Surface damage and the optical reflectance of single-crystal silicon P. J. Zanzucchi and M. T. Duffy The reflectance of silicon measured at 4.3 eV can be used to determine the surface quality of silicon. Crys- tallographic damage, which occurs with abrasive polishing, and texture, which occurs with epitaxial film growth, can be detected. The effect of surface damage on the optical reflectance of silicon measured at 4.3 eV is reported. The reflectance measurement is nondestructive, simple, fast (on the order of seconds), and sensitive. The technique is readily adaptable to quality control inspection in silicon device manufactur- ing facilities. Introduction Semiconductor devices are fabricated on oriented crystalline surfaces, which have been polished to a high degree of smoothness and flatness. To obtain smooth surfaces, successively finer abrasive polishes are used in all but the final polishing step. Abrasion produces surface damage and this must be removed by a final polish, which removes surface material by chemical rather than abrasive means. 1 ' 2 If the final chemical polish does not remove all the damaged surface mate- rial, local variation in the crystallographic orientation of the surface material can exist. McFarlane and Wang 3 have reported that polishing-related defects on the surface of sapphire or spinel substrates cause mi- soriented epitaxial growth of semiconductor films. Mendelson 4 reported that similar defects occur for ep- itaxial silicon grown on silicon substrates with residual polishing damage, such as scratches. The presence of lattice damage at the surface of a semiconductor has a significant effect on the optical properties of the material. For example, it is evident from comparison of the optical constants for crystalline, polycrystalline, and amorphous silicon 5 that loss of lattice order reduces the magnitude of the optical con- stants, particularly at photon energies well above band gap. With damaged silicon surfaces, this effect is evi- dent in the various optical properties, such as reflec- tance when measured in the uv region of the spectrum. The magnitude of the reflectance decreases with in- creasing surface damage. Thus, the quality of silicon The authors are with RCA Laboratories, Princeton, New Jersey 08540. Received 8 April 1978. surfaces with respect to surface damage can be deter- mined by using nondestructive optical reflectance techniques. The factors influencing the optical reflectance from a surface are complex. Crystallographic disorder is one, but other factors such as physical light scattering must also be considered. Donovan et al. 6 have shown that the optical reflectance of variously polished germanium surfaces is related to the residual surface damage after polishing. Crystallographic disorder is apparently the principal factor affecting the optical reflectance of polished germanium. Other factors, such as light scattering, have been studied by Bennett and Porteus 7 who showed that the residual surface roughness of polished glass surfaces can significantly reduce the specular reflectance of these surfaces when they are metallized to form mirrors. These studies show that the presence of surface defects of various types can be determined from optical reflectance measurements. Systematic studies of the effects of surface damage on the optical reflectance of silicon surfaces have not been reported. A correlation between the reflectance of silicon measured at 4.3 eV and surface quality is re- ported here. These data suggest the reflectance mea- surement is readily adaptable to the quality control inspection of silicon. The effects of crystalline disorder, surface roughness, and surface texture due to epitaxial deposition processes are discussed. Experimental Materials Both 1.5-in. (3.75-cm) and 2-in. (5-cm) dia (111) oriented single-crystal silicon wafers, damage-free on both sides, were obtained from commercial vendors. These substrates were repolished at RCA Laboratories to introduce various levels of damage, using commercial 1 November 1978 / Vol. 17, No. 21 / APPLIED OPTICS 3477
Transcript
Surface damage and the optical reflectance ofsingle-crystal silicon
P. J. Zanzucchi and M. T. Duffy
The reflectance of silicon measured at 4.3 eV can be used to determine the surface quality of silicon. Crys-tallographic damage, which occurs with abrasive polishing, and texture, which occurs with epitaxial filmgrowth, can be detected. The effect of surface damage on the optical reflectance of silicon measured at4.3 eV is reported. The reflectance measurement is nondestructive, simple, fast (on the order of seconds),and sensitive. The technique is readily adaptable to quality control inspection in silicon device manufactur-ing facilities.
Introduction
Semiconductor devices are fabricated on orientedcrystalline surfaces, which have been polished to a highdegree of smoothness and flatness. To obtain smoothsurfaces, successively finer abrasive polishes are usedin all but the final polishing step. Abrasion producessurface damage and this must be removed by a finalpolish, which removes surface material by chemicalrather than abrasive means.1'2 If the final chemicalpolish does not remove all the damaged surface mate-rial, local variation in the crystallographic orientationof the surface material can exist. McFarlane andWang3 have reported that polishing-related defects onthe surface of sapphire or spinel substrates cause mi-soriented epitaxial growth of semiconductor films.Mendelson4 reported that similar defects occur for ep-itaxial silicon grown on silicon substrates with residualpolishing damage, such as scratches.
