mounting of a specimenassembly in the testing machine. The moveable grip is clamped to the base of the device with a special clamp that can be tighted and released with minimal displacement. Some of the 250-um-wide coupons have been tested individually, by manually

cutting off 3 of the 4 coupons in the specimen assembly (fig. 2)

at this stage of the test. Then the sides of the silicon frame are chipped out manually using a stylus. This operation usually takes

15 m to Vi h. When the clamp is released, the moveable grip

returns to its cantilevered position, so that friction between the moveable grip and the base is avoided. The specimens are usually observed to be slightly buckled at this point. This

buckling comes from two sources: buckling present in the specimens before the frame was severed, and additional buckling

introduced when the moveable grip is undamped. The clamp is arranged so that if this grip moves at all upon release, it moves in the direction to compress and buckle the specimen.

In the results tabulated below, the initial specimen area is used in all stress calculations. The areas were determined as the product of specimen thickness and specimen width. The

thicknesses were measured with a commercial profilometer of the type commonly used in semiconductor facilities. Specimen

widths and gage lengths were measured with a traveling

microscope. Reported elongation values are elongation at maximum load. In some tests it seemed like a useful value of the

strain to failure could be recorded, but in many tests the load decreased gradually to nothing with increasing imposed

displacement. Therefore strain to failure could not be defined from the load-displacement record. A final measurement of the

total length of the two parts of the fractured specimens, as is commonly done for bulk specimens, is impossible for these

thinfilm specimens, because of large out-of-plane deformations at

failure.

RESULTS

Thin Film stress-strain behavior

Stress-strain curves forTiAITi and TiCuTi layered specimens

are shown in figures 4 to 7. The Ti layers make up less than 10 percent of the specimen thickness. Figures 4 and 5 show

constant-deformation-rate tests. These stress-strain curves look a

lot like bulk metal stress-strain curves; one important difference

is that the elongation axis extends only to 2 or 3 percent, instead of the 50 percent or more that would be needed to display the behavior of bulk pure-metal specimens. Figures 6 and 7 show

cyclic stress-strain behavior. Nothing in these plots is significantly different from general behavior observable in bulk

metals. However, the extent of the imposed cyclic strain was

limited to avoid failure.

250

200-

150-

100

50

Specimen OldAI3 3 coupons

TiAffi-0. 1-2.0-0.1 urn

Microtensile tester #2

-500.000

FIGURE 4.

350

300

250

CL 200

to 150

eo

CD is 100 w

50

0.005 0.010

-50

-0.005

FIGURE 5.

0.015

Strain 0.020 0.025 0.030

TYPICAL STRESS-STRAIN CURVE OF

TiAITi FILM, 2 pm THICK, SHOWING

THAT THE STRESS-STRAIN CURVE

CAN BE FOLLOWED PAST THE

MAXIMUM LOAD, AND THAT THE

POINT OF FINAL FAILURE IS NOT

CLEARLY DEFINABLE.

/ iTi-Cu-Ti Cu05 i /

i 4 coupons

0.000 0.005 Strain

0.010 0.015

TYPICAL STRESS-STRAIN CURVE OF

TiCuTi FILM, 1.7 pm THICK, SHOWING

THAT Cu FILMS ARE TYPICALLY

STRONGER THAN THE Al FILMS, AND

HAVE SIMILAR ELONGATION TO

MAXIMUM LOAD.

21
-0.002

FIGURE 6.

0.002 0.004 0.006 Strain

o.ooe 0.01

CYCLIC STRESS-STRAIN RECORD FOR

TiAITi THIN FILM, SHOWING

HYSTERESIS SIMILAR TO WHAT

WOULD BE OBSERVED WITH

CONVENTIONAL MACROSCOPIC

SPECIMENS.

Figure 8 compares the loading and unloading slopes of the same TiCuTi specimen to the Young's modulus of bulk

polycrystalline Cu. As noted in the introduction, there is no

question but that the thin metal films should have the same elastic moduli as bulk metal. In this figure, the initial modulus, meaning,

the apparent modulus for initial loading, is less than the handbook

modulus value. This figure illustrates that the final modulus may

not be observed on initial loading because of plastic deformation, and that the present apparatus and specimens are capable of producing the correct result.

130

125

120

0.o

«„-115

D.OW

110

105 -I

100

I Handbook value of Young's modulus

Ti-Cu-Ti Specimen Cu 08

3 coupons

12 3 4 5 6 7 8 9 10 11 12 13 14 Loading number

FIGURE 8. SLOPES OF STRAIGHT-LINE

PORTIONS OF THE STRESS-STRAIN

RECORD SHOWN IN FIGURE 7. THE X- AXIS IS LOADING NUMBER,

INCLUDING INITIAL LOADING,

UNLOADINGS, AND RELOADINGS.

0.005

FIGURE 7.

0.010 Strain

0.015 0.020

CYCLIC STRESS-STRAIN RECORD FOR

A TiCuTi FILM, PLOTTED ON A

COARSER STRAIN SCALE THAN THE

PREVIOUS FIGURE, SHOWING SIMILAR

HYSTERESIS AND REPEATABLE

CYCLIC BEHAVIOR.

Copper and aluminum thin films vs bulk metal

The behavior of TiAITi and TiCuTi films can be compared

to that of bulk Al and bulk Ti based on multiple tests. Some of

the TiAITi results were reported previously (Read and Dally, 1993), and are introduced here for purposes of comparison. The

testing device on which these data were obtained, called microtensile tester number 1, was more susceptible to errors of

displacement measurement than tester number 2, though its load measurement device was quite accurate. Tables 1 and 2 give the TiAITi and TiCuTi results. The statistical summary of the TiAITi and TiCuTi results is given in Table 3. From Table 3 we see that the strength values

for TiCuTi are higher than those for TiAITi. The large standard

deviation of the yield strengths of TiAITi films is believed to result from yield strengths measured in the old tester that are generally too low. The new tester produced higher values. The

best average value is believed to be much closer to the higher

values produced by tester number 2 than to the average listed in Table 3.

22
TABLE 1

TENSILE TEST RESULTS FOR TiAITi FILMS 2.0 pm THICK.

TABLE 2. TENSILE PROPERTIES FOR TiCuTi

FILMS 1.72 pm THICK.

Yield strength, MPa

Ultimate strength, Mpa

Strain at maximum load, %

121* 199* 1.2*

115* 186* 1.1*

96* 191* 1.8*

71* 156* 1.3*

120* 144* 0.54*

91* 146* 0.85*

186 191 0.8

195 197 0.8

Obtained with microtensile tester number 1.

Yield strength, MPa

Ultimate strength, Mpa

Strain at maximum load, %

207* 240* 1.1*

276* 330* 0.5*

276 308 0.84

302 333 0.87

269 302 0.95

249 270 0.55

264 346 1.8

253 335 1.4

265 323 1.2

Chained with microtensile tester number 1.

TABLE 3. AVERAGE AND STANDARD DEVIATIONS OF MEASURED VALUES OF STRENGTHS OF 2-nm-THICK

TiAITi AND TiCuTi THIN FILMS

Material Value Yield strength, MPa

Ultimate

strength, MPa

Strain at maximum load, %

TiAITi Average: 124 176 1.0

Standard dev: 41 22 0.37

TiCuTi Average: 262 310 1.0

Standard dev: 24 33 0.39

Table 4 lists the handbook tensile property values for pure

bulk polycrystalline copper, aluminum, and titanium. Literature values for the yield and ultimate strengths of pure copper and

aluminum vary. This variability indicates the influence of grain

size, very low levels of impurities, and prior deformation on the flow behavior of pure face-centered-cubic metals. The yield and ultimate strengths of the pure aluminum

specimens are given in Table 5, and summarized statistically in Table 6. They agree with those of the TiAITi specimens within

the standard deviations of the measured quantities. Especially

since tester number 1 seemed to produce low values of the yield

strength, the disagreement between the yield strengths of the pure Al specimens and the TiAITi specimens does not seem

statistically significant. The elongation of the thinner, pure specimens seems lower than that of the thicker, layered

specimens, however. It is argued below that this lowered

elongation is a real thickness effect.

23
TABLE 4. HANDBOOK TENSILE PROPERTIES OF PURE POLYCRYSTALLINE Al, Cu, AND Ti.

Material Source Yield Strength, MPa

Ultimate

Strength, MPa

Elongation, %

Al ASM Handbook

(1979)

15 to 20 40 to 50 50 to 70

Smithells (1983)

20 55 55

Cu ASM Handbook

(1979)

33 209 60

Smithells (1983)

48 216 48

Ti ASM Handbook

(1979)

140 235 54

Smithells (1983)

103 241 55

TABLE 5. TENSILE RESULTS FOR PURE Al FILMS, 0.44 ^m THICK.

Yield strength, MPa

Ultimate

strength, MPa

Strain at maximum load, %

161 166 0.5

168 174 0.5

110 211 0.3

162 210 0.3

TABLE 6. AVERAGE AND STANDARD

DEVIATIONS OF MEASURED VALUES

OF STRENGTHS OF PURE Al THIN

FILMS 0.44 \im THICK

Yield

strength, MPa

Ultimate

strength, MPa

Strain at maximum

load, %

Average: 150 190 0.45

Standard

dev:

24 20 0.09

Aspect ratio Figure 9 shows the yield and ultimate strengths of a set of

TiAITi specimens from a single wafer, plotted against crosssection aspect ratio, that is, the ratio of specimen width to specimen thickness. The specimen thickness was constant. The

width was varied during specimen fabrication, by subdividing the

original wide tensile coupons into arrays of parallel narrower

coupons. In testing such arrays, the average strength of the whole

array is measured. The figure indicates no strong, consistent

trends of strength with aspect ratio. Observations of the appearance of the specimens as they deformed and failed also yielded no indications that the narrower specimens were behaving any differently than the wide ones. Therefore, the strength results reported here are believed to be insensitive to the cross-section aspect ratio, at least over the range tested.

Figure 10 plots elongation to maximum load against the same

aspect ratio. The scatter is larger than for the strengths, but again, no consistent tendency is shown for these specimens, which had

different widths but the same thickness. The received wisdom

from macro-scale studies of thin wide sheets of aluminum seems

to be that care is required in maintaining uniform gripping, and in keeping the specimen gage length sufficiently long compared to the width. But, when sufficient care is taken with these items, near-normal values of tensile elongation at maximum load are expected. Dieter (1976) discusses geometric corrections of the conventional elongation to failure, which includes a significant contribution from deformation occuring during necking This

contribution, of course, occurs after maximum load.

24
300

250

CO 9=200 I £150

I 100

CO 50 T

Yield

Ultimate

Wafer W3

TiATTi

20 40 60. 80 100 120 140

Aspect ratio, Width / Thickness

FIGURE 9. YIELD AND ULTIMATE STRENGTHS

FOR TiAITi SPECIMENS FROM A

SINGLE WAFER, PLOTTED AGAINST

CROSS-SECTION ASPECT RATIO.

0.025

0.020

| 0.015

O) 1 0.010 4

0.005

0.000

| Wafer W3 | TiAITi

20 40 60 80 100 120 140 Aspect ratio, Width / Thickness

FIGURE 1 0. ELONGATION TO MAXIMUM LOAD FOR

TiAITi SPECIMENS FROM A SINGLE

WAFER, PLOTTED AGAINST CROSSSECTION ASPECT RATIO.

Most studies of the mechanical behavior of thin films have

reported low values of elongation to failure, consistent with the values around 1 % reported here. One study (Steinwall, 1992) found much higher values in some specimens. Those specimens had a lower cross-section aspect ratio than most of those tested

here. But, those specimens also had ultra-fine grains, much

smaller than those in the specimens used in the present study.

The average grain diameter of these ultra-fine-grained specimens was less than 0.1 um. It is argued below that Steinwall's higher

ductility values relative to the present study are a result of his

ultra-fine grain sizes, and not a result of his lower cross-section aspect ratio.

DISCUSSION

Why is the measured ductility so low?

