1AD-A125 342 STUDY OF MECHANO-CHEMICAL MACHINING OF CERAMICS AND THE IIIEFFECT ON THINN FILM BEHAYIOR(U) HONEYWELL CORPORATETECHNOLOGY CENTER BLOOMINGTON MN H VORA ET AL. JAN 83
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MICROCOPY RESOLUTION TEST CHARTN A IO N A L 1U R EA U O F T N A D S 1 6 -
Technical Report No. 2
., Contract N00014-80-C-0437; NR 653-002
' STUDY OF MECHANO-CHEMICAL MACHININGF .- OF CERAMICS AND THE EFFECT ONL- THIN FILM BEHAVIOR
H. Vora and R.J. StokesHoneywell Inc.Corporate Technology Center10701 Lyndale Avenue SouthBloomington, MN 55420
January 1983
Final Technical Report
Reproduction in whole or in part ispermitted for any purpose by theUnited States Government
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c.. Office of Naval Research WYr- 800 North Quincy Street LSLJ Arlington, VA 22217
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Study of Mechano-Chemical Machining of Final Technical ReportCeramics and the Effect On Thin Film 1 May 1980-30 November 1982Behavior. 6. PERFORMING ORG. REPORT NUMBER
7. AUTHOR(S) 0. CONTRACT OR GRANT NUMBER(S)
O H. Vora and R.J. Stokes NOOO14-80-C-0437
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Reproduction in whole or in part is permitted for any purpose by theUnited States Government.
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18. SUPPLEMENTARY NOTES
19. KEY WORDS ( CONTINUE ON REVERSE SIDE IF NECESSARY AND IDENTIFY BY BLOCK NUMBER)
Alumina, Boron Carbide, Gallium Arsenide, Mechanochemical Polishing,Silicon, Silicon Carbide, Silicon Nitride, Strength, Thin Film Adhesion,Zirconia
20bBSTRACT (CONTINUE ON REVERSE SIDE IF NECESSARY AND IDENTIFY BY BLOCK NUMBER)
Efforts have been made in this program to develop mechanochemicalpolishing technique for a number of materials and to determine theconsequences of mechanochemically polished surfaces on the physical
' performance of ceramics. The objective of the mechanochemical polishing<I efforts is to produce scratch-and damage-free surfaces by finding suitable
soft abrasives that remove material from the workpiece through chemicalreactions. Mechanochemical polishing effects were observed when hot-
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IUpressed and r action bonded S 3 N 4 were polished with two oxidesof ion, Fe 203 and Fe 304 Mechanochemically polished surfaces of
2 33£4Si3 4 were analyzed using the techniques of Auger electro spectroscopy,
* profilometry and interferometry. Data showing the f4fp4ts of mechano-
chemical polishing on the strenght of hot-pressed Si N4 and on the
* adhesion of thin titanium films on this material:have been obtained.
Also, preliminary efforts have been made to find suitable soft
- abrasives for mechanochemical polishin3 of GaAs, Spinel (MgO.A1203)
SiC, _IC, and partially stabilized ZrO2
4 2
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*Preface
This report and the previous annual technical report cover work performed duringthe period May 1, 1980 to November 30, 1982 under Office of Naval Research Con-tract No. N00014-80-C-0437. The authors would like to thank Mr. D.J. Sauve, Jr.,for excellent technical support throughout this work, Dr. R. Wagner for evaluatingthe effects of surface finishing on the adhesion of thin titanium films on hot-pressedSi3N4 , and Mr. E. Fisher of the Ford Motor Company, Dearborn, Michigan, forproviding the fracture strength data on ground and mechanochemically polishedhot-pressed Si3N4 . Auger electron spectroscopy analyses of mechanochemically.polished surfaces were performed by Dr. T. Orent, and RHEED work was performedby Mr. C. Knudson.