The presence of lattice damage at the surface of asemiconductor has a significant effect on the opticalproperties of the material. For example, it is evidentfrom comparison of the optical constants for crystalline,polycrystalline, and amorphous silicon5 that loss oflattice order reduces the magnitude of the optical con-stants, particularly at photon energies well above bandgap. With damaged silicon surfaces, this effect is evi-dent in the various optical properties, such as reflec-tance when measured in the uv region of the spectrum.The magnitude of the reflectance decreases with in-creasing surface damage. Thus, the quality of silicon
The authors are with RCA Laboratories, Princeton, New Jersey08540.
Received 8 April 1978.
surfaces with respect to surface damage can be deter-mined by using nondestructive optical reflectancetechniques.
The factors influencing the optical reflectance froma surface are complex. Crystallographic disorder is one,but other factors such as physical light scattering mustalso be considered. Donovan et al.
6 have shown thatthe optical reflectance of variously polished germaniumsurfaces is related to the residual surface damage afterpolishing. Crystallographic disorder is apparently theprincipal factor affecting the optical reflectance ofpolished germanium. Other factors, such as lightscattering, have been studied by Bennett and Porteus7
who showed that the residual surface roughness ofpolished glass surfaces can significantly reduce thespecular reflectance of these surfaces when they aremetallized to form mirrors. These studies show thatthe presence of surface defects of various types can bedetermined from optical reflectance measurements.
Systematic studies of the effects of surface damageon the optical reflectance of silicon surfaces have notbeen reported. A correlation between the reflectanceof silicon measured at 4.3 eV and surface quality is re-ported here. These data suggest the reflectance mea-surement is readily adaptable to the quality controlinspection of silicon. The effects of crystalline disorder,surface roughness, and surface texture due to epitaxialdeposition processes are discussed.
Experimental
Materials
Both 1.5-in. (3.75-cm) and 2-in. (5-cm) dia (111)oriented single-crystal silicon wafers, damage-free onboth sides, were obtained from commercial vendors.These substrates were repolished at RCA Laboratoriesto introduce various levels of damage, using commercial
Fig. 1. Reflectance of undamaged and damaged silicon surfaces inthe 2-5.5-eV range.
alumina and diamond polishes ranging from 0.1 Am to6 m in average particle diameter. To insure thequality of damage-free surfaces on the control samples,some substrates were repolished with colloidal silica.For studies of as-grown epitaxial films, 0.6-Am thicksilicon films were grown on damage-free (1102) orientedsapphire. The epitaxial growth technique using silanehas been reviewed by Cullen8 and more recently byManasevit. 9
Method
The optical band gap of single-crystal silicon is 1.1 eV.The optical absorption of silicon increases above theband gap. The reflectance shows only a small increaseuntil about 2 eV, then increases rapidly and shows amaximum at 4.3 eV, which corresponds to the energyfor the X 4-X1 transition in silicon (see Fig. 1). Themagnitude of the reflectance at 4.3 eV is determined bythe crystalline perfection and the roughness of thesurface material.
The specular reflectance at 4.3 eV of variously pol-ished (111) silicon surfaces and of variously groNwn epi-taxial silicon films on sapphire has been measured.Both abrasion damage and defects on epitaxial films,such as texture, are associated with the presence oflattice disorder. A common relative parameter, the sizeof a polishing abrasive, is used to characterize surfacedamage.
To provide a quantitative measure of abrasion dam-age, the root-mean-square (rms) value a- of the surfaceroughness has been determined for variously polishedsilicon surfaces prepared at RCA Laboratories by thetechnique reported by Cunningham and Braundmeierland also by Endriz and Spicer."i For this purpose, athin silver film, 75-150 nm in thickness, is evaporatedonto the variously polished silicon surfaces. The re-flectance of the silver near the surface plasmon band of
silver (0.35 Ain) is measured and is correlated with therms surface roughness of the underlying silicon as shownin Fig. 2. The calibration data for this technique aregiven by Cunningham and Braundmeier and by Endrizand Spicer. In this work, the data of Cunningham andBraundmeier are used. The difference between thesedata and that of Endriz and Spicer has been discussedby Cunningham and Braundmeier. They suggest it isrelated to the characteristics of the polishing processesused. In our work, the roughness parameter a- corre-lates with surface damage observed in SEM micro-graphs of the abraded silicon surfaces. However, in-dustrial polishing processes often are not reproduciblewith respect to the degree of surface damage unlesscarefully controlled, and, for the same polishing con-ditions, we find significant scatter in the experimentallydetermined a values.