Possible causes of the observed low ductility for thin films

include: a real thin-film effect, a grain size effect, an aspect ratio effect, non-uniform specimen cross section, rough specimen edges, and other unknown effects or artifacts. The tensile test results presented in this paper indicated low ductility in pure and

multilayer thin film tensile specimens of various thicknesses and

widths. The thinnest specimens tested had the lowest ductility. The low ductility values for thin films reported here and in many other studies are believed to be real, and not an artefact of

the test procedure or the specimen preparation. Paper after paper on thin film mechanical properties has reported low ductility. The

only counterexample known to the authors is found in Steinwall's (1992) results, which showed low ductility for some thin-film specimens with grain sizes similar to those used here, but higher

ductility for specimens of ultra-fine grain sizes, prepared and

tested by the same procedures.

The argument that low ductility in thin films results from some deficiency in the specimen-preparation procedure must be

considered carefully. Evidence against the specimen-preparation

argument is that ductilities in the neighborhood of 1 % are found

for a variety of specimen-preparation procedures. It is clear that small tears or cracks in thin-film tensile specimens can drastically reduce their elongation to failure. Because the specimens used

here are produced by microlithography, the edges contain some

undulations on the scale of 1 jim, but no mechanical damage. If these extremely small irregularities were the cause of the low

observed ductility, this observation would be significant in itself. However, it is hard to believe that all of the many thin film studies in the literature have been subject to the same sort of edge

irregularities. An additional piece of evidence in favor of the low ductility of thin films is that some studies have found low elongation is some specimens and higher ductility in others, all prepared with

similar techniques. Kim and Weil (1989) reported systematic decreases in elongation to failure as they made their monocrystalline nickel electrodeposited specimens thinner. Their

plot of elongation results is consistent with values of 1 % near specimen thicknesses of 1 urn. The results of Steinwall (1992)

mentioned above also fall in this category, because he found low

ductility for films of some grain sizes, but high ductility for

others.The aspect ratio argument was covered above. Though the data are few, it appears that the aspect ratio effect on thin film elongation is weak.

There is good evidence that very fine grain size alone does

not reduce ductility to the extent commonly observed in thin

films. Studies of the mechanical behavior of very-fine-grained

specimens of Cu and of Al have been performed (Merz and

Dahlgren, 1975, and Alpas et al, 1990). These studies used thinfilm deposition techniques, but continued the deposition for extremely long times to produce specimens several hundred um

thick. These studies indicated ductilities approaching 10 %, much

higher than the values observed here. Two factors that strongly influence ductility have been

identified: specimen thickness and grain size. A companion study

25
to this one (Keller, Phelps, and Read, 1994) has shown that intra- grain dislocation generation and motion are suppressed in pure Cu

films 0.25 urn thick. This was attributed to the tendency for disloctions to be trapped at grain boundaries, where displacement

accumulated, causing the film to literally "unzip" and fail along

grain boundaries. The picture that emerges is that ultra-fine grains (Steinwall, 1992) enhance the grain-boundary slip, and that this deformation mode can provide substantial elongation to failure in thin films. Normally fine grains, of the order of 0.1 to 1 urn, do not allow

significant grain-boundary slip. In these films, dislocations accumulate at grain boundaries, causing them to fail with low

tensile elongation. The thinner the film, the stronger this effect, and hence the lower the elongation.

CONCLUSIONS

The mechanical behavior of thin films is influenced by their thickness and grain size. Yield and ultimate strengths are elevated over bulk values by the very-fine grain size found in normally-deposited thin films. Impurities trapped in the specimens during the deposition process probably play a role here

as well. Ductility is reduced below bulk values because the atomic-scale deformation process is influenced by the thin-film

surface.. However, ultra-fine grains can introduce an additional

slip mechanism, some sort of grain boundary slip, and thus can have higher elongations than the specimens tested here (Steinwall, 1992). Thinner fine-grained specimen sthan those tested here should have even lower ductility.

REFERENCES

Alpas, A. T., Embury, J. D., Hardwick, D. A., and Springer,

R. W., 1990, "The mechanical properties of laminated microscale composities of A1/A12 3 ", Journal ofMaterials Science 25, 1990,

pp. 1603-1609. ASM Handbook Committee, 1979, Metals Handbook Ninth

Edition Volume 2, Properties andSelection: Nonfermus alloys and

pure metals, America! Society for Metals, Metals Park, Ohio

44073.

Baral, D., Ketterson, J. B. and Hilliard, J. E., 1985,

"Mechanical properties of composition modulated Cu-Ni foils," Journal of Applied Physics, 57, pp. 1076-1083.

Davis, B.M., Li, D.X., Seidman, D.N., Ketterson, J.B., Bhadra, R., and Grimsditch, M., 1992, "Elastic and nanostructrural

properties of Cu/Pd superlattices," Journal ofMaterials Research,

7, Number 6„ pp. 1356-1369.

Dieter, George E., 1976, Mechanical Metallurgy, Second

Edition, McGraw-Hill, New York, pp. 346-348. Dunn, Robert, 1993, Keynote address at ASME International Conference on Electronic Packaging, Binghampton, New York.

Guess, Tommy R. and Burchett, Steven N., 1992, "An

experimental / analytical study of strains in encapsulated

assemblies," in A dvances in Electronic Packaging 1992, Volume

1, Proceedings of the First Joint ASME/JSME Conference on

Electronic Packaging, EEP-Vol.1-1, Milpitas, California, April 9-

12, 1992, edited by W. T. Chen and J. Abe', American Society of

Mechanical Engineers, New York, pp. 543-550.

Hardwick, D. A., 1987, "The Mechanical Properties of Thin

Films: A Review," Thin Solid Films 154, pp. 109-124. Hoffman, R. W., 1966, "The Mechanical Properties of Thin Condensed Films," in Physics of Thin Films, edited by Georg

Hass and Rudolf Thun, Academic Press, New York.

Keller, R. R., Phelps, J. M., and Read, D. T., 1994, "Tensile Deformation-Induced Microstructures in Free-Standing Copper

Thin Films," Materials Research Society Proceedings, in press. Kim, Ingon and Weil, Rolf, 1989, "The Mechanical

Properties of Monocrystalline Nickel Electrodeposits," Thin Solid

Films, 169, pp. 35-42. Lau, John H., and Rice, Donald W., 1992, "Thermal fatigue

life prediction of flip chip solder joints by fracture mechanics

method," in Advances in Electronic Packaging 1992, Volume 1, EEP-Vol.1-1, Proceedings of the First Joint ASME/JSME

Conference on Electronic Packaging, Milpitas, California, April

9-12, 1992, edited by W. T. Chen and J. Abe', American Society of Mechanical Engineers, New York, pp. 385-392. Mehregany, Mehran and Senturia, Stephen D., 1988,

"Anisotropic Etching of Silicon in Hydrazine," Sensors and

Actuators, 13, pp. 375-390. Merz, M. D., and Dahlgren, S. D., 1975, "Tensile strength and work hardening of ultrafine-grained high-purity copper," Journal of Applied Physics 46, pp. 3235-3237.

Nix, W. D., 1989, "Mechanical Properties of Thin Films,"

Metallurgical Transactions, 20A, pp. 2217-2245.

Pao, Yi-Hsin, Chen, Kuan-Luen, and Kuo, An-Yu, 1991, "A

nonlinear and time dependent finite element analysis of solder

joints in surface mounted components under thermal cycling," Materials Research Society Symposium Proceedings Vol. 226,

Materials Research Society, Pittsburgh, pp. 23-28. Read, D. T., 1992, "In situ measurements of strength and

ductility of thin films", in A dvances in Electronic Packaging 1992,

Volume 1, EEP-Vol.1-1, Proceedings of the First Joint ASME/JSME Conference on Electronic Packaging, Milpitas, California, April 9-12, 1992, edited by W. T. Chen and J. Abe',

American Society of Mechanical Engineers, New York,, pp. 463-

468. Read, D. T. and Dally, J. W., 1993, "A new method for measuring the strength and ductility of thin films," Journal of

Materials Research, 8, No. 7, pp. 1542-1549.

Smithells, 1983, Smithell's Metals Refrence Book, Sixth Edition, edited by Eric Brandes, Butterworths, London.

Steinwall, J. E., 1992, Microfabrication and Mechanial

Properties of Free Standing Thin Films, Ph.D. thesis, Cornell

University.

26
Appendix C. 1995: Fatigue of Microlithographically-Patterned

Free-Standing Aluminum Thin Film Under Axial

Stresses

27
David T. Read

Materials Reliability Division, National Institute of Standards and Technology,

Boulder, CO 80303

James W. Dally

Mechanical Engineering Department,

University of Maryland,

College Park, MD 20742

Fatigue of

Microlithographically-Patterned

Free-Standing Aluminum Thin

Film Under Axial Stresses*

Fatigue life as a function of number of stress cycles has been determined for three-layer

titanium-aluminum-titanium thin-film specimens. For nominal stresses ranging from 50

to 90percent ofthe upper-bound ultimate strength, the fatigue lives ranged up to 76 cycles. In all specimens, active cracks were observed after only a few stress cycles. These cracks grew with repeated cycling. The fatigue life was reached when the specimen could no

longer sustain the maximum cyclic stress.

Introduction

This study is motivated by the need to extend the methods of

fatigue-life prediction [1-5] to thin films. Deterministic evaluations of the reliability of multilayered thin-film structures require an understanding of the fatigue lives of the individual

layers. One source of stresses in multilayered structures is differential thermal expansion in adjoining materials. Electronic devices are commonly subjected to thermal cycling

during a manufacturing screening test to reveal defective

devices. Manufacturing screening is from 1 to 20 cycles, while operation can involve thousands [6-8] of cycles. The specimens tested here were free-standing, as will be

described below, and so their behavior does not directly simulate that of films adhering to substrates. The tensile stress in free-standing films is easier to apply and control than

stresses in films on substrates. These results are presented as a simpler first step toward understanding the more complicated

cases of films on substrates and thin films in multilayer

structures. They may also be relevant to cases where free- standing films are used in special devices. In these experiments we used a relatively new technique [9], which enabled us to conveniently fabricate and test a series of

thin-film specimens that were nearly identical. However,

because of the extremely small size of these specimens, and

because the specimens were produced by vapor deposition, the experimental procedure differed from the current practice of

strain cycling to investigate the fatigue behavior of bulk

materials at low cycle life. The differences were: we could not use a standard specimen geometry, for example, hourglassshaped; we could not impose compressive stresses, because the

films were free-standing; and we controlled the applied cyclic

'Contribution of the U.S. National Institute of Standards and Technology; not

subject to copyright in the U.S. Contributed by the Electrical and Electronic Packaging Division for publication in the Journal of Electronic Packaging. Manuscript received by the EEPD August 11, 1992; revised manuscript received December 20, 1994. Associate Technical Editor: A. Dasgupta.

stress, whereas in conventional testing strain is often controlled. In these experiments, we controlled the cyclic stress applied to the specimens, and observed the number of cycles to

failure in the range of 1 to 100 cycles. The prediction of fatigue life of conventional structures with

macroscopic dimensions has reached a high state of development [1-5]. However, the methods are based entirely on

empirical observations of the fatigue resistance of either uncracked or cracked specimens. Two typical descriptions of

the fatigue life of initially uncracked specimens are commonly

used: low-cycle fatigue and high-cycle fatigue. The tests reported below were carried out according to the general

methods of high-cycle fatigue testing, in that the control parameter was stress, and the stress was restricted so that the

elastic part of the strain was generally greater than the plastic part. However, the resulting cycle lives were much less than

those typical of high-cycle fatigue. In this paper, we describe: (a) the techniques used to prepare thin-film specimens; (b) the loading system that was

designed to accommodate these very small specimens; and (c) the procedures used in testing the films. The results are presented in the form of a segment of an S-N curve. We also discuss the relationship of these results to the context of a modern treatment of fatigue resistance.

Specimen Preparation

The techniques used to prepare specimens are relatively new [9]. In a batch-production process, each thin film sample is fabricated on a silicon frame, which supports it until it is ready

to be tested. The specimen structures, consisting of films suspended from supporting frames, are referred to as speci- men assemblies. The thin film on each frame is patterned to form four parallel tensile strips or coupons. Each coupon has a gage section 1 mm long by 0.25 mm wide by 2.45 n-m thick. The

central Al layer is 2.25 |j.m thick. The two titanium layers on

the specimen surfaces are each 0.1 (irn thick. The gage sections

Journal of Electronic Packaging 28 MARCH 1995, Vol. 117/1
sio. ^

^mm

Silicon 3 mr ^HM

Aluminum

(a)

7l$k.s"^''.

