Properties of chemical-vapor-deposited silicon carbide for optics applications in severe environments

Properties of chemical-vapor-deposited silicon carbidefor optics applications in severe environments

Jitendra S. Goela, Michael A. Pickering, Raymond L. Taylor, Brian W. Murray, and Arthur Lompado

Important data on chemical-vapor-deposited (CVD) SiC concerning the elastic modulus, polishability,scattering measurement, thermal and cryogenic stability, and degradation owing to the effects of atomicoxygen and electron beams have been obtained with the aim of assessing the suitability of SiC as an opticalsubstrate for severe environments. These measurements show that CVD SiC substrates exhibit excellentpolishability (<0.1 nm rms) with low scatter, good retention of mechanical properties up to 1500'C, superiorthermal and cryogenic stability (-190° to 13501C) and high resistance to atomic-oxygen and electron-beamdegradation. These results suggest that CVD SiC optical substrates will perform extremely well in severeenvironments such as outer space, and when used in lasers, combustion, and synchrotron x rays.

Key words: SiC optics, CVD SiC, optical properties, space optics, laser optics, x-ray optics.

1. Introduction

Silicon carbide is a good candidate material for use assubstrates for laser mirrors, solar collectors and con-centrators, astronomical telescopes and optics in thevacuum-ultraviolet and x-ray regions.1-17 Effortshave been made to use SiC in the fabrication of opticalmounts, benches, and baffles with an aim to develop-ing optical systems with minimal thermal distortion.For synchrotron mirror applications the SiC sub-strates must withstand high x-ray fluxes without caus-ing surface degradation or excessive thermal distor-tion. For space applications the optics mustdemonstrate adequate cryogenic stability and high re-sistance to degradation from atomic oxygen, trappedcharges such as electrons and protons, and high-energyphotons. For applications in other hostile environ-ments, such as in a combustion or laser system, theoptics must be able to withstand high temperatureswithout appreciable surface degradation or figure dis-tortion.

Silicon carbide is a good material for optical sub-strates because of its many attractive properties such

J. S. Goela, M. A. Pickering, and R. L. Taylor are with MortonInternational, Inc./CVD Incorporated, 185 New Boston Street, Wo-burn, Massachusetts 01801. B. W. Murray and A. Lompado arewith Spire Corporation, Patriots Park, Bedford, Massachusetts01730.

Received 30 August 19900003-6935/91/223166-10$05.00/0.© 1991 Optical Society of America.

as low density, low thermal expansion coefficient, highthermal conductivity, and high thermal shock resis-tance.1-17 In addition, SiC is also an extremely hardand stiff material, making it particularly suitable forfabricating a precision surface figure and finish in thematerial. SiC, made by sintering or hot pressing SiCpowders or by the reaction-bonding process, is usuallya porous and multiphased material that, when pol-ished, does not produce a good optical surface. Fabri-cation of an optical figure with these forms of SiCrequires an overcoat of chemical-vapor-deposited(CVD) SiC, Si, or any other suitable material. Recent-ly, SiC produced by the CVD process has exhibitedsuperior mechanical, thermal, physical and opticalproperties and excellent polishability (<0.1 nm rms).CVD SiC is theoretically a dense, polycrystalline mate-rial that is free from voids and microcracks, resulting ina substrate with superior properties for mirror applica-tions. Further, the CVD process is scalable. Mono-lithic 0.5-m-diameter lightweight mirrors have beenproduced, and currently scaling of the process to pro-duce lightweight 1.5-m-diameter mirrors is under way.

Some properties of CVD SiC of direct relevance foroptics applications have been reported in the litera-ture.1-17 However, the properties of CVD SiC dependconsiderably on the specific CVD process parametersused, e.g., deposition temperature, furnace pressure,specific chemical reagents and their composi-tion.4-6,14,15 Further, in most studies CVD SiC has beenused as a coating on base mirror materials such as hot-pressed, sintered, reaction bonded, or recrystallizedSiC and graphite. Rehn et al.