II
iii A.I|
Table of Contents
Section Page
1 INTRODUCTION 1
2 RESULTS 3
2.1 Alumina 32.2 Silicon Nitride 8
2.2.1 Mechanochemical Polishing 82.2.2 Thin Film Adhesion 142.2.3 Flexural Strength 19
2.3 Other Materials 22
3 DISCUSSION 25
INDEX OF PUBLICATIONS 27
DISTRIBUTION LIST (BASIC) 28
DISTRIBUTION LIST B (SUPPLEMENTARY) 30
iv
OF
i
U Ust of Illustrations
Figure Page
1 Profile of Ground Surface of Kyocera Sapphire 4
2 RHEED Pattern of Meller Sapphire 4
3 Weight Loss of Kyocera and Meller Sapphire Samples During DryMechanochemical Polishing with SiO2 5
4 Weight Loss of a Polycrystalline Alumina Sample DuringDry Mechanochemical Polishing with Si0 2 6
5 Surface Profiles of Coors ADS 995 Alumina (a) Before and(b) After Mechanochemical Polishing 7
6 Weight Loss of Hot-Pressed Si3N4 (Wesgo SNW-1000) DuringWet Mechanochemical Polishing with Fe2O3 Under VariousPressures 9
7 Nomarski Micrographs of an Si N Substrate (a) Before*. and (b) After Mechanochemical Polishing 11
8 -Surface Profiles of an Si 3N4 Substrate (a) Beforeand (b) After Mechanochemical Polishing 12
9 Surface Figure of a 2.5 x 2.5 cm Si3N4 Substrate 12
10 Auger Spectra of an Si3N4 Sample Mechanochemically Polishedwith Fe20 3. View (a) shows the polished surface and(b) depicts it after suputter-etching. 13
'11 Weight Loss During Wet Mechanochemical Polishing with Fe203e of a 19mm Diameter Disc of Ceradyne Hot-Pressed Si3N4 15
12 Weight Loss of Reaction Bonded Si3N4 During WetMechanochemical Polishing with Fe20 3.Sample cross-section = 9 x 6.3mm 16
13 Uniaxial Pulltest for Measurement of Thin Film Adhesion 16
14 Auger Profile of the Ti/Si3 N4 Interface. View (a) is hypothetical,assuming no reaction between Ti and Si3N4. View (b) showsactual profiles. 18
15 Si2 XPS Peak of Si Single Crystal, Si3N4 and theSi3N4/Ti Interface 20
V
6"
List of Illustrations (Concluded)
Figure Page
16 Weibull Plot of Standard Test Bars of Hot-Pressed Si3N4 21
17 Weibull, Plot of Mechanochemically Polished Bars of Hot-PressedSi3N4 21
18 Optical Microgra ph and Electron Channeling Pattern ofMechanochem'ically Polished GaAs 23
vi
List of Tables
Table Page
I Pulltest Results of Ti Metallization on Si3N4 Substrate 17
2 Fracture Strengths of Standard and Mechanochemically PolishedTest Bars of Hot-Pressed Si3N4 20
3 Potential Soft Abrasives for Mechanochemical Polishing of
Various Materials 24
I
viiThe reverse of this page Is blank.
U Section 1Introduction
Efforts have been made in this program to develop mechanochemical polishing tech-niques for a number of materials. In this technique, a hard brittle material (mostlyceramics and some semiconductors, such as silicon) is polished with suitableabrasive whose hardness is less than that of the workpiece. Since such an abrasivecannot damage or scratch the workpiece, the technique can yield damage- andscratch-free surfaces. Even though the abrasive used in mechanochemical polishingis soft, the observed rate of material removal is often high and comparable to thatachieved with harder abrasives. One proposed explanation is that chemical reactionsoccur at the contact points between the abrasive and the workpiece.1 These trans-formed regions, which are believed to be very small (of the order of 100 A), then de-tach themselves from the workpiece during polishing.
b Mechanochemical polishing is performed in a manner very similar to conventionalpolishing (using conventional polishing machines). However, in order to promote thechemical reaction, it is often necessary to generate high temperatures at the contactpoints between the abrasive and the workpiece. A simple way to accomplish this isto apply pressure in the order of 7 X 104 N/m 2 (10 psi) on the workpiece and to per-form the mechanochemical polishing "dry." In some cases, chemical reactions can beinduced at moderate temperatures and significant removal rates are observed, evenwhen the mechanochemical polishing is performed "wet." The terms dry and wet re-fer to the use of a loose powder of soft abrasive as the polishing medium in the for-mer case and a slurry of a soft abrasive in a liquid (usually water) as the polishingmedium in the latter case.