". V " ' ' jj -__^_

Silicon

SIOj ^TM"

Aluminum

(b)

pis

Silicon <ao.=

Z , I

1

— iii

Transverse direction

(d)

E O d

SIO, I Silicon

~v - gnn]

SiO, on Silicon

Fig. 1 Schematic diagrams showing specimen preparation sequence. Thicknesses of Si02 and aluminum films are exaggerated relative to the thickness of the silicon wafer, (a) Window patterned in the Si02 on the

silicon wafer, (o) Aluminum thin film deposited over the whole wafer, (c) Cross section along the tensile direction showing the aluminum film covering the whole wafer, (d) Tensile coupons within the windows on the

silicon wafer, (e) Tensile coupons suspended on the frame, after removal

of the silicon within the window. ( f ) Plan view of specimen assembly, at a

smaller scale.

of the tensile specimens are freely suspended across the

interior of the frame. Thirty-six of these frames are produced on a single 51 mm (2 in.) diameter silicon wafer. The specimens were prepared by employing a number of processes common to the microelectronics industry. Figure 1 shows the specimen preparation steps schematically. We

began with a 51 mm (2 in.) diameter silicon wafer, polished

and oxidized on both sides. The first step, Fig. 1(a), was to pattern, by semiconductor-style microphotolithographic tech- niques, windows in the SiC>2 on both sides of the wafer. We

then deposited the metal film by successive electron-beam

evaporation of controlled amounts of titanium, aluminum, and

again titanium. The base vacuum was 10~ 6 torr. The evaporation rate was set at 10 A per s for aluminum, and 3 A per s for titanium. Figures 1(b) and 1(c) show the metal film, labeled "aluminum" for simplicity, covering the whole wafer, including

the bare silicon in the previously defined windows, and the

silicon still coated with SiC>2 surrounding the windows. We

then defined the geometry of the individual tensile coupons by

another standard lithography step. Figure 1(d) shows the

transverse cross section of the individual tensile coupons. The

pattern in the metal film is aligned to the windows in the Si02 layer using a mask aligner. The manual aligner we used is, at best, capable of alignment to within a few u.m. We estimate

that we aligned to better than 10 u.m. This was more than adequate for our specimens, which had a length over 1 mm.

Finally, the wafer was etched in aqueous hydrazine to remove

areas of silicon not protected by Si02

. Figure 1(e) shows the suspended films in cross section. Figure 1(f) shows a plan view

of the specimen assembly.

8mm

Fig. 2 Portion of the completed wafer with 9 specimen assemblies, each

supporting four parallel tensile coupons. A: Etched grooves separate

individual specimen frames. B: Aluminum thin film. C: Windows etched through the silicon wafer. D: Lines where frames will be fractured to allow

testing of the free-standing films. E: Aluminum tensile coupons sus- pended from the supporting frames.

/ ; / ;—7

t

Fig. 3 Drawing of loading system used for the fatigue tests

Figure 2 shows a portion of the completed wafer with nine

specimen assemblies, each supporting four parallel tensile coupons.

Loading System

The loading system was designed for tensile testing of

thin-film specimens [9], and was adapted for low-cycle-life

fatigue experiments. The loading system has two special features. First, is the ability to rigidly clamp the specimen

frame while it is severed and then to release it without

imposing tensile displacements that produce loading. This

control of displacements during the frame fracture process is a

stringent requirement because extension to failure of the

thin-film tension specimens is only about 10 u.m. The second

special feature is the ability to impose very small forces that are

precisely controlled. The mechanical features of the loading system, illustrated in Figs. 3 and 4, include a fixed platen and a sliding platen with a pair of specimen-mounting plates, a calibrated spring, and a series-connected pair of micrometers. The mounting plates are

fixed to the two platens with small screws so that they can be

removed and recycled. After alignment, the specimen frame is bonded to the mounting plates with a quick-setting methyI-2-

cyanoacrylate adhesive. The load is applied by displacing one end of the spring with one or the other of the two micrometers.

One micrometer is a differential type with graduations of 1 ixm, and the other is a conventional type with graduations of 10

u,m. The other end of the spring is fastened to the sliding

platen. As the spring is extended, the spring force is transmit- ted through the sliding platen to the specimen array. For these

tests, only the single-screw micrometer was used, so that the

2 / Vol. 117, MARCH 1995 29 Transactions of the ASME
Fig. 4 Photograph of the loading system used for the fatigue tests

loads were imposed with a precision of about 5 mN, or 1 percent of the ultimate strength.

The travel of the sliding platen was retarded by friction; however, this effect was mitigated by continually tapping the support structure of the fixture with a light striker during the loading cycle. Measurements of the remaining frictional forces showed that these were less than 1 mN.

The extension of the loading spring was measured using an

eddy-current sensor that was fixed to the micrometer-mounted

end of the spring. Its target was an aluminum flag attached to the sliding platen. With this mounting arrangement, the sensor

is insensitive to simple translation of the spring as a whole, but responds to spring extension. The tensile elongation of the specimen was measured using

a second eddy-current sensor mounted to the fixed platen. The

same flag served as the target for this sensor. It was required to be much more sensitive, because of the very low tensile elongations of these specimens, around 10 u.m. The tapping of

the fixture to mitigate friction limited the resolution of the sensor to ±0.25 u,m, although the specified precision of the instrument system is ±0.0025 u,m. During the test the signals for load and displacement were digitized and stored 4 times per second. Pairs of successive readings were averaged for analysis and for plotting force-elongation curves. The whole

loading system, shown in Fig. 4, was mounted under an optical

microscope equipped with a closed-circuit television system.

Test Procedures

The test procedure, adapted from that developed previously

for tensile testing [8], was to adhesively bond a specimen

assembly to the sliding and fixed platens, and then to clamp the

sliding platen in place. We next fractured the frame at the

locations D indicated in Fig. 2 and released the clamp. Finally, the specimen was inspected to ensure that the specimen was undamaged and that the frame was completely severed. The fatigue loads were applied by turning the single-action

micrometer by hand. The specimen was first loaded to a preselected maximum fatigue load, then cycled between this maximum and the minimum load, which was 10 percent of the maximum load. Each load cycle required approximately 1 min.

Hysteresis loops were recorded on an xy plotter, and the measured values of applied force and specimen extension were

stored on the hard disk of a laboratory computer. Specimen

failures could be observed in the microscope, and recorded on

videotape. The system was stress-controlled, so once a speci- men could no longer sustain the maximum load, it failed catastrophically. The failure was by high-velocity crack propagation.

Results and Discussion

Two specimens were tested to determine the tensile strength

and 12 other specimens were tested in fatigue. Loads were

250

225;

200-

J?| 175

1 '»

w 125 2 100-

TlAfTi (Km. arrays of 4 specimens, each 0.0025 % 0.2S x 0.6 mm

R • Minimum stress / maximum stress 0.1

10

Cycles to failure 100

Fig. 5 S-N, curve for HATTi thin-film specimen array, showing the maximum cyclic stress as a function of number of cycles to failure

250

Fig. 6 Stress-strain record for a TiAm specimen array that failed on the 17th cycle

selected to ensure that failure occurred in 100 cycles or less. The results, given in Fig. 5, show the number, Nf, of cycles to

failure as a function of the maximum applied stress S. There were clearly imperfections in the specimen geometries. We

often observed crack-like features even before loading.

The results of Fig. 5 show a bimodal pattern with 11 of the 14

tests giving a S-Nf curve with relatively small scatter. The

remaining three points are also consistent but indicate lower

strength by about 30 percent. We will divide the data into two

populations (high and low strength) for the discussion. The

S-Nf curve for the high-strength population shows a knee at about 16 cycles.

Stress-Strain Behavior. The stress-strain behavior of one

thin-film specimen under cyclic load is illustrated in Figs. 6, 7, and 8. The important features in each of these plots are the

stress-strain hysteresis loops. As noted above, the force- displacement data were acquired as discrete readings, at a rate of 4 readings per second. For plotting, these were smoothed by

interval averaging over two readings, or approximately Vi s. In

Figs. 6-8, the discrete data points were connected by straight

lines. The duration of each loop is 1 minute or more.

In Fig. 6, a continuous record of stress-strain is shown over

17 cycles of loading. We observe two important features in these results. First, a hysteresis loop occurs, and the size of the loop changes with the number of cycles—large, smaller, then

large again. Second, a ratchetting occurs because a permanent

strain is imposed on the specimen with each cycle of load.

Stability of the cyclic stress-strain curve was not achieved in these tests, where the cyclic loading was stress-controlled and

the life was always less than 100 cycles. The irreversible part of

the strain in Fig. 6 is attributed entirely to plastic strain in the specimen. The distribution of this strain over the gage length

Journal of Electronic Packaging 30 MARCH 1995, Vol. 117/3
250

200

TiATTi Sm. array o) 4 specimens, each 0.0025 x -3S x 0.6 mm

Cycle 1 Cycle 9 Cycle 16

0.005 0.01

Strain

0.015 0.02

Fig. 7 Stress-strain record for the same specimen as Fig. 6, showing

cycles 1,9, and 16

250

0.02 0.04 0.06 0.08

Strain, shifted to separate load cycles

0.1

Fig. 8 Test record for the same specimen as Figs. 6 and 7, with individual cycles offset to higher strains to show all cycles separately

was not measured; however, the specimens were observed

using the optical microscope. No sign of inhomogeneous

deformation was observed. Standard alignment checks, such as loading a specimen with four gages situated around its tensile

axis, were not possible in the present test procedure. However,

the fixture was compliant to lateral motion of the sliding grip;

this provided some self-alignment. Also, certain observations not available in standard fatigue tests were possible here. These specimens would be expected to buckle or wrinkle if significant nonaxial loading occurred. For example, the out-of- plane settling of the specimens when the silicon frame was

broken was often evident from their appearance. Further- more, four individual tensile specimens were present in each

test. Nonaxial loading would be expected to load the individual specimens differently. But no sign of nonaxial loading or in-plane bending was observed. The two interior specimens

behaved just the same as the two to the outside. Figures 6-8

are believed to represent axial strains only, with no detectable

contributions from misalignment or bending.

The changes in the hysteresis loop are more clearly depicted

in Fig. 7, where the loops corresponding to only the first, ninth,

and sixteenth cycles are shown. In the first cycle, we see a stress-strain curve that is representative of a tensile test, except

that the specimen was not loaded to failure. In spite of the

discontinuities in the curves due to friction, we observe a linear

elastic region, followed by a plastic region with significant strain hardening. Precise measurements of the constitutive constants such as the elastic modulus and the strain hardening

coefficient are not possible for several reasons. The flag, which

translates with the moving end of the specimen, also rotates, introducing an error into the strain measurement. Also, the

0.25 mm

Fig. 9 Crack in thin-film fatigue specimen prior to failure

thin-film specimens are not fiat: after etching a transverse curl

is evident. Returning to Fig. 7, the hysteresis loop is much smaller for load cycle 9 than it was for cycle 1. Strain hardening has

occurred, and the yield strength of the thin film has been

enhanced by cyclic stressing above the original yield strength.

The hysteresis loop for the sixteenth cycle, which was the

last complete cycle before the specimen failed, has markedly

increased in size over the ninth. This increase is due to cracks which initiated during previous cycles and extended during the 16th cycle. The crack extension at the maximum applied stress results in large apparent strains. The elastic recovery curve for

all three hysteresis loops was nearly identical. Another display of the stress-strain curves for this specimen

is presented in Fig. 8. In this representation, the individual loops have been separated by adding an artificial offset to the

strain for each cycle. These results show the marked reduction

of the size of the hysteresis loop on the second cycle, compared

to that of the first. The loops remained essentially constant in size until the fourteenth cycle. At this point, cracks, which had

apparently initiated during previous cycles, began to extend. These cracks, as shown in Fig. 9, increased the compliance of

the specimen and produced permanent extensions in the gage

length. Crack growth was very large on the sixteenth cycle and

specimen failure occurred just as the maximum stress was

reached on the seventeenth cycle. Figure 9 is typical of the observations available during the tests. Cracks were seen to

initiate early in the tests, and to grow gradually to the size (half the width of the specimen) shown in Fig. 9. Failure occurred soon after the cracks reached this size. The slopes of the initial portion of the loading curves in these figures do not agree with handbook values of Young's

modulus for the specimen materials. We attribute this to problems with the design of the testing device, specifically, with the measurement of strain. Here we report no results that are based on measurement of strain. We note that the difficulty of measuring accurate values of Young's modulus in tensile testing is well-known even in tests of macroscopic specimens.