2 and Rehn and Choyke3

reported that CVD SiC can be superpolished to a sur-

3166 APPLIED OPTICS / Vol. 30, No. 22 / 1 August 1991

Table 1. Comparison of CVD SIC with Other Potential Mirror Materlals

ULEMaterial Property CVD SiC Mo Al Be 7971 Zerodur

Density, p (kg m-3 X 103) 3.21 10.2 2.7 1.85 2.20 2.55Coefficient of thermal expansion 2.4 5.4 25.0 11.4 0.03 0.15

(K-1 X 10-6)Specific heat, C (J kg-' K-1) 700 250 899 1880 708 820Thermal conductivity, K (W m'1 K-') 200 134 237 216 1.3 6.0Elastic modulus, E (GPa) 466 250 76 303 67 90Thermal distortion parameter 8.3 2.5 0.95 1.9 4.3 4.0

Ka-1 (W M-1 x 107)Inertia loading parameter 145 24.5 28.1 164 30.4 35.3

(N m kg-' X 106)Thermal stress parameter 1.8 1.0 1.25 0.63 6.4 4.4

Ka-' E-' (W m N-' X 10-4)

face roughness of 0.3 nm rms. Kelly and West4 report-ed fabrication of composite synchrotron mirror blanksconsisting of a layer of CVD p3-SiC upon a polishedblank of hot-pressed SiC. These authors did not pro-vide any quantitative data on the properties of CVDSiC, but they commented that CVD SiC has thermaland chemical properties similar to those of the hot-pressed and reaction-bonded form of SiC but withsuperior polishability. Engdahl6 reported measure-ments of polishability (0.4-1.9-nm rms), total integrat-ed and angle resolved scattering, and reflectance in thevacuum-ultraviolet and x-ray regions for CVD SiC andcoated CVD SiC samples. Maguire et al.13 reportedmeasurement of some mechanical, thermal, and physi-cal properties of relevance for large space mirrors ontheir CVD SiC. These authors reported a surfacefinish of as low as 0.1 nm rms, although most SiCsamples exhibited surface finish in the 0.5-1.7 rmsrange. Finally, Pickering et al.14"1 5 reported impor-tant data on physical, mechanical, thermal, and opticalproperties of CVD SiC fabricated at CVD Incorporat-ed.

A comparison of important properties of CVD SiCwith those of the other mirror materials, such as mo-lybdenum, aluminum, beryllium, ultralow-expansionsilica, and Zerodur, is shown in Table I. The impor-tant figures of merit are (i) density, (ii) pressure andbowing distortion parameter E, which is the modulusof elasticity, (iii) thermal distortion parameter K/a,which is the ratio of thermal conductivity to coefficientof linear expansion, (iv) natural frequency and inertialoading parameter E/p, and (v) thermal stress parame-ter K/daE. It is desirable to have high values for allthese figures of merit except density. Aluminum is anattractive choice as a mirror material because of itshigh thermal conductivity and low cost, but has thedeficiency of a high value of thermal expansion coeffi-cient. Beryllium is lightweight and has high stiffnessand thermal conductivity but it is a toxic material andits coefficient of thermal expansion is large, althoughnot so large as that of aluminum. In addition, both ofthese metals require alternate plated layers into whichthe actual optical surfaces are worked.

The ultralow-expansion the silica and Zerodur glass-es possess low values of coefficients of thermal expan-sion and density, but these materials also possess low

values of thermal conductivity and elastic modulus.Further, ULE glass is compositionally and microstruc-turally inhomogeneous leading to anisotropy in itsthermal expansion. These inhomogeneities becomeworse as the size of the ULE glass mirror blank in-creases. Although Zerodur is microstructurally homo-geneous, the conventional process for fabricating opti-cal components from Zerodur is quite slow. Further,there is also a concern regarding thermal hysteresis inZerodur.

CVD SiC is attractive because its combined proper-ties are superior to those of the other mirror materials.It can be seen from Table I that SiC has the highestvalues of the elastic modulus and the thermal distor-tion parameter and the second-highest value of theinertia loading parameter. These properties, com-bined with the fact that SiC is a lightweight materialwith moderate values of the thermal stress parameter,make it an attractive material for reflective optics ap-plications.

In this paper, effects of environmental parameterssuch as temperature, atomic oxygen, and electron-beam impingement on CVD SiC substrates are as-sessed. As is shown below, these measurements sug-gest that CVD SiC is an excellent substrate for use insevere environments such as space, combustion, lasers,and synchrotrons. In Section II a brief review of im-portant properties of CVD SiC is presented. Alsoincluded are additional data on the elastic modulus,polishability, and scattering measurements that havebeen recently obtained. In Section III the thermaland cryogenic stability of SiC is discussed. The effectsof atomic oxygen and electron-beam impingements arepresented in Sections IV and V. Finally, conclusionsare included in Section VI.

