The structure-mechanical property relationship of amorphous silicon monoxide thin films

Thin Solid Films, 154 (1987) 149-157

149

THE STRUCTURE-MECHANICAL PROPERTY RELATIONSHIP AMORPHOUS SILICON MONOXIDE THIN FILMS* R. W.

HOFFMAN,

Jr.?,

T. E. MITCHELLf

Case Wesrern Reserve University,

(Received

AND R. W.

OF

HOFFMAN

University Circle, Cleveland. OH 44106 (U.S.A.)

March 23, 1987)

Amorphous silicon monoxide (SiO,) thin films were produced by Joule heating and electron bombardment evaporative methods in high vacuum. Real-time force us. elongation curves were recorded for SiO, films 200-300 nm thick on an instrument known as the nanotensilometer. The elastic modulus, fracture stress and plastic deformation were determined from hard mode tensile testing. The elastic modulus varied from 53 to 75 GPa independent of film preparation method. No plastic deformation was detected for successive tensile pulls on a given specimen. Estimates of plastic deformation never exceeded 0.015% strain at the point of fracture. Failure occurred by brittle fracture and fracture stress ranged from 70 to 380 MPa. The film composition determined by Rutherford scattering gave an x value in SiO, within the range 0.9-1.0 for all films independent of evaporation method. The structure of the SiO, was determined to be of an amorphous character by electron diffraction and structure imaging using transmission electron microscopy.

1.

INTRODUCTION

Methods of studying bulk material mechanical properties include a variety of mechanical tests such as hardness and tensile testing. Such methods can be applied to thin films. For example, ultramicrohardness involves the measurement of penetration depth vs. the force on the indenter ‘. The triaxial stress distribution under the indenter is complex although, if known, it could yield flow stress measurements of the film materials. Difficulties with this technique include understanding the complex stress distributions below the diamond indenters during testing and separating the effects of the substrate on the measurements.

* Paper presented at the 14th International U.S.A., March 23-27, 1987. t Present address: 44130, U.S.A.

Sverdrup

Technology

t Present address: MS K-765 Alamos, NM 87545, U.S.A. 0040~6090/87/$3.50

Center

Conference

Inc., LERC for Materials

on Metallurgical

16530 Commerce Science,

Coatings,

San Diego, CA,

Ct., MS 302-1,

Los Alamos

National

0 Elsevier Sequoia/Printed

Cleveland, Laboratory,

OH Los

in The Netherlands

R. W. HOFFMAN,

150

Jr.,

T. E. MITCHELL,

R. W. HOFFMAN

An alternative is to use a miniature tensile-testing instrument which directly measures the force-elongation curves of samples subjected to uniaxial stress. The elastic modulus, flow stress, strain hardening and fracture stresses can be determined. The unique structures of thin films can then be related to their properties and, with modeling, the mechanical failure of the films can be predicted. Difficulties with this method of testing include sample preparation and handling. In this paper, results oftensile testing silicon monoxide (SiO,) films 200-300 nm thick are presented. SiO, films have been chosen for study because of their importance as antireflective coatings as well as passivating and insulating layers in semiconductor devices. In addition, previous mechanical data have been determined for SiO, by Pivot2 which have given an elastic modulus of 50 GPa. He also reported plastic deformation during the soft mode tensile testing which exhibited a strainhardening effect. Deposition parameters and their relationship to the resulting structures and internal stresses have been studied3-(j. Evaporation conditions in this study were chosen to produce stoichiometric amorphous SiO, films. The machine used for the tensile testing, termed the nanotensilometer, is of unique design and has been previously described’. It can be operated in either soft or hard modes and allows sample viewing, displaying and recording through the use of a video camera and recorder and a trinocular eyepiece microscope. The data to be presented are among the first inorganic material properties to be studied with the unique design of the nanotensilometer. 2.