These difficulties are even more severe here, where the total elastic elongation is approximately one wavelength of light. Our force data are much more accurate than our displacement

data, because we used a soft spring, described above, to convert force to a displacement for measurement. Comparison With Bulk Material Behavior. These results cannot be compared directly to typical results for the behavior

of bulk metals in fatigue, because the test procedures used

4 / Vol. 117, MARCH 1995 31 Transactions of the ASME
1.2

1.0 -

0.8 -

0.6

to o.4 h

0.2 -

-

I I

Thin Film /

I I

I I I I I I I / Unnotched Sheet

-

\ ^wn.

Severely >a

Notched Sheet

I I I

^^/////////A

^Se^giERs^?iS5S^^Ba^

i i i i 10"1 10° 10 1 1C 10J 10' 10= 10° 10' 10° 10* CYCLES TO FAILURE

Fig. 1 Comparison of the present results for TiAITi thin-films to results for fatigue of thin sheets of aluminum and stainless steel with stress concentrations [4]

here are unconventional. The size and thickness of the specimens and the available loading system precluded both

conventional low-cycle and high-cycle fatigue test procedures.

Also, because of the manual loading fixture, cycle lives beyond

100 cycles were impractical.

The present results are compared to previous results [4] on

the fatigue life of axially loaded notched and unnotched sheet specimens of aluminum and stainless steel in Fig. 10. The

notched specimen results included stress concentration factors of 2 and 4. The normalized S-N curves exhibited a knee similar

to that seen in the present results for thin-films. For the unnotched comparison specimens the knees occurred generally between 103 and 104 cycles. For the severely notched

specimens, the knee occurred between 10 and 100 cycles. The

present thin-film results show normalized failure strengths that are significantly lower than the bulk sheet results [4] for cyclic

life in excess of 5 cycles. The thin-film S-N curve is also below

the scatter band for axially loaded cylindrical specimens [4]. We conclude that the S-Nf curve for this set of thin-film specimens in tension-tension fatigue is lower than that in bulk

material by a factor of about 2. Low Versus High-Cycle Fatigue. In low-cycle fatigue, the

plastic strain range Ae plays the decisive role, whereas in high-cycle fatigue the stress is regarded as the controlling

factor. Analysis of the cyclic Ae-TV curve by Sandor [1] shows

that for a typical aluminum alloy the plastic and elastic parts of

the strain were equal at a fatigue life around 1500 cycles. So in Sandor's example alloy, 7075-T6 aluminum, the boundary

between low-cycle fatigue and high-cycle fatigue can be taken

as a fatigue life of 1500 cycles. For this alloy, the monotonic

plastic strain to failure is 11 percent [10]. For the thin-film TiAITi specimens tested here, the strains involved are much different. The average tensile strain to

failure is about 1 percent, and that includes elastic strains, plastic strains, and crack-opening displacements that produce

apparent strains. The force-extension curves in the present set of tests show that elastic strains are larger than the plastic strains on all cycles after the second. This result holds even if the elastic strain is calculated from the cyclic stress and the Young's modulus of bulk Al, rather than as the reversible part of the stress in Fig. 7, and if an effective gage length of 0.6 mm

is used. For the specimens that failed on the second cycle, the

nonelastic strains were comparable to the elastic strains. We think that the extremely short low-cycle fatigue life of the thin-film specimens is related to their brittle tensile behavior, that is, to the lack of an extended range of plastic strain before failure in tension. The elongation of these films is much lower (0.54 to 1.81 percent) than the elongation of either bulk aluminum or titanium, which are an order of magnitude

higher [10]. The mechanical behavior of thin films has been

reviewed by several authors [11-13]. Values of elongation to

failure of around 1 percent or even less are typical in free-standing films, in both uniaxial and biaxial tests. The

cause of this behavior is not understood. The fatigue lives of the thin films tested in this study were much less than might have been expected from bulk behavior,

as indicated above in Fig. 10. The cause must be sought in two main areas: intrinsic thin-film behavior; and, artifacts of the specimen-preparation process. The mechanical behavior of

thin films is known to be different from that of bulk material

[11-13]. Hoffman [11] notes observations of localized plastic

deformation leading to cracking and failure in tensile tests However, fatigue failures are extremely sensitive to defects, and the thin-film specimens tested here could not be subjected

to conventional specimen-preparation, including careful re- moval of surface irregularities by polishing. Therefore the present results cannot now be regarded as characteristic of

thin films in general. Additional testing is needed to clarify the mechanism of crack initiation in thin-film testing, and the role of intrinsic thin-film behavior in the fatigue resistance of thin

films. The process used here for fabricating the thin-film speci- mens is advantageous for an engineering assessment of thin- film fatigue characteristics. In particular, four elements of the present experimental procedures may affect the fatigue strength of these films: the etch in a hydrazine solution; the high ratio of specimen width to thickness; possible damage induced during

fracture of the frame; and added stress due to vibration from

the tapping used to mitigate frictional effects.

Summary

Techniques previously developed for preparation and test- ing of tensile specimens of thin films [9] have been successfully

adapted for fatigue testing. The low-cycle fatigue life of TiAITi

thin film specimens has been measured under stress-controlled

cycling. The consistency of the results given in Fig. 5 indicates that it is possible to prepare a set of nearly identical thin-film specimens and to test them under cyclic tensile stresses. Typical low-cycle fatigue behavior was not observed after 16

cycles. To achieve a life of 50 cycles, it was necessary to lower

the applied stress to 65 percent of the ultimate strength. The

specimens used here clearly contained imperfections, and so

the observed fatigue life behavior may not be characteristic of

thin films in general.

Acknowledgments

A variety of facilities and apparatus were required for fabricating tensile specimens on silicon. Use of facilities offered by R. E. Harris, of the Cryoelectronics Group of the Electromagnetic Technology Division of NIST, and by William

May of the Electrical Engineering Department of the University of Colorado at Boulder is gratefully acknowledged.

References

1 Sandor, Bela I., Fundamentals of Cyclic Stress and Strain, University of Wisconsin Press, Madison WI, 1972. 2 Fuchs, H. O., and Stephens, R. I., Metal Fatigue in Engineering, Wiley, New

York, 1980.

3 Dieter, George E., Mechanical Metallurgy, Second Edition, McGraw-Hill, New York, 1 976, pp. 403-447.

4 Hardrath, H. F., Landers, C. B., and Utley, E. C, "Axial Load Fatigue Tests on Notched and Unnotched Specimens of 61S-T6 Aluminum Alloy, Annealed

347 Stainless Steel and Heat-Treated 403 Stainless Steel," Nat. Adv. Co. Aero.

Tech. Note 3017 (1953), reviewed in Forrest, P. G., Fatigue of Metals, Pergamon

Press, Oxford, 1962, p. 162. 5 Manson, S. S., "Fatigue: A Complex Subject—Some Simple Approximations," Experimental Mechanics 5, No. 7, July 1965, p. 193.

Journal of Electronic Packaging 32 MARCH 1995, Vol. 117/5
6. Dally, James W., Packaging ofElectronic Systems, McGraw-Hill, New York, 10 American Society for Metals, Metals Handbook Ninth Edition, Vol. 2, 1990, p. 382-383. American Society for Metals, Metals Park, OH, 1979, 7 Mandel, C. E. Neil. Jr.. "Environmental Stress Screening," Electronic 11 Hoffman, R. W., "The Mechanical Properties of Thin Condensed Films,"

Materials Handbook, Vol. 1 Packaging, C. A. Dostal et al., eds., ASM Interna- Physics of Thin Films, Vol. 3, edited by G. Hass and R. E. Thun. eds., Academic

tional, Metals Park, OH, 1989, p. 876, 882. Press, 1966, pp. 211-273.

8 Robock. Paul V., and Nguyen, Luu T. "Plastic Packaging," Microelectronics 12 Brotzen, F. R., Rosenmayer, C. T., and Gale, R. J., "Mechanical Behavior Packaging Handbook, R. R. Tummala and E. J. Rymaszewski, eds.. Van of Aluminum and Al-Cu(2%) Thin Films," Thin Solid Films, Vol. 166, 1988, pp. Nostrand Reinhold, New York, 1989, p. 631. 291-298.

9 Read, David T., and Dally, James W„ "A New Method for Measuring the 13 Doerner, Mary F., and Nix, William D., "Stresses and Deformation

Constitutive Properties of Thin Films," Journal of Materials Research, Vol. 8, Processes in Thin Films on Substrates," CRC Critical Reviews in Solid State and

1993, pp. 1542-1549. Material Sciences, Vol. 14, 1988, pp. 225-268.

6 / Vol. 117, MARCH 1995 33 Transactions of the ASME
Appendix D. 1997: Tension-tension Fatigue of Copper Thin

Films

34
TENSION-TENSION FATIGUE OF COPPER THIN FILMS

D. T. Read

Materials Reliability Division

National Institute of Standards and Technology

Boulder, Colorado 80303

Tension-tension, load-controlled fatigue results extending

to over 100,000 cycles for electron-beam-evaporated copper are

reported. The specimen is fabricated as a single tensile coupon

with hundreds of grains across the width but only one or few

through the thickness, supported on a silicon frame. These

specimens had measured values of Young's modulus lower than

literature values for bulk polycrystalline values, but had high

values of the yield and ultimate strengths. Closed-loop feedback

control of the piezoelectric actuators allowed load-controlled

fatigue tests with a cycle duration of 15 s. Stress-endurance

results for this set of copper films is reported and compared to

previous results

.

Key words: Closed-loop testing, cyclic, displacement, endurance,

force, piezoelectric, S-N curve, servo, strain, stress.

Contribution of the U. S. Department of Commerce. Not subject to

copyright in the U. S. A.

35
INTRODUCTION

Electronic devices are tested for reliability by thermal

cycling. Modeling of the potential failure modes in such devices

requires data on material behavior. These data are lacking for

many of the unique materials that are used in electronic devices,

especially thin-film materials. Several excellent reviews on

thin-film mechanical behavior have appeared recently [1-8].

However these works have concentrated on test methods and general

behavior, rather than on specific property values. The data

presented here on the resistance of copper thin-films to failure

by tension-tension fatigue are intended to be useful in modeling

the reliability of electronic interconnects made of copper thinfilms and subjected to cyclic loading.

A previous paper on fatigue behavior of aluminum thin-films

[9] reported unexpectedly low fatigue resistance. In the present

study, the fatigue resistance of copper thin-films is found to be

considerably better than was reported for the aluminum films.

However, the data cannot be unambiguously interpreted to mean

that copper thin-films have better fatigue resistance than

aluminum, because of changes in the test procedures. The present

test procedures are improved over those used for the aluminum, in

that the cyclic loads are applied under closed-loop feedback

control [10], and tensile strips are tested individually, instead

of in groups of 4

.

SPECIMENS

All the specimens in this study came from the same silicon

wafer substrate. Windows were produced in the oxide layer on a

50-mm diameter silicon wafer by microphotolithography. The

substrate was clamped to a plate held at room temperature by

flowing cold water. Copper of 5-nines purity was evaporated from

a graphite crucible, using an electron beam in a chamber that had

been pumped to a base vacuum of less than 10" 6 torr. The

deposition rate averaged 10 A per second, as monitored by a

quartz -crystal thickness gauge. The specimen film consisted of

an adhesion layer of 0.05 /xm of Ti, 1.1 /xm of Cu, and a second,

cap, layer of 0.05 jum of Ti. These specimens are designated as

EBE Ti Cu Ti in the figures.