11. Properties of CVD SIC

CVD SiC was fabricated by the pyrolysis of methyl-trichlorosilane in excess hydrogen in a subatmosphericpressure CVD reactor. References 7-11, 15, and 17describe the CVD reactor and process conditions thatwere used to fabricate SiC. The CVD SiC process wasdeveloped in a small research reactor to produce free-standing SiC samples of up to 7.5-cm diameter X0.625-cm thickness. This process was then scaled to0.5- and 1.5-m-diameter reactors, and SiC plates of 0.5-

1 August 1991 / Vol. 30, No. 22 / APPLIED OPTICS 3167

Table II. Important Properties of CVD SIC

Property Typical Value

Crystal structure Face-centered cubic;polycrystalline

Sublimation temperature (C) -2700Grain size (m) 5-20Density (g cm- 3 ) 3.21Hardness (kg mm- 2 ) 2520

Knoop (500 g load)Chemical purity 99.999% SiCTrace elements Mn (1.2), Fe (7.3), Co (1.0),

(parts in 106) Ni (0.6)Cu (5.5), Zn (1.5)Fracture toughness,

KIC valuesMicroindentation 3.3

(M N m-'- 5)Controlled flow (M N m-' 5) 2.7

Elastic modulusSonic (GPa/10 6 psi) 466/68Four-point flexure 461/67

(GPa/10 6 psi)Poisson's ratio 0.21Flexural strength, four-

point (MPa/ksi)aAt 298 K 595/86At 1673 K 588/85

Coefficient of thermalexpansion (10-6 K-')At 133 K 0.4At 173 K 0.8At 273 K 1.9At 573 K 4.1At 1373 K 4.6

Thermal conductivity(W m- 1 K-')At 123 K 120At 173 K 180At 298 K 144

Heat capacity (J kg-' K-')At 123 K 250At 273 K 700At 773 K 1200

a 1 ksi = 103 psi.

m and 1.0-m diameter were produced. The materialwas extensively characterized for important propertiessuch as density, hardness, flexural strength, elasticmodulus, fracture toughness, thermal expansion coef-ficient, thermal conductivity, heat capacity, crystalstructure, chemical purity, morphology, and reflec-tance in the wavelength range from the near ultravioletto the infrared. These measurements were performedat CVD Incorporated; the University of Dayton Re-search Institute; the Naval Weapons Center, ChinaLake, Calif.; and several other commercial laborato-ries. These data have been extensively reported inRefs. 14 and 15 and essentially show that CVD SiC is atheoretically dense, highly pure, f3-phase polycrystal-line material possessing superior mechanical, optical,thermal, and physical properties of interest for opticsapplications. Important properties of CVD SiC aresummarized in Table II.

Additional measurements to determine the sonicmodulus as a function of temperature are shown in Fig.1. These measurements were performed at the Uni-versity of Dayton Research Institute, using a Grindo-sonic, MK 3 (J. W. Lemmers Company) instrument.

460

a0.

440

0

c. 420

-W

400 '0 500 1000 1500

Temperature (C)

Fig. 1. Elastic (sonic) modulus of CVD SiC.

It can be seen from Fig. 1 that the sonic modulusdecreases by only 10% from -0 0C to 1500'C. Thesedata together with the fact that the flexural strength ofCVD SiC is slightly larger at 1500'C than at roomtemperatures indicates that CVD SiC is a good mate-rial for high-temperature applications.