In connection with wet and dry mechanochemical polishing, it should be noted thatthe chemical reactions involved in surface finishing can be broadly classified intotwo types: (1) solid-solid and (2) solid-liquid. Dry mechanochemical polishing ofsilicon with soft abrasives such as CaCO 3, BaCO3 and MgO is an example of mecha-nochemical polishing involving solid-solid type of chemical reaction. Each of theseabrasives yields highly reflecting silicon surfaces showing few scratches. Such sur-faces are also obtained when silicon is polished with a slurry of any of theseabrasives in water. The observed material removal in this case can be due to bothsolid-solid and solid-liquid type of chemical reactions if, for example, the slurry dis-solves silicon at temperatures generated by the rubbing of silicon surfaces againstthe surfaces of the abrasive and the lap.
IN. Yasunaga, N. Tarumi, A. Obara and 0. Imanaka, in The Science of CeramicMachining and Surface Finishing II, B.J. Hockey and R.W. Rice, editors. NationalBureau of Standards Special Publication 562, 1979, 171.
li I I I I I I - III I .) . _ I . J .
Slurries of many soft abrasives in water are not neutral. Since some materials dis-solve in acidic or basic solution, at or slightly above room temperature, liquid-solidtype of chemical reactions can make significant contribution to the removal rates ob-served during wet mechanochemical polishing. An adjustment of the pH of the slur-ry provides an important means for controlling removal rates in such cases.
The emphasis of most of our efforts in this program have been on determining thefeasibility of surface finishing of a number of materials through solid-solid type ofchemical reactions. Typically, them efforts involve polishing a given material con-taining dull surface, such as that produced by grinding or lapping with hard andcoarse abrasives, in dry condition with a number of soft abrasives using pressures ofthe order of 3.5 x 103 to 7 x 103 N/m2 (5-10 psi). If an abrasive reacts chemicallywith the workpiece, it will produce scratch-free ard highly reflecting surfaces. Inmany cases, such as polishing of alumina with SiO2 particles or of silicon nitridewith Fe20 3, no significant changes in the removal rates occur when the polishingmode is changed from dry to wet. The latter is the preferred mode of mechanochemi-cal polishing because it minimizes chipping at the edges of the sample, results inquieter polishing operation, and makes it easier to control the flatness of the sample.
Some of our observations concerning the mechanochemical polishing effects in thecase of silicon and alumina were reported in the previous report.2 In this report, weare reporting-the results of our efforts to develop mechanochemical polishing tech-niques for Si3N4, evaluate the effect of mechanochemical polishing on the strengthand the thin film adhesion in the case of Si3N4. Also described are our preliminaryobservations of mechanochemical polishing effects in the case of GaAs, zirconia, SiC,B4C and spinel (MgO. A120 3).
2H. Vora and R.J. Stokes, Office of Naval Research Annual Technical Report No.NOOC1 4-80-C-0437-1, 1 81).
2
4
Section 2. Results
2.1 ALUMINA
There are many factors that contribute to the removal rates obtained in mechano-chemical polishing. Among these are the roughness of the sample and the damaged
layer introduced by previous grinding or lapping operations. To illustrate the effectsof these factors, removal rates obtained by dry mechanochemically polishing threedifferent types of alumina with SiO2 particles were measured. Two of these weresingle-crystal alumina of (1010) orientation, one purchased from Kyocera and theother from Adolf Meller Co. The third was polycrystalline alumina (COORS ADS995). Kyocera sapphire contained a thick damaged layer introduced by previousgrinding operation; its surface profile is shown in Figure 1. The surface of Mellersapphire was scratch-free and also appeared to be damage-free, as shown by itsRHEED pattern in Figure 2.
All alumina samples were 2.5 x 2.5 cm in cross-section and were dry mechano-chemically polished on a 25 cm diameter linen bakellite lap, using a submicron sizeSiO 2* as abrasive. The experiments were conducted on a Strasbaugh polishing ma-chine, model 6Y-1, using a lap rpm of 50 and a stroke frequency of 20 cpm.
