Application of the Raman Microprobe to Analytical Problems of

13 Application of the Raman Microprobe to Analytical Problems of Microelectronics Downloaded by CORNELL UNIV on September 7, 2016 | http://pubs.acs.org Publication Date: January 28, 1986 | doi: 10.1021/bk-1986-0295.ch013

Fran Adar Instruments SA, Inc., Metuchen, NJ 08840

The Raman microprobe MOLE consisting of a conven tional research microscope that is optically and mechanically coupled to a Raman spectrometer, is described. Microanalysis for quality control and materials development is performed non-destructively and requires minimum sample modification. Spatial resolution, determined by optical wavelengths and available microscope optics is approximately lμm laterally and 10μm axially in transparent materials; in opaque materials the axial resolution, determined by the optical skin depth, will be ca. 300-3000A. The minimum Raman detectable volume is dependent on the intrinsic Raman intensity. Examples of successful identification of organic contaminants on Si wafers include silicone oil, PET, EDTA, FIFE and cellulose. Elemental silicon has been identified in the vicinity of an Al wire bonded to Au. Stress in laser-annealed laterally seeded Si on silicon oxide has been measured quantitatively and stress-relief at the surface has been detected. The crystalline phase of a ZrO2 particle precipitating a break in an optical fiber has been identified; since two possible sources of ZrO2 contaminants exist in different crystalline polymorphs, only phase identification enabled elimination of this problem. Other potential uses of the Raman microprobe in a microelectronics analytical laboratory will be discussed. Raman scattering i s a vibrational spectroscopic technique that can fingerprint both organic and inorganic species and identify poly morphs of crystalline materials. By coupling an optical microscope to a conventional spectrometer, the technique becomes a microprobe with spatial resolution of 1pm, determined by the wavelength of the radiation (ca. .5pm) and the numerical aperature of the microscope objective (n.a anaiygig - organic Cfrntariflatipn Over the past years we have been asked to identify organic contaminants that appear on silicon wafers during processing operations. As the scale of integration of the circuits increases, and the size of the smallest features decreases, the size of contaminants that can effect a device's operation becomes more c r i t i c a l and the ability to identify foreign materials becomes increasingly important. The amount of material present i s often too small for analysis by infrared absorption or X-ray diffraction. The Auger and electron microprobes are incapable of yielding chemical (i.e., molecular) identification. The Raman microprobe i s unique in i t s ability to identify organic contaminants that appear as particles as small as lpm, or as films as thin as lpm.

Casper; Microelectronics Processing: Inorganic Materials Characterization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


Figure 1 - Instrument schematic showing the light path through microscope and monochromator. Digital data collection was achieved by computer-controlled scanning of the monochromator usually with 1 wavenumber increments and then counting individual photon pulses from an amplifier discriminator. Spectra are manipulated by the computer and plotted on a digital plotter.

Downloaded by CORNELL UNIV on September 7, 2016 | http://pubs.acs.org Publication Date: January 28, 1986 | doi: 10.1021/bk-1986-0295.ch013

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ED1A. A very large particle (XLOOvim) on a silicon wafer was identified as EDTA (ethylene diamine tetraacetic acid). Because of the size of the particle, i t was apparent that the EDIA had not precipitated on this wafer after inadequate rinsing (precipitated crystals would exhibit morphology of growth on a substrate) • It was inferred that this particle had inadvertently settled on the wafer when clean room gloves had not been appropriately changed. SILICONE. A silicone film about 5ym thick was identified on a silicon substrate. It would have been d i f f i c u l t by X-ray microanalysis to detect the additional silicon due to the film above the silicon X-ray signal from the substrate. Moreover, the X-ray spectrum would not have revealed the presence of surface silicon as organically-bound silicon, i.e., as a form of poly (dimethyl siloxane) • However the Raman spectrum, by fingerprinting the polymer, provides clues to the source of the contaminant. In this case, a degraded silicone-rubber gasket was suspected of introducing material that had settled on this sample. PET CHIP. A microscopic particle on a silicon wafer was fingerprinted as a piece of polyethylene terephthalate. The source of this contaminant was identified as a wafer handling basket. This result was especially surprising because these baskets had been "guaranteed" not to chip and produce debris. Fluorinated Hydrocarbon. The bottom of Figure 2 shows a spectrum of material that had settled on another wafer. The features at 521 and 950cm" are one- and two phonon bands of the silicon substrate. The other features come from the contaminant. The top of the figure shows a spectrum of teflon (polytetrafluorethylene) • While the two spectra i n the figure are not a perfect match, i t i s significant to note that there are no spectral features between 1400 and 1500cm" , which i s where most hydrocarbons exhibit a strong band due to the C H 2 deformation. The lack of this feature implies the absence of any alkane regions longer than two carbon atoms. Interpretation of this spectrum requires information on the history of the wafer. This specimen had been polished with a slurry containing organic solvents and then etched. The contaminant could be attributed to two sources. Low molecular weight polymer could have been carried from a teflon container. Or polymer could have been deposited on the wafer during etching in a C F 4 / H 2 plasma i f the plasma contained excess H 2 . 1

1

CWfffflFFf The upper spectrum of Figure 3 was recorded from a hazy film (150°C) • (2) Thus, this Raman spectrum enables one not only to identify the chemical composition of the contaminant, but also allows determination of the processing step during which the contaminant precipitated. It i s useful to note here that identification of crystalline polymorphs of many other polymers provides information on their thermal anchor stress history.