The surface roughness of many samples of CVD SiCpolished by different vendors has been measured byprofilometer techniques. All these measurementsranged in surface roughness from 0.035 to 14 nm rms.A typical surface roughness output from a profilome-ter measurement is shown in Fig. 2. This sampleexhibits a surface roughness of 0.049 nm (0.49 A) rmsand a peak-to-valley roughness of 0.252 nm, which isconsiderably better than previously reported forSiC.6 ,'3

The bidirectional reflectance distribution function(BRDF) of highly polished SiC samples was also mea-sured. The BRDF characterizes the directionality ofthe radiation reflected from a surface and is defined asthe ratio of the measured radiance L emitted from thesample surface to the irradiance E incident upon thesample. 8"9 Mathematically, this function is repre-sented as follows:

ERD = L c Pscat- (1)

where Pscat and Pinc are the scattered and the incidentpower, respectively, Q is the solid angle subtended bythe detector aperture, A is the area of the irradiatedregion, and A cos scat is the projected area of irradia-tion upon the sample surface at the scatter angle beingmeasured. The fact that the incoming radiation isfrom a single direction permits the substitution ofPscatlA cos 0 for L and Pinc/A for E. The units of BRDFare inverse steradians (sr-'). For an ideal planar in-terface, the scattered power, Pscat, is zero since theBRDF function becomes the intensity profile of theincident beam.

The BRDF was measured from the specular peak toh15° with a fully automated scatterometer.20 Mea-

surements were performed at two wavelengths, 0.6328and 10.6 m, using specular angles 0o equal to 100, 300,


.*Autaa'mwr I anac

Funat on

-. h- 1A A A 1

1

A

SpeatralDon. I tyFunat 1anGoomet r I 0Speou 1 arRefleatana.

1

1

D

Average of 2 runsC.V.D. B-SIC OFF CENT

ER *2/2

0.49 RMS 2.52 P-VANGSTROMS

22 Feb 1990 10:49:42

Fig. 2. Profilometer trace of the CVD SiC polished surface.


.- *.4 iO

8.

x x

He I oghtD1 0tr I butI'Funot I on

0. 48 R I

apeI but iont Ion

1E4

1000

100

10

1E-1

1E-2

1E-3

1E-4

1E-5-18-15-12 -9 -6 -3 0 3 6 9 12 15

9

9.0

-1. go -0. 30 .00 03 g.. jgo

(a)18

Theta - Thetao (Degrees)(a)

1. 00

8. e

- I

1 E-6

-18 -15 -12 -9 -6 -3 0 3 6 9 12 15

Theta - Thetao (Degrees)18

(b)

Fig. 4. OPD map of 0.25-m-diameter lightweight SiC substrate (a)before heating and (b) after it has been heated to 13501C for 60 h.

(b)

Fig. 3. Comparison of BRDF data for two SiC samples polished by(a) United Technology Optical Systems and (b) General Optics.The specular angle of incidence is 100.

or 45°. For SiC, the BRDF data were determined to beessentially independent of the specular angle. Conse-quently, only BRDF data for a specular angle of 100 arereported.

Figure 3 shows a comparison of BRDF of two SiCsamples, 1 and 2, which were polished by two differentvendors: United Technology Optical Systems, WestPalm Beach, Fla. (sample 1) and General Optics,Moorpark Calif. (sample 2). We see that the peakBRDF value of sample 2 is considerably less than thatof sample 1 at both wavelengths. When off-peakBRDF values are compared, there is an order of magni-tude improvement in the visible region, but this im-provement is small in the infrared region. These dataare in conformity with the surface roughness data sincethe roughness of sample 1 was measured to be -1.2 nmwhile that of sample 2 was -0.05 nm rms.

Ill. Thermal and Cryogenic Stability

Thermal stability measurements were performed on a0.25-m polished-SiC substrate. This substrate con-sisted of a SiC faceplate -2.5 mm thick, upon thebackside of which a lightweight SiC backstructure wasdeposited. The SiC substrate was a monolithic struc-ture fabricated by the CVD process. Additional de-tails about this substrate are given in Ref. 15.

An optical path difference (OPD) map of the pol-

ished SiC substrate was first obtained at United Tech-nology Optical Systems [Fig. 4(a)]. The optical profi-lometer measurements indicated that the surfacefigure was 0.10X rms (peak-to-valley = 0.85 X, where X= 0.6328 nm). The radius of curvature was measuredto be 7.44 m. This sample was then mounted in a CVDchamber, heated to 13500C, and maintained at thattemperature continuously for 60 h. The lightweightSiC substrate was cooled to room temperature, and itsOPD map was again obtained as shown in Fig. 4(b).The profilometer measurements showed that the sur-face figure has changed to 0.41X rms (2.63X peak tovalley) and that the radius of curvature has changed to7.48 m. This change amounts to a figure change of0.3X rms ( = 0.6328 ,um) and a radius of curvaturechange of only 0.54%. These data show that a light-weight SiC substrate is stable to thermal cycling up to13500 C.