EXPERIMENTAL

TECHNIQUE

SiO, films were prepared in a conventional high vacuum chamber by both Joule heating and electron bombardment techniques. Base pressures of 4 x lo- ’ Pa were achieved by oil diffusion pumping. Liquid nitrogen cold traps were employed to condense the water vapor from the residual gas. Quartz crystal monitors sampled the film thickness during growth. A shutter was situated between the sources and the substrates so that a uniform evaporation rate of 1 nm s ’ could be established before deposition onto the substrates. Samples suitable for mechanical testing require uniform dimensions. The sample size, because of the constraints of the nanotensilometer, required a crosssectional area of less than 80 urn’. The sample width was controlled by depositing the SiO, onto the fractured edge of a thin (about 150 urn) glass cover slide. The glass slides were coated with a water-soluble parting agent before SiO, deposition. Freestanding films were floated on water and transferred to Teflon pads for drying before mounting in the nanotensilometer. Film characterization was accomplished by transmission electron microscopy and Rutherford backscattering spectroscopy (RBS). The microscopy was performed on samples before and after mechanical testing. The microscopes employed were a Philips EM 400T and a JEOL 200CX. Electron diffraction patterns and structure images were recorded in a JEOL 200CX microscope operated at 200 kV with a point-to-point resolution of 0.236 nm. The composition of the films was determined by RBS of 2 MeV He+ ions from samples which were either floated or directly

MECHANICAL

PROPERTIES

OF SILICON MONOXIDE

FILMS

151

deposited onto carbon substrates. The silicon-to-oxygen ratio was determined by the ratio of the areas of the respective peaks. Assuming a bulk density also allowed calculation of the sample thickness. The film thickness was measured with a Tencor Alpha step surface profilometer for comparison with the RBS and crystal monitor measurements. The nanotensilometer has been described in detail in previous work’. Briefly, the instrument is described as two inverted pendula to which glass-sample-holding “jaws” are attached. A coil is rigidly attached to each pendulum which interacts with a permanent magnet attached to the base when current passes through the coils. The separation of the pendula is measured with a three-terminal ac. capacitor. The control circuit allows ramping the separation of the “jaws” (elongation) while the current in the coils is a measure of the force required to separate the jaws (tensile force). Thus a hard mode machine with a prescribed elongation rate results. The instrument is free from seismic disturbance because of its symmetrical design. The dried free-standing films are mounted on the glass jaws. Gripping the sample is performed by inducing intimate contact of the film to the jaw after wetting with alcohol. The sample is aligned before the alcohol evaporates. The sample is finally allowed to dry and is held in place by van der Waals bonding. The nanotensilometer is allowed to stabilize thermally before testing. Force-elongation curves under hard mode tensile testing at an elongation rate of 4 x 10-l nm s-i were recorded successively by increasing the maximum force and relaxing to zero strain until failure occurred in an attempt to determine an elastic limit. The nanotensilometer has an optical microscope mounted above the jaws which is used to view the sample during testing. In addition, a television camera is mounted on a trinocular eyepiece and the signal is recorded on a video tape recorder. These two features allow measurement of sample dimensions, monitoring of sample alignment and observing fracture (by rerunning the video tape in slow motion). The aforementioned methods of characterization were applied to samples from many different evaporations. Sufficient data were obtained from four evaporations, two ofelectron-beam and two from Joule-heated evaporations, to allow comparison of the deposition method. 3.

RESULTS

AND DISCUSSION

3.1. Elemental analysis The stoichiometry of the SiO, films was determined by RBS using 2 MeV a particles as the projectile. The value of x in SiO, was calculated from the areas of the respective peaks by the relation*

x

=

Ao

oSi(EO)

A,i

cr,(E,)

where Asi and A, are the respective peak areas and usi and CS~are the scattering cross-sections evaluated at the incident alpha particle energy E,. The average value of x was 0.93 f 5% for electron-beam-evaporated films and 0.97 f 5% for Jouleheated evaporated films. An example of the RBS data is shown in Fig. 1. The individual peaks for silicon, oxygen and the carbon of the carbon substrate are

152

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Fig. I. Typical RBS spectrum from SiO, films on a carbon substrate (. .)with a simulated spectrum of Au/C/C-Si-O.lC (---) using RUMP9 overlaid. The respective edges for silicon, oxygen and carbon are labelled. The large peak at low energy is due to the carbon substrate.