MgiQariaj-ysj-s - Preek i n Wire Connect An integrated circuit package that had failed during use was submitted for analysis. The ceramic package had been opened and i t was found that some of the aluminum wire connects had broken not far from the gold bonding pads. It was requested that material deposited on the gold around the broken aluminum wire be identified - the material was suspected to be organic. Numerous scans taken of the deposited residue did not indicate the presence of an organic material. However, a band at approximately 519cm" was observed on every pass - and i t was not observed when the instrument was focussed on the gold pad or aluminum wire. We were forced to the conclusion that the "contaminating" material was elemental, semi-crystalline silicon. (The silicon phonon frequency down shifts when the crystallite size i s reduced (1Q) or when the lattice i s under tensile stress 1

(11)). From infrared analysis, i t was known that significant amounts of s i l i c a were present. It was subsequently argued that s i l i c a gel had contaminated the package and acted as a carrier for corroding chemicals. The Raman microprobe did not find evidence for s i l i c a in significant amounts. We argued that the failure could have been caused by the aluminum wires themselves. These often have a small percentage of micro-crystalline silicon added as a hardening agent. If the silicon i s inadequately dispersed there might be local heating during use which would promote electron-migration of the silicon and could end ultimately i n a break i n the wire. In order to reconcile the Raman microprobe results with those of infrared, i t i s important to recognize the limitations of the techniques. The Raman-active phonon mode in silicon i s infrared inactive. However, a l l silicon i s coated to some extent with a thin film of a thermal oxide to which infrared absorption i s very sensitive. While the oxide i s also active in the Raman effect, i t s signal i s much weaker than that of the silicon i t s e l f .

Stress i n tarer Annealed S i l i c o n on Silicon Qxite In collaboration with Hewlett Packard Laboratories lateralepitaxially regrown films of laser-annealed silicon on silicon oxide islands were examined. (1Q) This study i s motivated by the



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attempt to produce high quality single crystal silicon films isolated by oxide from the silicon crystal wafer. Such materials could then be used to manufacture devices with increased component density per package. It i s known, however, that mismatch at the silicon/oxide interface w i l l usually result in stress in the annealed material that causes dislocations and grain boundaries which w i l l effect the silicon's conductivity. The BMP study (1Q) discribed here was an effort to combine the ability of the Raman signal to monitor stress with the high spatial resolution of the technique. (11) The measured stress could then be correlated with the geometry of the sample. The samples were prepared by depositing .55pm of polysilicon over a (001) silicon wafer patterned with 480ym square islands of Si02 that were 1.4pm thick. The edges of the island were oriented along (110) directions. The polysilicon was encapsulated with 60A of silicon nitride. The samples were held at 500°C during recrystallization which was accomplished by scanning an 11W, 80pm diameter Gaussian argon ion laser beam across the sample at a rate of 25cm/sec, raster stepping 10 m between scans. The Raman active phonon of high quality single crystal silicon occurs at 520.7cm"". Under tensile stress the band shifts to lower energies; under compressive stress the band shifts to higher energies. (12) Because of the values of the thermal expansion coefficients of silicon and i t s oxide, the silicon film re-crystallized over the oxide w i l l experience tensile stress. In order to maximize the accuracy with which the stress could be measured, the Raman spectra were recorded digitally with 0.1cm"" between data points. (Instrumental repeatability was also 0.1cm-"). Figure 4 shows a plot of the Raman phonon frequency as a function of distance from the edge of the oxide island. The figure also shows the dependence of the stress (as extrapolated from the Raman frequency) on the position of the probe. Two sets of data are illustrated i n the figure; Raman spectra were excited at wavelengths 514.5 and 457.9nm. Because the optical penetration depth of silicon i s different at these two wavelengths, the information generated reflects different thicknesses of samples. Both sets of data indicate an increase in stress as the distance from the oxide edge increases. A striking difference i s the indication of a drop i n stress at approximately 25ym from the edge that i s detected in the 457.9nm-excited data. This i s to be attributed to the shallower penetration depth of this radiation; the conclusion to be drawn i s that after the sample breaks into polycrystalline grains approximately 11pm from the edge (as measured by Nomarski contrast micrographs), there i s relief of stress at the surface of the recrystallized material. 1

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Other Areas of Applicability of the BMP Optical Fibers. Several manufacturers of optical fibers have suggested that a Raman microprobe could be a useful tool in characterizing fibers. The literature shows that i t i s possible to monitor the concentration of additives to s i l i c a down to the 1 mole percent level. (13-17) Polished sections of preforms or drawn fibers have been monitored in the microprobe in this laboratory.