The cryogenic stability of SiC was also assessed.First, an interferometric plot and an OPD map of ahighly polished SiC sample were obtained as shown inFigs. 5(a) and 5(b). The surface figure at room tem-perature was measured to be 0.045X rms and 0.26Xpeak to valley. This sample was then cooled to-190'C, and the interferometric plot and OPD mapswere again obtained as shown in Figs. 5(c) and (d).The difference in change of figure from room tempera-ture to -190'C was measured to be X/125 rms in thevisible (X = 0.6328 Am) region. This change is quitesmall and is within the uncertainty of the measure-ment. Next, the SiC sample was brought to roomtemperature, and its interferometric plot and OPD


LL0CY}

m

0-0 .6328jurn0 *- 10.6 am

000

.0_0-0-0-'00 000'.0.S0-0

r . - e*

1E-1

1E-2

1E-3

1E-4

1E-5

U)

LL

m

i_-~ (b

(a)

(c)

(e)Y

Fig. 5. OPD and interferometric plot of SiC sample at (a), (b) room temperature, initially; (c), (d)-190 0 C; (e), (f) room temperature aftercooling to -1900C.


25-

20-

I

a)C)0

4-J

a)_4

4-a,

15-

10-

5 -

0

Back

FFrnt I I

I , , I , .200 600 1000 1400 1800 2200

X (nm)--X

Fig. 6. Comparison of reflectance of exposed (front) and unex-posed (back) surfaces of CVD SiC samples to energetic oxygen flux.

map were again obtained [Figs. 5(e) and 5(f)]. Thechange in figure from -190 0C to room temperaturewas again measured to be /70 rms which is quitesmall. These data show that CVD SiC exhibits excel-lent cryogenic stability. Thus both thermal and cryo-genic stability tests show that CVD SiC possesses ex-cellent stability over the -190-1350'C temperaturerange.

IV. Atomic-Oxygen Effects

Silicon carbide samples of 2.5-cm diameter were nomi-nally polished on both sides. One side of these sam-ples was exposed to thermal energy atomic oxygen in aconventional plasma system called asher (Tegal) atNASA Marshall Space Flight Center, Huntsville, Ala.,while the other side was used as a reference. The ashercontrol parameters were 800-m Torr pressure and 60-W rf power. The temperature of the specimens was100 C. The samples were exposed to atomic oxygen ata nominal flux of 2 X 1018 atoms cm-2 S-1 for a cumula-tive duration of 6 h. Inspection of the specimensrevealed no visible change in appearance even undermagnification. Examination of the samples with in-terferometric surface profiling revealed no obvious evi-dence of oxidation, erosion, or surface reaction. Theellipsometer measurements indicated small differ-ences between the exposed and unexposed sides of thespecimens, but these differences could not be correlat-ed to any defined oxide, contaminants, or roughenedsurface layers.

The same specimens were then exposed to an ener-getic atomic-oxygen flux having a broad energy spec-trum (0-10 eV) with a peak at 5 eV. The atomic fluxused was 1 X 1016 atoms cm- 2 s1 for 6 h, therebyproviding an integrated flux of 2 X 1020 atoms cm-2.Although the energetic oxygen flux was 2 orders ofmagnitude less than that used in the conventionalsystem, the energy (0-10 eV) associated with theseatoms made this impingement more damaging. This

Table Iil. Refractive Index and Extinction Coefficient of SIC SamplesExposed to Energetic Atomic Oxygen as Determined by Ellipsometry ( =

0.6328 ,um)

Refractive Index Extinction CoefficientSurface (N) (K)

Unexposed 2.654 0.294Exposed 2.15, 2.56-2.68 -0.6, -0.24 to -0.47

exposure was performed at the Plasma Physics Lab-oratory, Princeton University. Inspection of samplesindicated a visible change in appearance. The reflec-tance of both sides (exposed front, unexposed back) ofCVD SiC samples was measured and is compared inFig. 6. It can be seen that there are small changes inreflectance in the wavelength region 200 nm < X < 600nm. No measurable change in surface roughness with-in 2.5 nm rms (which was the surface roughness of theunexposed side) was observed. The ellipsometer mea-surement on SiC samples indicated differences in re-fractive-index and extinction coefficients between theunexposed and exposed sides as shown in Table III.