immediately visible. A small surface carbon peak inherent to RBS in oil-diffusionpumped scattering chambers is also found. The only other impurities found in the films were trace amounts of tungsten, iron, sodium and chlorine. 3.2. Structure All electron diffraction patterns recorded were those of a typical amorphous material. As seen in the example in Fig. 2, they consisted of at least one diffuse ring corresponding to a Bragg spacing of 0.37 nm. This agrees with the results of Hass6

Fig. 2. Diffraction pattern recorded amorphous film should be noted.

with the Philips

EM 4OOT microscope.

The typical

“halo” of the

MECHANICAL

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OF SILICON MONOXIDE FILMS

153

for similar evaporation techniques where the value of the Bragg spacing lies roughly halfway between the first strong reflection for silicon at 0.3 15 nm and that of silica at 0.410 nm. Images recorded with the Philips microscope in bright field conditions showed the films to be continuous and free of holes and cracks. They did contain some remnants of the parting agent or at least a replica of surface roughness of the substrate. Structure images from the thin SiO, films were recorded in the JEOL 200CX high resolution microscope under phase contrast conditions. Figure 3 shows a structure image representative of the SiO, film where it is clear that no crystallinity is visible. Hass has shown a thermal decomposition of the amorphous SiO, films upon heating above 700°C. He found crystalline silicon and amorphous SiO, above 900°C. The effects of a highly localized electron beam could cause heating and destruction of phases. It may have also contributed to the areas of mixed crystalline This does not conclusively eliminate silicon and SiO, seen by Kaito and Shimizu”. the possibility of a microcrystalline structure but the microcrystals would have to be at the unit-cell level. The evidence gathered here points to the result that no longrange order exists, and thus these SiO thin film materials should be termed amorphous.

Fig. 3. High resolution transmission electron microscopy structure the structure is of an amorphous character. The image was recorded

image of the SiO, film showing that at approximately 50 nm underfocus.

3.3. Mechanical testing Force-elongation curves for numerous evaporations of each type were recorded. Of those recorded, many curves were considered as poor quality because of artifacts of mounting or non-uniform loading. In these cases, the load-bearing cross-sectional area of the samples would continuously increase with increasing elongation, creating a somewhat parabolic force-elongation curve (Fig. 4). This type

154

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2

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6 8 IO ELONGATION

1.2 urn

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HOFFMAN,

Jr.,

T. E. MITCHELL,

R. W.

HOFFMAN

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Fig. 4. Examples of force-elongation curves discarded from the discussion (L12E sample 1): curve a, from a sample whose load-bearing cross-sectional area increased during testing; curve b, gross sample slippage from the jaws during elongation (vertical line), successive lesser amounts of slippage are also evident as the curve continues from that point.

of behavior continued until sample failure and accurate stress-strain calculations were impossible. Another type of curve eliminated from the discussion was obtained when the sample slipped from the jaws. An example of gross slippage is also shown in Fig. 4 as sharp drops in force and concave deviations from linearity until the sample was completely released from the jaws. This occurred rather infrequently; however, it still eliminated a number of samples from the mechanical testing data. The elastic modulus was calculated by measuring the slope of a segment of the linear portion of the tensile curves and multiplying by the appropriate values as in the following equation: + 0

where E is the calculated modulus, m the slope of the line, I, the original gauge length and A, the original cross-sectional area. The cross-sectional area was calculated from the width of the sample and its thickness. The sample thickness was measured with quartz crystal monitors, step profilometer and RBS. Average values of the elastic modulus are shown in Table I. The first item to be pointed out is the reproducibility within a given evaporation. The thermal stability does not become a factor in the force measurement as long as it is monitored and the TABLE AVERAC;F

I VALUES

Ol- THE ELASTIC

Eiecrron-km-hrcrted L4E LllE Joule-heated .snmples L13 L15

MODULUS

Modulus (GPa)

(G Pa)

64.8 52.7

0.28 0.14

ill * 10

53.9 75.0

0.21 0.20

*12 +11

Standard

deoiarion

samples

MECHANICAL

PROPERTIES

OF SILICON MONOXIDE FILMS

155

instrument is not operated during large drift rates. Within a given tensile specimen (Fig. 5), successive tensile tests were drawn on top of each other by the nanotensilometer, demonstrating the excellent reproducibility. In this example, Ll 1E sample 5, the fourth tensile test gave a modulus of 52.9 GPa and the fifth pull E on this sample was found to be 52.2 GPa. The vertical line on the curve from the point of maximum load for the fifth run was the fracture as interpreted on the nanotensilometer.