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M I C R O E L E C T R O N I C S PROCESSING: INORGANIC M A T E R I A L S C H A R A C T E R I Z A T I O N

STRESS (x10 d y n c m - ) 9

2

514.5 nm EXCITATION: 5/13/82 • , 12/28/82 • 1/5/83 o

521.0 -

SINGLE CRYSTAL SILICON

O

•

O


•

•

-20

-10

SEED

SOI

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DISTANCE FROM SEED / SOI BOUNDARY (fim) Figure 4 - Raman phonon frequency o f l a s e r - r e c r y s t a l l i z e d l a t e r a l e p i t a x i a l l y annealed s i l i o n on i n s u l a t i n g oxide (SOI) and the derived s t r e s s , as a function o f distance from the seecl/SOI i n t e r f a c e : Spectra were acquired with l a s e r wavelengths 514.5 and 457.9nm. In c o l l a b o r a t i o n w i t h L . Soto at B e l l Laboratories, we have recently succeeded i n i d e n t i f y i n g the cause o f a break i n a cxxonunications f i b e r . (18) By X-ray fluorescence zirconium was known t o be present i n the 2um p a r t i c l e p r e c i p i t a t i n g the break the Raman spectrum showed c l e a r evidence f o r the monoclinic polymorph. Because the furnace used t o p u l l the f i b e r s contained monclinic and tetragonal z i r c o n i a s i n d i f f e r e n t l o c a t i o n s , i t was only the polymorph i d e n t i f i c a t i o n by the Raman microprobe which provided the information necessary t o r e f i n e the manufacturing process. I I I - V Semiconductors. The use o f Raman spectroscopy t o characterize c r y s t a l l i n e f i l m s o f compound semiconductors has been reviewed. (12) Effects that can be monitored are o r i e n t a t i o n , c a r r i e r concentration, charge c a r r i e r s s c a t t e r i n g times, mixed-crystal composition (12) and Group V deposits i n the native oxides.(20) 1

Literature Cited 1. Adar, F. In "Microbeam Analysis - 1981", Geiss, R.H., Ed.; San Francisco Press 1981; pp. 67-72. 2. Delhaye, M. and Dhamelincourt, P. J. Raman Spectrosc. 1975, 3, 33.


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9a. 9b. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

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Dhamlincourt, P. In "Microbeam Analysis - 1982:; Heinrich, K.F.J., Ed.; San Francisco Press, 1982; pp. 261-269. Hirschfeld, T. J . Opt. Soc. Am. 1973 , 63 , 476-7. Rosasco, G.J., Etz, E.S., Cassatt, W.A. Appl. Spectrosc. 1975, 29, 396. Rosasco, G.J. In "Advances in Infrared and Raman Spectroscopy Volume 7"; Clark, R.J.H. and Hester, R . J . , Eds.; Heyden: London 1980; Chap. 4. Dhamelincourt, P. PhD. Thesis, L'Universite des Sciences et Techniques de Lille, L i l l e 1978. Adar, F. and Clarke, D.R. In "Microbeam Analysis - 1982"; Heinrich, K.F.J., Ed.; San Francisco Press 1982; pp. 307-310. Atalla, R.H., Dimick, B.E., Nagel, S.C. In "Cellulose Chemistry and Technology"; Arthur, J.C. J r . , Ed.; ACS Symposium Series No. 48, American Chemical Society. Atalla, R.H., Appl. Polym. Symp., 1976, No. 28, 659-669 Igbal, Z. and Veprek, S., J . Phys. C. Solid State Phys. 14, 0000(1981) Zorabedian, P., Adar, F. Appl. Phys. Lett. 1983, 43, 177-179. Anastassakis, E, Pinczuk, A, Burstein, E . , Pollack, F.H., Cardona, M. Solid State Commun. 1970, 8, 133. Walfrafen, G.E. and Stone, J., Appl. Spectroscopy 1975, 29, 337-344 Lan, G.-L., Banerjee, R.K., and Mitra, S.S., J. Raman Spectrocs. 1981, 11, 416-423 Sproson, W.A., Lyons, K.B, Fleming, J.W. J . Non-Crystal. Solids 1981, 45, 69-81 Shibata, U . , Horiguchi, M., Edahiro, T. J . Non-Crystal. Solids 1981, 45, 115-126. Noguchi, K . , Murakami, Y., Uesugi, N. and Ishihara, K . , Appl. Phys. Lett. 1984, 44, 491-493 Soto, L, and Adar, F. In "Microbeam analysis 1984" Romig, A.D. Jr. and Goldstein, J. I., Eds.; San Francisco Press, 1984, pp. 121-124. Abstreiter, G, Bauser, E. Fischer, A . , Ploog, K. Appl. Phys. (Springer Verlag) 1978, 16, 345-352. Schwartz, G.P., Gualtieri, G.J., Griffiths, J . E . , Thurmond, C.D., Schwartz, B. J. Electrochem. Soc. 1980, 127, 2488.

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Application of the Raman Microprobe to Analytical Problems of

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