V. Electron-Beam Irradiation

A sample of SiC was irradiated with a pulsed, low-energy electron beam (SPI-Pulse 300, Spire Corpora-tion).21 This device can produce a pulsed beam ofelectrons of energy up to 20 J with an average pulsewidth of 60 ns. Sample surfaces may be irradiatedwith a fluence of up to 1.0 cal/cm2 with a pulse that canbe tailored both spatially and temporally.

The SiC sample was irradiated with the electronbeam to determine the response of the material todifferent electron fluences. The virgin surface wascharacterized by profilometry, optical photomicros-copy, and BRDF measurements at 0.6328 and 10.6 Am.The polished surface (0.05-nm rms roughness) of thesample was then irradiated at average fluence levels(tk) of 0.2,0.4,0.5, and 0.6 cal/cm2. The beam diameter(1/e2) at the surface was -9 mm, allowing for multipleirradiations across the 38-mm sample diameter. Thesample was irradiated in fluence level increments of 0.2cal/cm2 until visible surface damage occurred at 0.6cal/cm2. To determine the threshold for visible dam-age, the fluence was then reduced to 0.5 cal/cm2 andthe sample was irradiated at a fourth position.

Examination of the sample revealed damage in theform of clouding (i.e., diffuse light scattering) of thepolished surface beginning at a fluence level of 0.5 cal/cm2. The damage site was approximately circular,with a diameter of 2 mm for the 0.5-cal/cm2 spot and of3-4 mm for the 0.6-cal/cm2 position. Optical micros-copy of the irradiated sites showed a surface morpholo-gy change that progressed from a polished surface withno distinguishable features to a mosaic of grain struc-tures. Although it was not visible to the unaided eye,slight damage was observed at the 0.4-cal/cm2 irradia-tion site when an optical microscope was used. Degra-dation of the surface was generally proportional to the


e= 0.5 cal/cm2

(C)

e= 0.4 cal/cm2

(b)

e= 0.6 cal/Cm2 -

(d)

Fig.7. Nomarski photomicrographs (95x) of virgin and e-beam irradiated sites on a CVD SiC sample: (a) virign, (b) 0.4 cal cm 2, (c) 0.5 calcm- 2 and (d) 0.6 cal cm- 2.

incident fluence, and photomicrographs of all damagesites, taken at 95X magnification, are shown in Fig. 7.The change in surface morphology appears to be afunction of the incident fluence. The damage waslimited to the surface plane of the sample, which re-mained in focus across the whole damage site. Theabsence of any cavities or protrusions and the appear-ance of grain structure suggest that the change in sur-face morphology seen in Fig. 7 is due to the ejection ofcontaminants or other loosely bound material from thegrain boundaries in the surface.

The BRDF measurements at 0.6328 and 10.6 ,mtaken before and after irradiation at 0.5 and 0.6 cal/cm2 electron-beam fluences are shown in Fig. 8. Notethat the data for the positions that received irradiationfluences of 0.2 and 0.4 cal/cm 2 were indistinguishablefrom the virgin-surface data and are omitted for clar-ity.

To quantify the change in BRDF and make mean-ingful comparisons of such changes between irradiatedsites, we can use the following definition of the normal-ized ABRDF:


Virgin

(a)

1 E!

lE,

100(

1 ((1

I- 1

L-3 1E-1

m 1E-31E-3

1E-4

1E-E

1E'

1E'

1 OOC

U)1_1 1C

L lE-1

C 1E-,

1E-4

1E-5

-16 -12 -8 -4 0

- O (Degi

(a)

0 Virgin Surface a

0.5 cal/cm2

0.6 cal/cm2

O OA A .