Microphotolithography was used again to subtract ively

pattern the film into tensile coupons. Then the wafer was etched

in aqueous hydrazine at 100 °C for approximately 4 h to remove

the Si from the regions behind the tensile strips, leaving them

suspended across windows in the silicon wafer. Aqueous hydrazine

is a hazardous material because it is highly toxic and corrosive.

It is not explosive.

36
Each finished specimen consists of a Si frame about 8 by 3

mm, with a window through the center about 1 mm long by 1.5 mm

wide. Suspended across the window is a single tensile strip.

The suspended strips hve a gage section 2 00 jiim wide by 600 /xm

long and wider sections at the ends. The Si frame protects the

specimen from damage during handling, and allows the specimen to

be aligned to the tensile axis before the frame is glued to the

grips of the tensile apparatus. Figure 1 shows a schematic of

the specimen, including the silicon frame and the suspended

tensile strip. Figure 2 shows the design of the tensile coupon.

The last step in preparing the specimen for testing is the

removal of the sides of the Si frame. We simply chip out the Si

manually with a sharp tool. This must be done slowly and

carefully, but it can be done with a success rate that is well

over 50 percent, and takes up to half an hour per specimen.

Specimens that were ruined during this step are reported as

ruined during preparation.

Material evaporated under similar conditions was examined in

cross-section by transmission electron microscopy (TEM) and found

to have an average grain size of 0.98 fim. Figure 3 is a tracing

of a TEM micrograph showing the grain morphology of the crosssection. The 2 00-/Lim-wide tensile specimens have hundreds of

grains across the width, but only one or a few through the

thickness. The distribution of grain size in as-deposited thinfilms is discussed in detail by Zielinski, Vinci, and Bravman

[11]. The distribution is generally quite broad.

APPARATUS

The microtensile tester used in these tests was constructed

according to a new design especially for closed-loop testing of

thin-films. It uses two piezoelectric stacks as actuators. Each

stack is 60 mm long by 15 mm in diameter. The moving grip is

carried on a flexible load-sensing beam, which is attached at its

ends to a reference beam, which is analogous to the moving

crosshead of a conventional full-size tensile testing machine.

All the moving parts are cantilevered, so that there is no

friction. The tester is mounted in an optical microscope, so

that the specimens can be observed during testing.

Two non-contacting eddy-current sensors measure the

displacement of the moving grip relative to the fixed grip.

These are mounted in the plane of the specimen. Another like

sensor measures the displacement of the load-measuring beam

relative to the reference beam. A closed-loop control system is

formed by routing the sensor outputs to a computer, which

maintains target values for the control parameter, which in this

series of tests was load. An analog output from the computer

controls the voltage applied to the piezoelectric actuator. The

37
computer program calculates the difference between the desired

load and the actual load, and changes the piezoelectric voltage

to eliminate the error. This update occurs about 40 times per

second.

TECHNIQUE and OBSERVATIONS

Load control was selected because reversible strain control

requires compression as well as tension, and these freely

suspended thin-films could not be compressed. The fatigue cycles

were tension-tension with a minimum load nominally equal to 10

percent of the maximum load. Before each test a value of the

maximum load was selected, with the goal of filling in the

complete S-N curve. These load values ranged generally from 40

to nearly 100 percent of the ultimate tensile strength.

Voltages corresponding to load and grip displacement are

recorded about 4 times per second for the beginning of the test.

The duration of each fatigue cycle was 15 s. For tests above a

few tens of cycles, it became impractical to store every forcedisplacement data point. Therefore, an alternative recording

schedule was adopted where the maximum and minimum force,

displacement, and piezoelectric voltage were recorded for each

fatigue cycle. This produced a manageable volume of data, while

still recording the important quantities in the test.

Failure was indicated when, after many cycles of loading

to the target value and unloading to 10 percent, the load first

failed to reach the target value. In seeking for the target load

value, the closed-loop control routine would increase the voltage

output to the piezoelectric drivers to the maximum capacity,

which was 250 V, as indicated by the manufacturer of the piezo

stacks. Thus, failure was unmistakable.

Plastic ratchetting occurred in all the fatigue tests,

because the specimens were continually in tension. The specimen

strain would increase on each load cycle, and would not fully

recover. This effect was more pronounced at the higher load

levels.

Some problems were experienced with specimens failing in the

grip section at the edge of the frame. The failures seemed to

occur more often when the specimen was fully loaded within the

first few cycles. Therefore a preconditioning procedure was

adopted in which the specimens were cycled for 10 cycles at 1/3

of the full load, then cycled for 10 cycles at 2/3 of the nominal

load, and then cycled at the full load. The successful tests

performed before this explicit preconditioning procedure was

adopted also experienced a similar preconditioning, because the

load was increased gradually over several cycles to the

38
preselected value. As the tests became more routine, the ramp-up

to the selected load was quicker, and failures at the frame

became more common. And, all failures at the frame were excluded

from the final results. Therefore, the preconditioning is not

believed to contribute any detectable degree of variability to

the final results.

Some specimens survived many fatigue cycles only to fail

through an obvious malfunction of the test equipment. For

example, in one case, a test had to be stopped because the

computer's disk was getting filled. During the restart, the

specimen was inadvertently overloaded and failed. In another

case, the data file was not properly recorded, so we relied on

manual notes of the endurance, which were made every few hours.

The actual fatigue failure occured at an unknown cycle number,

but the notes allowed us to report a lower limit on the endurance

in that test. One test was intentionally terminated after over

100,000 cycles. All these tests that were stopped before a

fatigue failure are designated as runouts, to indicate that the

test result provides only a lower limit of the endurance for that

specimen at its load level

.

After failure we noted the location of the failure using the

microscope included in the test apparatus. Specimens that failed

in the gauge section, including those near the change in width at

the end of the gage section shown in Figure 2, were accepted.

Specimens with the failure at or outside the change in width were

excluded as failing outside the gage length. Table 1 shows the

outcome distribution for all the specimens that we tested or

tried to test.

Table 1. Outcome distribution for all specimens.

Test outcome Count

Fatigue failure (endurance result) 15

Runout (lower-bound endurance result) 4

Used for tensile test 3

Failed outside gage length (not reported) 3

Ruined during preparation (not reported) 8

Failed during preconditioning (not reported) 3

Total 36

39
RESULTS and DISCUSSION

Typical values of force-displacement and stress-strain for

this set of specimens are shown in Figure 4 . Table 2 gives the

tensile properties. The magnitudes of force and displacement

evident in Figure 4 are much smaller than those found in

conventional tensile testing. Clearly, special equipment is

required to test thin-films. The strains displayed in Figure 4

are derived from grip-to-grip displacement, and therefore

overestimate the actual elastic behavior of the specimen. The

displacement within the gage section of a specimen that had been

loaded and unloaded several times was measured by electronic

speckle pattern interferometry (ESPI) ; this measurement gave a

more accurate value of the Young's modulus, which is shown in

Table 2 . The strength of this material is much higher than

values of yield and ultimate strength for pure Cu found in

handbooks. This finding is typical of thin-films, and is

explained by the very fine grain size of these specimens as

compared to that of pure bulk Cu. The low values of elongation

are characteristic of thin-films tested in tension. Results of

the tensile test results are listed in Table 2, including results

for a specimen that endured over 150,000 fatigue cycles at a

maximum stress of 197 MPa.

Table 2. Tensile test results for electron-beam-evaporated thinfilm tensile specimens.

Condition Young '

s

modulus,

GPa

Yield

strength,

MPa

Ultimate

strength,

MPa

Elongation

to

failure,

percent

New 107

it 332 423 1.6

ii 345 409 1.3

it 314 364 1.2

Average

,

new

330 399 1.4

After

152,098

fatigue

cycles

at 197 MPa

343 400 1.2

40
The fatigue results are shown in Figure 5. Plastic

ratchetting occurred in all of the fatigue tests, to a much

greater degree in the specimens tested at the higher loads. Even

though these tests were all run under load control, the fatigue

failures at the highest loads appeared to be of the low-cyclefatigue type. The specimens failed when the plastic strain

ratchetted up to a sufficiently high value. Failure strains were

generally consistent with the tensile test results, typically

around 1.4 percent, as shown in Table 2.

The progress of a typical high-load test was as follows. As

the specimen was cycled, the total displacement increased. Some

hysteresis was recorded, especially in the earlier cycles. As

the test progressed, the appearance of the surface of the

specimen changed. The originally flat and shiny surface became

rougher, and many short, linear, crack-like features gradually

appeared. Eventually, these would grow, looking more and more

like cracks. Finally, the specimen would fail.

At lower loads, the appearance would change much more

gradually, but in a similar fashion.

Comparison to previous work

Figure 6 compares the present results to a trend curve given

by Chalmers [12] as generally representative of the stresscontrolled fatigue of metals. The present results appear to fit

the trend, with some scatter. Several sets of room-temperature

fatigue data for copper bars are summarized by Reed [13].

Unfortunately most of these only include data for 10 cycles and

higher. For most cases, the fatigue strength at 10 5 cycles is

about 200 MPa, and still declining. Figure 6, above, shows a

similar fatigue strength at 10 5 cycles. It might be argued that

since the present specimens have a higher tensile strength than

the typical material, around 400 MPa for the present material vs

about 3 00 MPa for many of the materials reported by Reed, the

present specimens should also have a higher fatigue strength.

This seems not to be the case.

Figure 7 compares the results of the present work for Cu

thin-films with previous results for Al thin-films [9], and

literature results for sheet metal [14]. These thin-films of Cu

clearly have better fatigue resistance than that previously

reported for a set of Al specimens. The present Cu data fall

between the data for unnotched and notched stainless steel and Al

sheet metal. The present results appear to indicate an endurance

limit at around half of ultimate, similar to the data for

unnotched sheet metal. The notched (kt = 4) sheet metal had an

endurance limit of around one quarter of ultimate. In the

present data the plasticity decreases significantly as the stress

drops from around 3/4 to 1/2 of ultimate. This occurs at an

41
endurance of about 5000 cycles. Thus, the transition between low

cycle and high cycle fatigue can be estimated to lie somewhere in

the neighborhood of 5000 cycles for this set of specimens.

The results shown in Figure 5 have variability at both low

ahd high loads, in comparison to data such as those presented by

Oshida and Chen [15]. However, Oshida and Chen were able to use

fully-reversed bending, their Cu films were supported by thicker

polymer layers, and they report their results as a function of

strain. In the present study, the variation in endurance as a

function of stress at high stresses can be understood through

consideration of the stress-strain curve (Figure 4) . The high

stresses are in the region of plastic strain, where small changes

in stress produce large changes in strain. It is anticipated

that if the cyclic strain in the present films could be

controlled, the apparent scatter in Figure 5 would be transformed

into a dependence of endurance on strain range, with less

scatter.

For low stresses, the variability in the present study

appears to be consistent with expectations. However, the

endurance limit, converted to strain, is lower than 0.2 percent,

while Oshida and Chen report a value around 0.6 percent at

comparable cycle counts. The causes for this difference is are

believed to lie both in the specimen material and in the test

technique. The electrodeposited specimens [15] were 13 fiw. think,

while the present specimens are 1.2 jum thick. The present

specimens are electron-beam-evaporated, while Oshida and Chen's

were electrodeposited. And, Oshida and Chen used bending of Cu

layers reinforced by by polymer layers, while the present study

used tension of unsupported films.

Specimen Geometry

Specimens as shown in Figure 2 , with sharp changes in width

at the ends of the gauge section, might be expected to fail

consistently at the corners. However, the microphotolithography

process used to fabricate the specimens produces rounded corners

in the actual specimens. Furthermore, slight defects too small

to characterize occur along the length of the specimens. These

real-world effects, rounded corners and other defects, explain

the relatively low number of failures at the end of the gauge

section.

Microstructural Phenomena

These specimens were different from bulk sheet metal

specimens in that they have only one or a few grains through the

thickness. Although a full investigation of the detailed

metallurgical behavior during these tests is beyond the scope of

this paper, some observations are presented here. Surface

42
features that appear during fatigue of these specimens, as shown

in Fig. 8. The density of the crack-like features and the

rumpling of the surface appear correlated with accumulated

plastic strain. Similar features, but not as clear or dense,

appear in tensile testing. However, transmission electron

microscopy (TEM) of a fatigued specimen, Fig. 9, revealed that

the microcracks shown in Fig. 8 are in the surface layer only,

and not in the Cu. In this figure, the black region is a cross

section through the fatigued specimen, and the gray surface layer

is the thin layer of Ti on the specimen surface. The openings in

the Ti layer correspond to the visible surface microcracks in

Fig. 8.