A Cary 14 spectrometer was used for all silicon re-flectance measurements, using as a reference an alu-minum-surface mirror obtained from the NationalBureau of Standards.' 2 The near-normal incident,specular reflectance jigs for the Cary 14 spectrometerwas designed at RCA Laboratories.13
Results and Discussion
Reflectance and Surface Roughness
Bennett and Porteus7 have shown that the reflectanceof a metal surface can be related to surface roughnessin terms of a, the rms surface roughness, and m, the rmsslope of the roughness profile. The relevant equationderived by Bennett and Porteus is valid only for metalsurfaces and cannot be applied to a semiconductorsurface. The dependence of the reflectance on the
CO
50
40
30
20
I10
0 10 20 a (1)g r s *i AA Sal~~~~0 4
Fig. 2. Estimated rms surface roughness for variously polished sil-
AND EXPERIMENTAL SILICON ON SAPPHIRELINDE A POLISH
.10 SYTON POLISH
0.1 1.0 10
( DIAMETER POLISHING GRIT )2 M2
Fig. 3. Plot of measured reflectance as a function of equivalentabrasive size for abrasively polished silicon surfaces.
crystalline quality of the material is a further compli-cation in the development of an adequate mathematicalmodel.
The measured rms surface roughness a can be relatedto the reflectance of abraded silicon surfaces using thedata of Fig. 2. Determining ar is a time-consumingmeasurement not suitable for production quality controlapplications. Without explicitly determining a, we finda useful correlation can be made between surfaceroughness and the average diameter of the polishingabrasive.
Abrasively Polished Surfaces
The reflectance of bulk silicon with a damage-freesurface is about 72% at 4.3 eV (Fig. 1). Introducing lightdamage on silicon surfaces by polishing with, e.g., LindeA, an alumina polish with a 0.3-gm diameter averageparticle size reduces the specular reflectance to about70%. Surface damage from the larger diameter abra-sives is visible and, for l-gm diamond, reduces the re-flectance of silicon to about 1/2 its maximum value. Inaddition, the reflectance spectrum is featurelesssuggesting a loss of crystallinity at the polished surface.5
The measured reflectance of abraded silicon surfacesis found to correlate empirically with the average di-ameter of the polishing grit, as shown in Fig. 3. Thelogarithmic scale and square of the term are chosen toprovide the best spread of the data. Reflectance isgiven in units of loglo (RO/R), where Ro is the reflectanceof an aluminum surface mirror, and R is the measuredreflectance of silicon. The data of Fig. 3 give an em-pirical quantitative parameter, the polishing grit di-ameter, which can be used to express the degree ofsurface damage. For example, if the measured reflec-tance is 0.40 the surface damage is equivalent to thatobtained by polishing with 1 -gm diamond grit. Simirlarly, the measured reflectance of unabraded texturedsilicon surfaces can be quantitatively correlated to apolishing grit diameter, which would cause the equiv-alent surface damage. The open circles (Fig. 3) repre-sent unabraded silicon-on-sapphire surfaces. As will
be discussed, these samples have a texture, which, interms of reflectance, is equivalent to abraded siliconpolished with 0.3-gm Linde A to 1-gm diamond pol-ishing grit.
Textured Unpolished Surfaces
Epitaxially grown silicon-on-silicon and silicon-on-sapphire are grown free of direct abrasive damage andare not polished after deposition. However, siliconepitaxial films often exhibit a surface texture, whichis evident from the micrographs obtained from SEMmeasurements [Figs. 4(a), 4(b), and 4(c)]. This textureis associated with degraded crystalline quality of thesemiconductor. Diminished uv reflectance from thesesurfaces is due to complex factors, including the effectof degraded optical constants and, in the case of severelytextured surfaces, light scattering. The effect of textureon reflectance is similar to the effect of abrasion.
With well established deposition conditions, surfacetexture is ordinarily undetectable at 20K magnification[Fig. 4(a)]. The reflectance of these surfaces is similarto the reflectance of damage-free bulk silicon surfaces.
0. 5.mM
II
(a)
(b)
(C)
Fig. 4. Scanning electron micrographs of (a) undamaged, (b) slightlytextured, and (c) textured epitaxial silicon surfaces.