-16 -12 -8 -4 0

0 - O (Degr

(b)

Fig. 8. BRDF scans of electron-beam irr-4;An __+ iti~ -A. f_ wnn~frt f) 0n -,

Table IV. Change In BRDF Values After and Before Irradiation (BRDF)with an Electron Beam at Two Different Fluence Levels, 0

k (cal/cm 2 )0.5 0.6

X(Um) e<e0 e>e 0 e<0 0 e>e0

0.6328 4.29 7.51 113 14510.6 1.01 2.18 6.58 6.46

4 a 12 6 drawn: (i) The elastic modulus of SiC degrades byonly 10% when the temperature of the SiC is raised

rees) from room temperature to 13500C. (ii) SiC can bepolished to a surface roughness of less than 0.1 nm rms.(iii) BRDF of a highly polished SiC sample is 1 X 10-5

at 10.6- and 0.6328-Am wavelengths for angles 30-180from specular. (iv) Minimal figure change (X/125 =

3328 ,um Radiation 0.005 Am rms) occurs when a SiC sample is cooled fromroom temperature to -190'C. (v) No appreciablesurface degradation occurs when thermal atomic oxy-gen flux of 2 X 1018 atom cm-2 s-l is directed onto theSiC sample for 6 h. (vi) Small changes in surfacereflectivity in the wavelength range 200 < X < 600 nm

A A A occur when an atomic-oxygen beam of broad energy.*- - - . . spectrum (0-10 eV) with a peak at 5 eV is directed onto~'qooco 0 0 CVD SiC. (vii) Electron-beam irradiation of SiC

0 -I O yields a surface damage threshold of 0.5 cal/cm-2 .

These results indicate that CVD SiC is an excellentoptical substrate for applications in severe environ-

rees) ments such as outer space, combustion, lasers, andsynchrotron x rays. The CVD process to fabricatethese materials has also been scaled to large size, per-

, lated AVD P SQlQ wh mitting lightweight mirrors in the 1-m-diameter range.

ABRDF - BRDF(av) postirradiation - BRDF(av) preirradiationBRDF(av) preirradiation

(2)

BRDF(average) was calculated for the angular regionbetween 4° and 150 (forward scatter) and also between-4° and -15° (backscatter) from the specular angle.The ABRDF values for the irradiation sites that re-ceived fluence levels of 0.2 and 0.4 cal/cm 2 were veryclose to zero [i.e., there was no measured deviation inthe BRDF(avrg)]. Table IV shows the measuredABRDF for each wavelength. The ABRDF valuesobtained are all greater than unity, indicating a degra-dation in the optical performance of the sample at bothwavelengths. Significant optical degradation at theshorter wavelength is apparent, but degradation of lessthan a factor of 10 for the infrared wavelength wasobserved.

VI. Conclusions

Measurements of elastic (sonic) modulus, polishabi-lity, surface scatter, thermal and cryogenic stability,and surface degradation from atomic oxygen and elec-tron-beam impingement were performed to assess theeffects of severe environments on CVD SiC. Fromthese measurements the following conclusions are

The authors acknowledge the assistance of the fol-lowing individuals and institutions for providing themeasurement data on CVD SiC: Roger C. Linton,John Reynolds, Joe Norwood, and Jason Daughn, ofthe NASA Marshall Space Flight Center, Huntsville,Ala., for atomic-oxygen exposure; William Langer, ofthe Plasma Physics Laboratory, Princeton, N.J., forenergetic atomic oxygen exposure; George Graves, ofthe University of Dayton Research Institute, Dayton,Ohio, for elastic modulus measurements; Terry Dono-van, of Naval Weapons Center, China Lake, Calif., forcryogenic stability; Ken Scribner, of General Optics,Moorpark, Calif., for polishing; and Pete Weber'sgroup at United Technologies Optical Systems, WestPalm Beach, Fla., for optical fabrication and OPDmaps.

References1. W. J. Choyke, R. F. Farich, and R. A. Hoffman, "SiC, a New

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Choyke, "Total Integrated Optical Scattering in the VacuumUltraviolet: Polished CVD SiC," Appl. Opt. 16, 1111-1112(1977).

3. V. Rehn and W. J. Choyke, "SiC Mirrors for Synchrotron Radia-tion," Nucl. Instrum. Methods 117,173-175 (1980).


_ 0 Virgin Surface Q 10.6 Jim Radiation

0 0.5 cal/cm2A

0.6 cal/cm2 0

-A A A AAAAAA A

-0 00'- 0"118 '

5

t

I

I

i

I

4. M. M. Kelly and J. B. West, "Fabrication and Use of SiliconCarbide Mirrors for Synchrotron Radiation," Proc. Soc. Photo-Opt. Instrum. Eng. 315, 135-139 (1981).

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Properties of chemical-vapor-deposited silicon carbide for optics applications in severe environments