Figure 10 shows a TEM of the fatigued specimen. Two

significant types of features are seen: a few individual

dislocations; and bands of debris lying at approximately 45° to

the plane of the specimen. No dislocation cells as reported in

[16] were seen in this specimen. The size of such cells is

typically 5 jum, which is much larger than the present specimen

thickness. A previous paper [17] presented observations of

dislocations in tensile tests of thinner Cu films produced in the

same way as those reported here. In that paper it was noted that

although only a few dislocations were seen, the density of

dislocations was reasonable for the level of plastic strain and

the volume of material observed. Similarly, a few dislocations

can be seen here in the fatigued specimen. It is believed that

the debris bands are remnants of interactions between mobile

dislocations and obstacles to their motion.

The absence of clear steps on the specimen surfaces,

combined with the absence of dislocation cells in the fatigued

material, indicate that the dislocations move individually or in

small groups and escape at the specimen surface. This is

different from the behavior of bulk materials, where slip bands

and dislocation cells are commonly reported.

SUMMARY

A set of electron-beam-evaporated Cu thin-film specimens

have been tested in load-controlled tension-tension fatigue. The

fatigue resistance is superior to that previously reported for a

set of Al thin-film specimens, and intermediate between unnotched

and notched sheet metal results found in the literature. The

transition from high cycle to low cycle fatigue occurred at

around 5000 cycles, accompanied by a decrease in the stress from

three-fourths to half of ultimate. Transmission electron

microscope observation suggest that dislocations associated with

the plastic strain traverse the specimen in a direction at

approximately a 4 5° angle to both the load axis and the throughthickness direction, and then escape from the specimen surface.

No cells or walls of dislocations were observed.

43
ACKNOWLEDGEMENTS

Xochitl Zamora-Thompson performed many of the tests reported

here. John Phelps made the TEM observations. This work was

supported financially by the Office of Microelectronics Programs

of NIST.

REFERENCES

1. D. A. Hardwick, "The Mechanical Properties of Thin-films: A

Review," Thin Solid Films 154, 1987, 109-124.

2. P. S. Alexopoulos and T. C. O' Sullivan, "Mechanical

Properties of Thin-films," Ann. Rev. Mater. Sci. 20, 1990,

391-420.

3. F. R. Brotzen, "Mechanical testing of thin-films,"

International Materials Reviews 39, No. 1, 1994, 24-45.

4. Jan-Ake Schweitz, "Mechanical characterization of thin-films

by micromechanical techniques," Materials Research Society

Bulletin July 1992, 34-45.

5. A. Fartash, Ivan K. Schuller, M. Grimsditch, "Vibrating

membrane elastomer for reliable measurement of mechanical

properties of metallic films," Rev. Sci Instrum. 62 (2),

February, 1991, 494-501.

6. J. A. Ruud, D. Josell, F. Spaepen, A. L. Greer, "A new

method for tensile testing of thin-films", J. Mater. Res.

Vol. 8, No. 1, Jan. 1993, 112-117.

7. W. D. Nix, "Mechnical Properties of Thin-films,"

Metallurgical Transactions A 20A, November 1989, 1989-2217.

8. I. Kim and R. Weil, "The Mechanical Properties of

Monocrystalline Nickel Electrodeposits, " Thin Solid Films

169, 1993, 35-42.

9. D. T. Read and J. W. Dally, "Fatigue of

Microlithographically-Patterned Free-Standing Aluminum Thinfilm Under Axial Stress," Journal of Electronic Packaging

117, March 1995, 1-6.

10. D. T. Read, "Piezo-Actuated Microtensile Test Apparatus",

submitted to ASTM Journal of Testing and Evaluation.

11. E. M. Zielinski, R. P. Vinci, and J. C. Bravman, "Effects of

barrier layer and annealing on abnormal grain growth in

44
copper thin films," Journal of Applied Physics 76, 1994,

4516-4523.

12. B. Chalmers, Physical Metallurgy, John Wiley and Sons, New

York, 2 07.

13. R. P. Reed and R. P. Mikesell, Low Temperature Mechanical

Properties of Copper and Selected Copper Alloys: A

Compilation from the Literature, National Bureau of

Standards Monograph 101, U. S. Government Printing Office,

1967.

14. P. G. Forrest, Fatigue of Metals, Pergamon, Oxford, 1966,

162.

15. Y. Oshida and P. C. Chen, "High and Low-Cycle Fatigue Damage

Evaluation of Multilayer Thin Film Structure," Journal of

Electronic Packaging 113, 1991, 58-62.

16. C. Laird, P. Charsley, and H. Mughrabi, "Low Energy

Dislocation Structures Produced by Cyclic Deformation,"

Materials Science and Engineering 81, 1986, 433-450.

17. R. R. Keller, J. M. Phelps, and D. T. Read, "Tensile and

fracture behavior of free-standing copper films," Materials

Science and Engineering A214, 1996, 42-52.

45
oaDo

E° EE

(DED

Co

•H

-P•H

•H -O <W

-p

C d) O £ O

T3 P C

(0 -M 3

•H -i0)e - •

<tf P w ^ s a co

0) co

•H £! £

•Hw

w o

•H +J

G <D TJ •H g Q) TJ fl P

rH <H

0)

H £ 3Oe

rH (0 fl • 0)

a) o h w a A3 Do w c

0) <l) +J IT" gH-HC H -H -H O W C -P

Q)

CO

W

a a) £ a) w p > +j

id coo c

CO

•H

CO o

46
o

CM coaoo

a> •H

10c

a> p

a) .cp

«MOcH

CM

•H

fa

47
Ti

<D PUa

(0> s

Q)

W I H go id IcO +J U o

+J Q) U CO

d) I rH W

a)

wo

nO

•H (0 a-w o

(d <m o 0)o

d) IT} 3 -Me a po

^1 rH (d P -H U o I ^ c

•H X3-H

r>

O

0)u&

•H

En

48
0.08

Strain

0.000 0.002 0.004 0.006 0.008 0.010

1 —

0.06

8 0.04

o

LL

0.02

0.00

/

s****°***'

r

/

Ti Cu Ti thin film

I J I L J L

400

300

CL

1200 co

(/)

CO - 100

1 2 3 4 5 6 7

Displacement, pm

Figure 4. Typical force-displacement and stress-strain record of

a thin-film from this set of specimens. One N is about

100 gram- force. The 0.2 percent yield strength, the

ultimate tensile strength, and the failure point are

indicated.

49
500

^400

$300

w

| 200

£

x

CO

100

Failure

i _. »Q . Runout

i_i

EBE Ti Cu Ti

i—i m i ml 1—i i 1 1 iii| 1h-+-HH-H i—I II I lll| 1—

I

I I I Ml| 1—I I I Mil 1E0 1E1 1E2 1E3 1E4

Cycles to Failure

1E5 1E6

Figure 5. Plot of maximum cyclic stress against number of cycles

to failure (S-N plot) for electron-beam-evaporated

copper thin-film specimens.

50
D)C

CD

\_

•*-> CO

CD 0.8

"co c

1® 0.6

h-

-»>».

t/> 0) 0.4

CD

i_

+-• CO

E

0.2

D

E

X

CO

41 """^

i % • "^mm,

n 1 : 1 i:ii:j 1 i : llllll i j 1 Hill; 1 : I '111:1 : 1 1 l::llj 1 j 1 :||:j 1E0 1E1 1E2 1E3 1E4 1E5 1E6

Cycles to Failure

Figure 6. Cyclic stress normalized to ultimate tensile strength

plotted versus cycles-to-failure for electron-beamevaporated Cu (present study) , compared to the general

trend for metals given by Chalmers [ 12 ]

51
CO

\-

1/5

CO

<D *—

CO

E

E

x

03

Stainless steel

and Al sheet

Unnotched

Notched

EBETi Al Ti

EBE Ti Cu Ti

Failure

Runout a

1E0 1E1 1E2 1E3 1E4 1E5 1E6

Cycles to Failure

Figure 7. Cyclic stress normalized to the ultimate tensile

strength versus cycles-to-failure for electron-beamevaporated Cu (present study) , electron-beam-evaporated

Al[10], and unnotched and notched sheet metal

specimens [14]

52
JHHH^HHHH 40/vm

Figure 8. Optical micrograph of the surface of a Ti-coated Cu

specimen after fatigue.

53
Figure 9. Transmission electron micrograph of a cross- section of

a Ti-coated Cu specimen after fatigue. The large dark

region is Cu. The light gray at the surfaces is Ti.

The openings in the Ti are not correlated with cracks

or notches in the Cu.

54
0.25#m

Figure 10. Transmission electron micrograph of a crosssection of a fatigued specimen.

55
Appendix E. 1997: Piezo-Actuated Microtensile Test

Apparatus

56
PIEZO-ACTUATED MICROTENSILE TEST APPARATUS

David T. Read

Materials Reliability Division

National Institute of Standards and Technology

Boulder, Colorado 80303

ABSTRACT

The design and performance of a microtensile test device

designed for measuring the mechanical properties of thin films

are reported. The moving grip is cantilevered to avoid friction.

Piezoelectric actuation allows precise control of the applied

tension. Force is measured indirectly using an eddy-current

sensor near a flexible measuring beam, and grip displacement is

measured using two similar displacement sensors. The apparatus

is constructed for use with specially designed specimen

assemblies in which the thin film to be tested is suspended

across a silicon frame. The apparatus is rigid enough that the

frames can be cut without applying tension to the thin-film

tensile specimen. The tester is run in closed-loop control

through a laboratory computer, which records force, displacement,

and other parameters 5 times per second. Key capabilities are:

stroke, 50 /xm; maximum force, 1 N; minimum cycle time, 2 s;

displacement resolution, ±20 nm; and force resolution, ±200 juN.

Key words: Closed loop, displacement, force, piezoelectric,

servo, strain, stress, thin film.

Contribution of the U. S. Department of Commerce. Not subject to

copyright in the U. S. A.

57
INTRODUCTION

This paper describes a piezo-actuated microtensile test

machine suitable for testing specially fabricated thin-film

specimens. A variety of microtensile test devices have been

described in the literature, with varying advantages and

disadvantages. A detailed review is beyond the scope of this

report. Several excellent recent papers on thin-film mechanical

properties include discussions of the apparatus and additional

references [1-5]. Reported approaches to measuring the

mechanical properties of thin films can be classified generally

as to whether or not the specimen film must be manually parted

from the substrate and placed on the grips, and whether the

stress applied is pure tension, biaxial, or more complex. For

example, Ruud et al . [6] parted their specimens from the

substrate for mounting in a tensile device that applied pure

tension, while Small et al. [7] used silicon-framed membrane

specimens in a hydraulic bulge-test device that applied biaxial

stress.

The problem of safely handling very small specimens has been

addressed for both metal and silicon specimens by including a

removable reinforcement in the specimen design. Kim and Weil [8]

used electrodeposited metal films with a reinforcing section.

Ding, Ko, and Mansour [9] tested silicon-framed silicon. A

previously reported microtensile test apparatus [10], abbreviated

MT-I, was the first device shown to be capable of testing

silicon- framed metal thin-film tensile coupons, where the thin

film to be tested is suspended across a silicon frame for

convenient placement and alignment in the test fixtures. This

design eliminates the need to manually part the specimen from the

substrate, at the cost of a wet etch to remove the substrate from

underneath the gage section of the specimen. Detailed

descriptions of this specimen and the steps involved in

fabricating it have been given previously [10, 11]. The

difficulty with this procedure is that the frame has to be cut

before the specimen is tested. The operations involved in

cutting the frame can easily allow uncontrolled tension to be

applied to the specimen, destroying it. Imposed tensile

displacements of as little as 2 /urn can produce plastic

deformation in a thin-film tensile specimen. Such displacements

may reguire forces as small as 0.01 N (1 gram- force ), and thus

can easily occur when a cutting tool is applied to the frame or

when the frame rebounds after being cut. All known specimen

grips mass at least 1 g, so the specimen cannot support the

weight of the grips against gravity, as is normal in testing bulk

specimens. MT-I used a special clamp to hold the frame while it

was cut. Upon its release, this clamp left the specimen exactly

in place, without applying any force in the tensile direction.