Data on the amount of material removed as a function of polishing time for Kyoceraand Meller sapphire samples are shown in Figure 3. These data indicate that in thecase of Kyocera sapphire, which contained a thick damaged layer with a very rough
surface, the removal rate is high in the beginning but decreases rapidly, withpolishing time, from the average value of - 21Lm/hr for the first hour of polishing toa steady value of - 0.4,sm/hr, reached after about 5 hours of polishing at a pressureof 7 x 10 4 N/m2 (10 psi). Whereas in the case of Meller sapphire, which containedfew scratches and a thin or no damaged layer, the removal rate at a given pressuredoes not change with polishing time and increases with increasing pressure from avalue of - 0.3jsm/hr at 3.5 x 104 N/m 2 (5 psi) to - 0.6,m/hr at 1.4 x 105 N/m2 (20psi).
Observations made in the case of polycrystalline alumina (COORS ADS 995) weresimilar to those made in the case of Kyocera sapphire, although the steady materialremoval rate was slightly higher in the latter case, - 0.34Am/hr for polycrystallineA12 0 3, as compared to - 0.4jm/hr for Kyocera sapphire at a pressure of 7 x 104N/m2 (10 psi). The weight loss data of COORS ADS 995 alumina are shown in Fig-ure 4, and its surface profiles, measured before and after iechanochemical polishingwith SiO2, are shown in Figure 5. It can be seen that mechanochemical polishin re-duces the peak-to-valley roughness of COORS ADS 995 aluminum from -6000A to-300k.
*Cerac Incorporated, Milwaukee, Wisconsin
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Figure 1. Profile of Ground Surface of Kyocera Sapphire
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IFigure 2. RHEED Pattern of Meller Sapphire
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2.2 SILICON NITRIDE
2.2.1 Mechanochemical Polishing
Our initial efforts to develop a mechanochemical polishing technique for siliconnitride were made using Wesgo SNW-1000 hot-pressed Si3N4.* It contained 87 w/oSi3N4 , and 11 w/o A12 0 3 and 2 w/o Y20 3 for densification. It was purchased in theform of substrates of approximate dimensions of 2.5 x .2.5 x 0.1 cm. Threesubstrates were mounted on a 10 cm diameter sample holder and attempts weremade to mechanochemically polish them using several soft abrasives, CaC0 3, MgO,SiO2, Fe20 3 and Fe30 4 , on a 25 cm diameter linen bakellite lap rotated at speeds upto 50 rpm. These experiments yvere conducted using a Strasbaugh polishing machine(model 6Y-1) and loads of up to 10 kgs were applied to the sample holder. Underthese experimental conditions, scratch-free surfaces were readily obtained whenFe20 3 or Fe30 4 was used as the soft abrasive. Consequently, the efforts were concen-trated on these two abrasives. The Fe20 3 ** particle size was 3#sm or less, and theFe3 O4t was 5jtm or less.
Data on the amount of material removed as function of polishing time dnring wetmechanochemical polishing with Fe20 3 are shown in Figure 6 for three differentpressures. For these experiments, a single substrate was mounted at the center ofthe holder and mechanochemically polished on a 25 cm diameter linen bakellite lap,rotating at 20 rpm until it was scratch-free. The sample was then removed from theholder, and its weight was measured. The subsequent decrease in weight, with apolishing time, for pressures of 0.7 x 104 N/m 2 (1 psi), 3.5 x 104 N/m2 (5 psi) and 7.0x 104 N/m2 (10 psi) are shown in Figure 6. Increasing the pressure from 0.7 x 104N/m2 (1 psi) to 3.5 x 104 N/m2 (5 psi) increased the removal rate by more than a fac-tor of 10, but further increase in pressure had no significant effect on the removalrate, i.e., the removal rate reached a saturation value of - 1.6,sm/hr at a pressure of3.5 x 104 N/m 2 (5 psi). In an effort to determine whether the removal rate can be in-creased further, the Si3 N4 substrate was dry mechanochemically polished, using apressure of 7 x 104 N/n 2 (10 psi). This did not increase the removal rate; the meas-ured removal rate under this condition was - 1.5;sm/hr.
4
"Wesgo Division, GTE Products Corporation, Belmont, CaliforniaBuehler Ltd., Lake Bluff, Illinois
t Cerac Incorporated, Milwaukee, Wisconsin
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POLISING TOME (hriFigure 8. Weigh~tLoss of Hot.Pressed Si Nt Wesgo SN .1 0) urn Wtchemica 9 P is ng it e203 Under Various pressures
Mechanochemically polished Si3N4 samples were subjected to visual and optical mi-croscopic examinations and, in addition, their surface roughness was measured us-ing a Talystep profilometer.* In these tests, both Fe20 3 and Fe3O4 polished surfacesappeared to be of comparable quality.