.2

4

.6 .6 1.0 ELONGATION

1.2

1.4

1.6

pm

Fig. 5. Force-elongation curves showing the excellent reproducibility sample 5). The curves were successive pulls on the same sample.

of the nanotensilometer

(LllE

The second item to notice is the variation in the calculated elastic modulus from evaporation to evaporation. Results from evaporations Ll 1E and L13 represent essentially the same modulus of about 53-54 GPa. This agreement from both the electron-beamand Joule-heated evaporation sources suggests no dependence of the modulus on the evaporation techniques. In addition, this modulus agrees with the value of 50GPa obtained by Pivot’ for stoichiometric SD. However, if the evaporation loads of L4E and L 15 are also examined, then the modulus could be as high as 65-75 GPa compared with 72.9 GPa for bulk fused silica” and a modulus of 66 GPa found for SiO, films”. The variations in the modulus with these loads are believed to be real since they exceed the range of the error. The variations are not explainable by any systematic differences in the resulting structures or stoichiometry since essentially no differences would be detected. One item which is suspected to cause differences in the calculated modulus is the film density. It has been shown to be as low as 1510 _+20kgm~3(1.51gcm~3)andashighas2150_+30kgm~3byShiojirieta1.’3under similar evaporation conditions. Hass also reported density variation dependent on the evaporation rate where at a low rate of 0.5 nm s 1 the density dropped to about 1980kgme3.At 1 rims-’ condensation rates, the density of 2100 kg mm3 was found to be near the bulk density. All the calculations for thickness from RBS and the crystal monitor were based on the bulk density value of 2130 kg mp3. Certainly, the density could be adjusted in each calculation to produce any elastic modulus desired; however, there is no justification from these data for selectively adjusting density, and the density was not independently measured.

156

R. W. HOFFMAN, Jr., T. E. MITCHELL, R. W. HOFFMAN

Another contributing factor to the variation in measured elastic modulus could be due to the plane stress condition of the samples. If the gauge-length-to-width ratio was not large enough to create a large portion of pure uniaxial tension in the sample, then the strain was summed over the stress distribution. Large differences in stress state would translate into variations in measured mechanical properties. The variations in the measured elastic modulus do not scale with the length-to-width ratio which ranges from 2.5 to 3.5. The American Society for Testing Materials quotes standards14 for metal foil tensile testing as 10 to 1 for the length-to-width ratio. They also have standards15 for testing paint and free-standing coatings which allow gauge-length-to-width ratios of 1 to 1 for tensile testing. They do insist that the values of length and width be “mutually agreed upon” when testing free-standing coatings. Even though the length-to-width ratio employed in this research may not have been sufficient to call the elastic modulus Young’s modulus, the effects of the stress distribution were not a factor in the measured modulus variations. Plastic deformation was not observed in the tensile test results. Small deviations from linearity were found, however, to be no more than 0.015” by the time that fracture occurred. This is well below the typical yield point determination for bulk materials of 0.2% offset. All samples failed by brittle fracture. The crack propagated across the sample width of about 150 urn within one frame on the video tape which had a resolution of 1/3Os frame- ‘. Most fractures were completely perpendicular to the sample length and were within the separation of the jaws, with many samples fracturing in the center of the gauge length. No evidence of shear bands was observed. None of the samples had cracks which arrested or appeared visible through the optical microscope before fracture. In addition, no hardening mechanisms were observed on repeated tensile testing of the same sample, as was reported by Pivot. Stress at fracture in almost all samples was found to be above 120 MPa. The maximum fracture stress found was 380 MPa which is approaching 1% of the elastic modulus, a typical limit in bulk mechanical properties. The fracture stress was approximately one order of magnitude greater than that reported by Pivot. One concern for direct comparison of results with Pivot’s is the strain rate of the tensile testing. If the SiO, thin film material follows the deformation characteristics of bulk glass under small strains, then a model such as a Maxwell element could describe the viscoelastic properties below the glass transition temperature T,. In this model, a faster strain rate would yield a higher instantaneous elastic modulus and a lower fracture stress and, under continuous straining, less plastic deformation. A slower rate would produce the inverse case. The problem in comparing the data of Pivot with the present data is that the strain-rate used by Pivot is unknown. If it is assumed that the strain rate for the present study is slower, a higher fracture stress would be expected. This was found and was higher by approximately an order of magnitude. In addition, a slower strain rate would cause a lower elastic modulus and higher plastic strain as the sample has time to relax. This was not observed in the comparison with Pivot’s work and, in fact, the inverse case of higher elastic modulus and lower plastic deformation was observed. A faster strain rate than that used by Pivot could not explain the differing results.