58
Few piezo-driven microtensile devices have been described.

The current microtensile tester (MT-II) is piezo-driven,

preserves the main advantage of the previous one (MT-I)[11], and

overcomes its main disadvantages. The main disadvantages of MT-I

were: friction between the moving grip and its support plate;

off-axis displacement measurement leading to possible errors;

high compliance of the spring used for force measurement; and

actuation by a manual micrometer.

MT-II overcomes these disadvantages by: cantilevering the

moving grip, so it moves without friction; using a much stiffer

beam-type spring for load measurement; using two displacement

sensors, one on each side of the specimen; and using

piezoelectric actuation. Details are described below.

MT-II can be reproduced at a low cost. The mechanical parts

of MT-II include less than a dozen simple mechanical components

that were made from ordinary materials to ordinary tolerances in

a conventional machine shop, and the commercially purchased

piezoelectric stacks. These are driven with a commercial 500 V

DC power supply. The sensors were also purchased commercially.

The electrical signals from the instruments are connected

directly to the input terminals of a commercially available

analog-to-digital conversion board in a personal computer. The

tester fits into an elementary analytical probing station

designed for testing of electrical circuits on silicon wafers.

Such devices are widely used in the electronics industry. The

probing station microscope has a long working distance that

allows manipulation of the specimen and a trinocular head that

allows observation and photography of the specimens during

testing.

MECHANICAL DESIGN

The design of MT-II is shown in Figure 1. An enlarged

schematic of a silicon-framed tensile specimen, with the frame

cut, is shown in Figure 2. The specimen differs from the version

reported previously in that only one coupon is used, rather than

four parallel strips. The gage section length is 700 /xm, and the

width is 200 jLim. The fabrication techniques were reported

previously [10, 11] and will not be repeated here.

The piezoelectric stacks are 2 mm in diameter and 60 mm

long. They are restrained by eight nylon rods that hold the

moving head assembly to the base of the device. The moving head

assembly includes the reference beam, the force-measuring beam,

and the moving grip; these are all cantilevered above the base

plate to avoid friction. The clearance is about 2 5 /im and is not

critical. (The term "grip" is used loosely here. The grips are

really more like platforms, as shown in Figure 3. In practice

the specimen frame is glued onto aluminum plates that are screwed

59
to flat surfaces on both the fixed part and the moving part of

the device.

)

The device is 25 mm thick. The grip surfaces are centered

vertically, as are all the sensors and the clamp screws. A small

aluminum plate is attached with screws to each of the grips. The

specimen frame is glued to these plates. The plates can be

removed and cleaned or replaced between tests. The clamp screws

can be run in against the measuring beam to prevent displacement

while the specimen frame is being cut.

The displacement of the moving grip is measured using the

commercially-supplied eddy-current displacements sensors [10]

shown in Figure 1. The tension applied to the specimen is

measured using a third displacement sensor, through the

displacement of the force-sensing beam shown in Figure 1. This

beam is fixed at both ends, and bends elastically in response to

force applied to the moving grip. This bending can be

characterized by a single spring constant relating applied force

and displacement, and therefore this part of the tester can be

thought of as a force-measuring spring. The calibration

procedures and sensitivities of these measuring devices are

discussed below.

CONTROL SYSTEM

The eddy current displacement sensors were described

previously [10]. The analog output of the displacement sensors

is introduced to the input of a commercially available analog-todigital board (A-to-D board) in a laboratory computer. Eight

double-ended channels with 16-bit precision are available. Three

channels are used for the two displacements and the load. Two

others are used for the current and voltage in a resistance

thermometer, and another is used to monitor the actual voltage

applied to the piezoelectric elements.

A custom-written program running on the computer manages

data acquisition and storage, and control of the device. The

microtensile tester is operated in digital closed-loop control.

This is necessary even in the displacement-controlled modes

because the piezoelectric stacks exhibit hysteresis and creep in

their response to applied voltage. The program currently offers

a choice of fixed value ("set point") , ramp, sine wave, or square

wave control of the displacement, the force, or the applied

voltage. The program evaluates the error as the difference

between the programmed value of the control quantity and its

actual measured value. The voltage applied to the piezo-stacks

is changed by the sum of a proportional response, a rate

response, and an acceleration response. A separate factor

relates each response component to its corresponding error. An

analog voltage proportional to the error is generated by a D-to-A

60
converter on the A-to-D board. This voltage is introduced to the

control input of a high voltage power supply. The maximum

allowed voltage is 250 V.

CALIBRATION

Force is calibrated routinely by tipping the device to a

vertical position and placing a known mass on the moving grip. A

typical value of the calibration factor is 1.5 N/V. Experiments

with springs, turning the device upside down to reverse the

effect of the weight of the moving grip, and so on indicate

differences of 3 percent or less between this calibration factor

in the range where it is calibrated and the range where data are

taken. A typical displacement of the measuring beam over the

course of a test is 2.5 /xm or less, while the thickness of the

measuring beam is 500 /xm. Although this procedure shifts the

effective zero-point of the force calibration range by the weight

of the moving grip, which is about 0.35 N, it is superior to

other methods that we have tried. Experiments with conventional

pulleys to convert gravitational force on known weights to

horizontal force have errors at least this large. Calibrated

springs in the proper range of force and displacement are not

commercially available.

Displacement calibration is performed using a commercially

available displacement-measurement system utilizing a timed

scanned laser beam. The displacement calibration factor is 25.4

jum/V to within better than 1 percent.

PERFORMANCE

The total stroke of MT-II is about 50 izm. Changing the

restraining rods from stainless steel to nylon improved this

value considerably. This stroke is sufficient to test to failure

thin metal films produced by physical vapor deposition (PVD)

.

Some electrodeposited films thicker than 5 /xm have strains to

failure approaching 10 percent, and so specimens with a 700 /xm

long gage section could not be tested to failure in this device.

There are a variety of rates involved in the data

acquisition and closed loop control cycles. The fastest rate is

the rate of conversion of individual analog voltages to digital

values. With the equipment now in use, each conversion requires

about 10 /xs. These conversions proceed independently

("asynchronously") , under control of the A-to-D board. At the

same time, the program is updating the control voltage based on

the previous readings.

The control voltage is updated 50 times per second with the

current laboratory computer, which is rated at a processor speed

of 25 MHz. Ten raw readings are averaged to get the "control"

61
values used to derive the error signal. Voltages are reported 5

times per second. Each reported value is the average of 10

control values. The reported values are stored on hard disk for

later retrieval.

The eddy-current displacement sensors have a sensitivity of

25 /xm/V and are operated with ± 2 V full scale. The raw voltage

readings have standard deviations of up to about 3 mV,

corresponding to 75 nm in the displacement measurement and 5 mN

in the force measurement.

The standard deviations for the reported displacements, with

the machine at rest, are 20 nm and 0.2 mN (0.02 gram- force ) . Of

these, the error in force is more significant. An improved

supplemental force sensor is being developed.

The response of the device to a step in the imposed voltage

is shown in Figure 4. The voltage setting was switched from 2

to 40 V. The power supply responded with a time constant of

about 0.1 s. The displacement followed, with a time constant of

about 0.45 s. These values were obtained by fitting the measured

response, indicated by the data points, to a simple exponential

function proportional to 1 - exp(-t / T) , where t is time after

the switch and T is the fitted time constant. Additive and

proportional factors and a starting time were also used in this

fit. The response of the device is slow because it is extended

physically, the measuring spring has some compliance, and the

piezo-stacks can exhibit hysteresis and creep. The minimum cycle

time quoted above, 2 s, was taken as four times the machine time

constant. For smooth sinusoidal cycles with about 100 data

points, a cycle time of about 20 s would be required.

No "overdriving" was used in this measurement. The actual

response of the mechanical system is shown, with no optimization

from the control system. The response time could have been

reduced by overdriving, that is, by applying a high voltage

momentarily to accelerate the motion of the device, then reducing

the voltage to its steady state value. The response in that case

would depend on the control parameters.

The response of the tester was documented by using a spring

in the form of a small steel "snap-ring", about 1 cm in

diameter, as a dummy specimen. This was found to be much more

convenient and reproducible than other possibilities such as fine

wires in tension or small coil springs. A wire with the

appropriate compliance would have to be less than 25 pm in

diameter. It would be delicate and difficult to mount, and its

elastic response would be difficult to verify. Small coil

springs also seemed difficult to mount in the tester. Snap-rings

are commonly found in mechanical systems such as automobile

carburetors, and are readily available in various sizes. Snap62
rings are compliant and springy in the diametral direction. In

normal use a snap-ring is forced open slightly, put in place in a

groove around a shaft, and released. It springs back to its

normal diameter and is retained by the groove. The ears of the

snap-ring were easily adapted to mount to the grips of the

tester, so that it provided a load that varied linearly with

imposed displacement.

Figure 5 shows the force on such a spring as a function of

time under load control. The force data are indistinguishable

from the sine function, while the piezo-voltage is seen to

decrease slightly over the time of this sample data.

Figure 6 shows a plot of force against displacement for the

spring for one cycle. The slight differences between the loading

and unloading records are due to friction and hysteresis in the

mounting of the spring, not in the testing machine.

SAMPLE RESULTS

Some sample test results are shown in Figures 7-9. These

results will be discussed in more detail elsewhere and are

included here to demonstrate the performance of the microtensile

test device.

Figure 7 shows the force-displacement record for an

electron-beam-evaporated (EBE) thin film of Cu with Ti layers on

both sides. The film is 1.2 /xm thick; the two Ti layers are each

0.05 jum thick. This plot demonstrates some of the difficulties

involved in testing thin films, because the elastic displacement

is less than 2 /im, the total displacement is less than 10 /xm, and

the ultimate tensile force is less than 100 mN (10 grams-force)

.

Figure 8 shows the engineering stress-strain plot from the

same test. Because the strains are so small, the engineering

strain accurately represents the true strain. The lines on the

plot indicate the loading slope, the yield strength, and the

ultimate tensile strength. The loading slope shown in this plot

does not represent the handbook value of Young's modulus for two

reasons: first, the strain is calculated from the grip

displacement, and so some deformation outside the gage length is

included; and, second, more accurate measurement of the strain in

the gage length, reported separately, indicates that the Young's

modulus of this specimen is less than the handbook value for bulk

copper. This film was deposited slowly on a substrate held at

room temperature, and such films have been reported to contain

many small voids. Such voids or other microstructural

differences between PVD films and bulk metal may lead to

differences in their elastic constants.

63
Figure 9 shows a set of fatigue data for the same thin-film

material. These data were obtained in load-controlled tensiontension fatigue. In the two longest tests, one specimen failed

at about 45 000 cycles, and one survived over 100 000 cycles.

DISCUSSION

The microtensile tester described above has been used for

tensile tests of PVD thin films and for electrodeposits. It has

been used for a series of load-controlled fatigue tests, the

longest of which ran for over 100 000 cycles. Its advantages

include mechanical rigidity that allows testing of silicon-framed

specimens, piezoelectric actuation for fine control, cantilevered

moving parts for frictionless operation, and closed loop control

so that load and displacement rates can be accurately controlled

and changed.

The tester as described here has two main shortcomings. One

is that it measures the grip displacement, whereas the

interesting quantity is the strain in the gage length. The other

is the limited resolution of the force measurement. Work is in

progress on using ESPI (electronic speckle pattern

interferometry) to measure the specimen displacement. This

approach appears to work and will be reported separately. The

load resolution is limited by the resolution of the eddy-current

displacement sensor and the stiffness of the force-measuring

beam. Reducing the rigidity of the beam would make it much more

difficult to break the specimen frame without damaging the

specimen. A more sensitive displacement-measurement technique

using an optical lever is being tested.

ACKNOWLEDGEMENTS

Xochitl Zamora-Thompson performed many of the tests reported

here. This work was supported financially by the Office of

Microelectronics Programs of NIST.