The heteroepitaxial films of Figs. 4(b) and 4(c) showsignificant texture due to various unusual depositionconditions, leaks, poor temperature control, impropersubstrate surface cleaning, etc. The sample shown inFig. 4(c) was rejected by production quality control forits poor visual appearance. The texture shown in themicrographs of Fig. 4 is typical of the range observed insilicon-on-sapphire manufacture.
Over a period of more than a year, the uv reflectanceof an estimated 500 epitaxial or bulksilicon surfaces hasbeen measured. For the heteroepitaxial films, mea-sured reflectance is shown in Fig. 3 by the open circles.The roughness of the surface due to texture can be re-lated to the size of the equivalent polished grit as dis-cussed previously. It is common to have epitaxial sili-con samples with texture when prepared by experi-mental development processes. Commercially avail-able silicon-on-sapphire films have also shown a widevariation in reflectance. Characteristic of a more de-veloped processing technology, few as-received bulksilicon substrates exhibited abrasion damage within thedetection limits of the reflectance method.
SensitivityTo improve detection of small changes in reflectance,
scale expansion, usually by a factor of 2 has been used.Multiple reflectance as used in sapphire surface eval-uation15 is not effective due to the relatively low re-flectance of a damage-free silicon surface, i.e., 72%.This limits the number of reflections in multiple re-flectance to less than six, i.e., (0.72)6 = 0.041. Withscale expansion, surface damage, or texture smaller thanthat typical of 0.3-,um Linde A polishing grit can beeasily detected. Irregular surface defects on siliconsurfaces such as hillocks strongly scatter light, and theoptical reflectance is greatly diminished.
Reflectance and Electrical Properties
The electrical properties of silicon damaged by pol-ishing degrade significantly, particularly near thedamaged surface.' Hall mobility data on a limitednumber of samples studied in this work show that latticedamage on sapphire substrates will degrade the carriertransport properties of silicon subsequently grown onsuch substrates as shown in Table I. These data alsoshow that the optical reflectance of the heteroepitaxialsilicon, in addition to other factors, is related to thesubstrate surface quality.
The optical reflectance measured at 4.3 eV gives in-formation very different from reflectance measured inthe visible region, such as with commercial laser scan-ners. With light surface damage the uv reflectance ofsilicon is determined primarily by the magnitude of theoptical constants, which are affected by the degree oflattice order. Since many electrical properties of
Table 1. Hall Mobility of Silicon Films Grown on Abrasively PolishedSapphire, Compared to Optical Reflectance Data
Hall mobilitya Relative(cm2 /V-sec) optical reflectance
Abrasive Controlb Abra- Control,polishing Abrasive chemical sive chemical
a Measured at room temperature.b The control is grown on the substrate, which is abrasively pol-
ished. A region of the substrate is carefully polished to obtain adamage-free surface. This procedure avoids effects due to variationin the silicon deposition process.
semiconductors also relate to the degree of lattice order,it is expected that the measured optical reflectance at4.3 eV will correlate with many electrical properties ofsilicon. In the visible region, however, the opticalconstants are not significantly affected by lattice dis-order, and the reflectance techniques measure primarilyphysical surface defects. Similar considerations applyto optical reflectance measurements on semiconductorsother than silicon.
Conclusions
The quality of silicon surfaces can be determinedfrom the magnitude of the optical reflectance measuredat 4.3 eV. Reduced reflectance is associated with sur-face damage of various types, including polishingdamage and deposition related surface morphology suchas texture. The presence of surface damage on Si isassociated with changes in the magnitude of the opticalconstants in the uv region of the spectrum. With severesurface damage, other effects such as light scatteringoccur and also contribute to reducing the specular re-flectance of the surface. Thus the optical reflectanceof silicon can be used for quality control inspection ofsilicon substrates.
We greatly appreciate the technical assistance of R.A. Soltis and D. A. Kramer for preparing samples andobtaining many of the measured values reported here.Technical assistance by B. J. Seabury in the SEMmeasurements and technical discussion with K. Gallo-way and W. M. Bullis and NBS have been helpful andare appreciated. Comments on this manuscript by R.E. Honig have also been helpful and are appreciated.
This research is funded by the Advanced ResearchProject Agency Order 2397 through the National Bu-reau of Standard's Semiconductor Technology ProgramContract No. 5-35915 and is not subject to copyright.Funding is also provided by RCA Laboratories,Princeton, N.J. 08540.
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(1961).8. G. W. Cullen, J. Cryst. Growth 9, 107 (1971).9. H. M. Manasevit, J. Cryst. Growth 22, 125 (1974).
10. L. J. Cunningham and A. J. Braundmeier, Jr., Phys. Rev. B 14,479 (1976).
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