MECHANICAL

PROPERTIES

OF SILICON MONOXIDE FILMS

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4. SUMMARY AND CONCLUSIONS Stoichiometric amorphous thin films of SiO, were produced by Joule heating and electron bombardment techniques. This was verified by RBS and electron diffraction. High resolution electron microscopy also showed the films to be amorphous. The mechanical properties were measured on a nanotensilometer which is found to have excellent reproducibility in the range of forces and elongations employed. The measured elastic modulus ranged from a low of 52.7 GPa to a high of 750GPa. No systematic variations in the elastic modulus with the evaporation method, the film stoichiometry and structure, and the length-to-width ratio of the tensile samples could be detected. Nevertheless, the variations in the calculated elastic modulus were real and were presumably due to density variations in the films or error in the measurement of film thickness due to the density variations. The films fractured in a brittle manner. This occurred at stress levels of 120-380 MPa which approached 1% of the measured elastic modulus. No plasticity could be detected even at the highest fracture stresses. ACKNOWLEDGMENTS

The authors would like to thank John Sears and Sabine Lanteri for their help with the electron microscopy and Kevin Chaffee for his aid with the Rutherford backscattering spectroscopy. We gratefully acknowledge the financial support of the Standard Oil Company and the Thin Film and Surface Science Laboratory, Case Western Reserve University. REFERENCES

I 2 3 4 5 6 7 8 9 10 11 12 13 14 15

W. C. Oliver, Ma&r. Res. Bull., ll(l986) 15. J. Pivot, Thin SolidFilms, 89 (1982) 175. J. Priest, H. L. Caswell and Y. Budo, Vacuum, 12(1962) 301. M. A. Novice, Br. J. Appl. Phys., 13 (1962) 561. A. E. Hill and G. R. Hoffman, Br. J. Appl. Phys., 18 (1967) 13. G. Hass, J. Am. Ceram. Sot., 33 (1950) 353. C. W. Hagerling, Ph.D. Thesis, Case Western Reserve University, Cleveland, OH, 1980. W. K. Chu, J. W. Mayer and M. A. Nicolet, in Backscattering Spectroscopy, Academic Press, New York, 1978, p. 109. L. R. Doolittle, Nucl. Instrum. Methods B, 9 (1985) 344. C. Kaito and T. Shimizu, Jpn. J. Appl. Phys., 23 (1984) L7-L8. S. Spinner, J. Am. Ceram. Sot., 37( 1954) 229. R. J. Jaccodine and W. A. Schlegel, J. Appl. Phys., 37 (1966) 2429. M. Shiojiri, Y. Saito, H. Okada and H. Sasaki, Jpn. J. Appl. Phys., 18 (1979) 1931. Tensile testing of metallic foil, ASTM Stand. E345, 3.01, 1985. Testing tensile properties of organic coatings, ASTM Stand. D2370,6.01, 1985.