REFERENCES

1. D. A. Hardwick, "The Mechanical Properties of Thin Films: A

Review," Thin Solid Films 154 (1987), 109-124.

2. P. S. Alexopoulos and T. C. O

1 Sullivan, "Mechanical

Properties of Thin Films," Annual Review of Matererials

Science 20, 1990, 391-420.

3. F. R. Brotzen, "Mechanical testing of thin films,"

International Materials Reviews 39, No. 1, 1994, 24-45.

64
4. Jan-Ake Schweitz, "Mechanical characterization of thin films

by micromechanical techniques," Materials Research Society

Bulletin July 1992, pp. 34-45.

5. A. Fartash, Ivan K. Schuller, M. Grimsditch, "Vibrating

membrane elastomer for reliable measurement of mechanical

properties of metallic films," Review of Scientific

Instruments 62 (2), February, 1991, 494-501.

6 J. A. Ruud, D. Josell, F. Spaepen, A. L. Greer, "A new

method for tensile testing of thin films" , Journal of

Materials Research Vol. 8, No. 1, Jan. 1993, pp. 112-117.

7. Small, M. K. , Daniels, B. J., Clemens, B. B. , and Nix, W,

D. , "The elastic biaxial modulus of Ag-Pd multilayered thin

films measured using the bulge test," Journal of Materials

Research 9, No, 1, January, 1994, 25-30.

8. I. Kim and R. Weil, "The Mechanical Properties of

Monocrystalline Nickel Electrodeposits, " Thin Solid Films

169, 1993, 35-42.

9. Ding, X., Ko, W. H. , and Mansour, J. M. , "Residual Stress

and Mechanical Properties of Boron-doped p

+ -Silicon Films,"

Sensors and Actuators A23, 1990, 866-871.

10. D. T. Read and J. W. Dally, "A new method for measuring the

strength and ductility of thin films," Journal of Materials

Research 8, No. 7, July 1993, 1542-1549.

11. D. T. Read and J. W. Dally, "Fatigue of

Microlithographically-Patterned Free-standing Aluminum Thin

Film Under Axial Stress," Journal of Electronic Packaging

117, March 1995, 1-6.

65
HHio>P

in

a) •P

a) rH•HWc

0) pouu

•H€oc

•HO

•HP

f0e

a)u

CO

•H

66
o

Cloo co

CO

CO o c

a)n

•H

a)w

a) H

•H

tfl G

0) • -p trc e-H

<H +J

<H d) c

•H

O

<H 73 Q)

0) £1 n^ C iC -P3 o u

-HH

C

•H <D

(A Q) O

(d

r-i £1 •H

«j a) p n

a) to Q U • a) CM £

-p

(DM

M 3 a) •H 1-4 h <3

67
£.on

u>cD

COOE

<boo x / / I ce

•HO

0)w

d)

•PoC

•H>o

(0

(0ft •H

d) xi

+j

«woco

•H

-MO

0)

Cfl

COWOU• <I) r> +JC

0)

p

u o 3 s

tr> •h

w

68
50

40

<D

D)

CD

o30

o

N

CD

20

10

Fits: ~ [1 - exp(- 1 / T)]

192 194

Displacement, T = 0.45 s

3 e

C

CD

2 E

o

03

Q.

(0

1 b

196 198

Time, s

200 202

Figure 4. Plot of piezo-stack voltage and moving grip

displacement against time, showing the response to a voltage

step.

69
0.42

0.4

0.38

O

£0.36

0.34

0.32

100

Measured

Sine function

Piezo voltage

Spring

60

50

40 °>

10

150 200

Time, s

250

>

CD

O)

CO

O

30 o

N

CD

20

'°-

Figure 5. Plot of force on a spring against time obtained with

the device in load control . The maximum and minimum values of

the force remain constant, while those of the voltage change

slightly. The time dependence is indistinguishable from a true

sine function.

70
0.42

0.4

z 0.38

<DO

£0.36

0.34

0.32

Spring

Measured data

Linear fit

9 10 11 12 13 14 15 16 17

Displacement, pm

Figure 6. Plot of force against displacement for the spring.

The spring is clearly linear. The small deviations result from

noise and friction in the mounts of the spring, not in the test

device.

71
0.10

0.08-

0.06

O

i£ 0.04 [

0.02 -

0.00

4 6 8

Displacement, ^m

10

Figure 7. Force-grip displacement record for a tensile test of

an electron-beam-evaporated (EBE) thin-film trilayer of Ti, Cu,

and Ti. The total thickness is 1.2 jum; the Ti layers are 0.05 nm

thick.

72
400

300

05

QL

co 200

a)

if)

100

0.000

®&?^

%&M^&$£*#&

EBE Ti Cu Ti

0.005 0.010 0.015

Strain

Figure 8. Stress- (engineering) strain record for a tensile test

of an electron-beam-evaporated (EBE) thin film trilayer of Ti,

Cu, and Ti. The total thickness is 1.2 [in; the Ti layers are

0.05 jum thick.

73
500

^400

(/) <D

-*—

CO

E

E

X

CO

300

200

100

Failure

Runout .

II — »i

L_J

. EBE Ti Cu

1 II I I Mil 1—I I I I Mil 1—M-++++H—II I I llll| 1—

I

I I llll| I—I I I Mil 1E0 1E1 1E2 1E3 1E4 1E5 1E6

Cycles to Failure

Figure 9. S-N curve for tension-tension fatigue of the TiCuTi

films. The minimum load was held at one tenth of the maximum

load. One specimen endured over 100 000 cycles without failure

74
Appendix F. Drawings

75
76
o £ «

t» S 3§

0^

i

i

<

n

\

OJo

CO *^

o ^^ 00

00 j ^r ^ rs-fl

c

<

c

1 , u

o I

i , K

o1 —H- — *

t

1

V )

p

1 1 A o| !

1 i

1 h

o

°: co ill Id —

1 1

'

' w !4

—

IT) OJ

^0O

0>

ojo

in — 00 "" O

IT) OJ

Q

o

© ©

m

in

CKL. UD

is

ff5 5S §£a^

5fe!I5

77
a. o c

lO O I) O • — — O O £* o o o o \

za -h 3d(/> <" _1 ujuj 7 i*go: IP ^1 X X X x

o

=>o

ono

00d

X-4-

1 r^ rJ \ o w v. o

o

r\ r\

o

1 ^

d

T

K j \. o

o

If)

CNI

1

o

o

m

CMd

o

o

o

.o

1CL,F=:o

Ml

z

78
5 t> m c 3l

in o m o

-H -H M M M M

£65 5I 3

UJuj

oc/i ~i

So X X 2 x x 5

PQ CD

0^ OJ CD

CD | CD

R u CD*" o z: CD

ID

CL LO "^

CD X CD

X ^ I—

1 in

CD

N—

1 |\

—\_

f

t

<—

CL \ __ _j— X ^aw^Q_

X

CD

CD

in

CD m

CD

CD

—c=.

i

CD

] Z33 if

1^ ^^

CD

CD 1

\

A CD 1

r 1 fc=D © C

^

f 1

f \

CD

OJ

CD

~1

I CD

0^

1^

CD

CD

m

CD OJ

CD

r—

CD

Ifc. §z

<_> =

"basis'5 — Q- t— O

79
* « o o o

31

E5 5• ys

•H -H -H -H

25 £

ES=^

Z

80
81
in o m o

5*

is

Jfe-

— Q. t— O

82
83
in o m o

.9 • S°E . s|F?E 5 X

X

X S °0 £ x. X. *> *. <

0_uo o

Q_Ld

LJ

(=1 Qiii in LJ

CD 1=1 X *

CD (^

00

CD

u CD 2~>

** CD X *-

sr "=*"

CD

in

0J -=3 1

CD

o>

CD

CD

in 1 r f • 1 I

*^o

o>

in

CD f f CD

in

0,1

<*0>

CD «a— cd

CD

inm

CD

CD

O *=3 czi

k84
85
VII rCT> ~o • • 7t m o « o •. .£• „o » - - o o TM

ro •< O

£ Zlj -H -H -H -H -H -H o

£ £ w O c 0) , i: Uu 7 [g

1 <A

AJI

dimen

unless

ot

TOLERANC

OTHERWIS

FRACTION,

.X .XX

.XXX

XXXX

ANGULAR

oj o

0) "52

ID ID fO ZD >n ju ~c CZ oo 3 HI

h- \¥

w> O

LO

0^ "O CD ce ^ O i—

-

-o

">

a.

•Ql DO

X 1 Eo 0J 1 £

i

Si k. LO LO ^s

[T^ OJ r~\

00 -

1 1 tj KD \aJ i 1 - - t

CD CD o

1 '

' Cl- ry CO

t

in

0J 1—O

00 VD 00 n oo —

~n

CD

CD

00 ^1

CD

a. O £

CD I'J L_ CD 00

CD

i

QC CD «t CD

CD

CD z

LaJCO

CJ3 CD

TATES NT

DF

INSTIT

NOLOGY E -to M~Q

§X§5

If

—

\

z

86
Appendix G. Computer Programs

These programs operate satisfactorily with the present tester and its data. However, they have

not been tested on other computers or by other users. Therefore, caution is in order regarding the use of these programs and any calculated results.

87

NISTTechnical Publications

Periodical

Journal of Research of the National Institute of Standards and Technology—Reports NIST research

and development in those disciplines of the physical and engineering sciences in which the Institute is active. These include physics, chemistry, engineering, mathematics, and computer sciences. Papers

cover a broad range of subjects, with major emphasis on measurement methodology and the basic technology underlying standardization. Also included from time to time are survey articles on topics

closely related to the Institute's technical and scientific programs. Issued six times a year.

Nonperiodicals

Monographs—Major contributions to the technical literature on various subjects related to the Institute's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes)

developed in cooperation with interested industries, professional organizations, and regulatory bodies.

Special Publications— Include proceedings of conferences sponsored by NIST, NIST annual reports, and

other special publications appropriate to this grouping such as wall charts, pocket cards, and

bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists,

engineers, chemists, biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical

properties of materials, compiled from the world's literature and critically evaluated. Developed under a worldwide program coordinated by NIST under the authority of the National Standard Data Act (Public

Law 90-396). NOTE: The Journal of Physical and Chemical Reference Data (JPCRD) is published bi- monthly for NIST by the American Chemical Society (ACS) and the American Institute of Physics (AIP).

Subscriptions, reprints, and supplements are available from ACS, 1155 Sixteenth St., NW, Washington,

DC 20056.

Building Science Series— Disseminates technical information developed at the Institute on building

materials, components, systems, and whole structures. The series presents research results, test methods, and performance criteria related to the structural and environmental functions and the durability

and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment

of a subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of

the subject area. Often serve as a vehicle for final reports of work performed at NIST under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures published by the Department of Commerce

in Part 10, Title 15, of the Code of Federal Regulations. The standards establish nationally recognized

requirements for products, and provide all concerned interests with a basis for common understanding of

the characteristics of the products. NIST administers this program in support of the efforts of private- sector standardizing organizations.

Consumer Information Series—Practical information, based on NIST research and experience, covering

areas of interest to the consumer. Easily understandable language and illustrations provide useful background knowledge for shopping in today's technological marketplace.

Order the above NIST publications from: Superintendent of Documents, Government Printing Office. Washington, DC 20402.

Order the following NIST publications—FIPS and NISTIRs—from the National Technical Information

Service, Springfield, VA 221 61. Federal Information Processing Standards Publications (FIPS PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register serves as the

official source of information in the Federal Government regarding standards issued by NIST pursuant to the Federal Property and Administrative Services Act of 1949 as amended, Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717 (38 FR 12315, dated May 11, 1973) and Part 6 of

Title 15 CFR (Code of Federal Regulations).

NIST Interagency Reports (NISTIR)—A special series of interim or final reports on work performed by

NIST for outside sponsors (both government and non-government). In general, initial distribution is handled by the sponsor; public distribution is by the National Technical Information Service, Springfield,

VA 22161, in paper copy or microfiche form.
U.S. Department of Commerce

National Institute of Standards and Technology

325 Broadway

Boulder, Colorado 80303-3328

Official Business

Penalty for Private Use, $300