The visual examination indicated that the mechanochemically polished Si3N4 sam-ples had mirror finishes and much higher reflectivities than as-received samplespolished conventionally by Wesgo with hard abrasives. The. optical microscopic ex-amination of the entire polished surface did not reveal any scratches. Figure 7 com-pares the Nomarski micrographs of an Si3 N4 substrate before (in the as-received con-dition) and after mechanochemical polishing with Fe20 3. The corresponding surfaceprofiles are shown in Figure 8, where it is seen that mechanochemical polishing re-duced the peak-to-valley roughness of Si 3N4 by a factor of 5 to a value of - 200.
" • The hot-pressed Si3N4 substrates used in this work were polycrystalline. Thus, theultimate smoothness that could be achieved by mechanochemical polishing was lim-ited by the grain boundaries, second phases and pores present in the material.
-Because of the high reflectivity, it was possible to determine the flatness ofmechanochemically polished Si3N4 substrates, using an interferometric technique.The surface figure of a 2.5 x 2.5 cm Si 3N4 substrate is shown in Figure 9. This fig-ure was obtained using a HeNe laser as a light source (X = 633 nm) and indicates aflatness of -3 wave.
The surface of an Si 3N4 substrate that had been mechanochemically polished withFe20 3 and rinsed in acetone was analyzed by Auger electron spectroscopy (AES) us-ing a scanning Auger microscope (3 keV, 1pA). The surface spectrum shown in Fig-ure 10(A) indicated that the major contaminates were oxygen and carbon. Repeatedscans across the region containing the strongest iron peak did detect approximately0.5 atomic percent iron in the surface layer. The peak positions and fine structureobserved in high-resolution scans of the Si (LVV), N (KLL) and 0 (KLL) Auger peaksclosely resembled those of an oxygen-rich silicon oxynitride. 3 The sample was sput-ter-etched with 4 keV argon ions until carbon was no longer detectable [Figure10(B)I. The roughness of the surface [see Figure 8(b)] made an accurate determina-*tion of the sputter depth impossible, but it was estimated that no more than 100A
* had been removed. No Fe was detected after sputter etching, but traces of Y and Al,which are constituents of the bulk material, were seen. The oxygen level had beenreduced by a factor of five, and the Si (LVV) and N (KLL) peaks had the character-istic energies and shapes of sputtered Si 3N4 .3-5
* Rank Taylor Hobson, Leicester, England
3R. Hezel and N. Liske, "Si (LVV) Auger Spectra of Amorphous Si-Oxide, Si-Nitride,and Si-Oxynitride," J. Appl. Phys. 51 (5) 2566-2568 (1980).
4S. Thomas and R.J. Mattox, "An Attempt at the AES Evaluation of the Compositionof Off-Stochiometric Silicon Nitride," J. Electrochem. Soc. 124 (12) 1942-1945(1977).
5P.H. Holloway, "Chemical Shifts in Auger Electron Spectra from Silicon in SiliconNitride," Surface Science 54 (2) 506-508 (1976).
10
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Figure 8. Surface Profiles of an Si3N4 Substrates (a) Before and (b) AfterMechanochemical Polishing
I Figure 9. Surface Figure of a 2.5 X 2.5 cm Si3N4 Substrate
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During the course of this work, hot-pressed Si3N4 samples manufactured by Norton(NC-132), Demetron, and Ceradyne, and a few samples of reaction-bonded Si 3N4 ,were also mechanochemically polished with Fe20 3 . Norton and Ceradyne hot-pressed Si3N 4 and the reaction-bonded Si3N 4 were observed to mechanochemicallypolish at significantly higher rates than Wesgo or Demetron Si3N4 . The measure-ments were made of the removal rates achieved by wet mechanochemical polishingwith Fe20 3 of Ceradyne Si3N4 and reaction-bonded Si 3N4 , using the same proceduredescribed earlier for Wesgo Si 3N4 . Weight loss data for Ceradyne and reaction-bonded Si3N4 are shown in Figures 11 and 12, respectively. Note that the measuredremoval rate at a pressure of 7 x 104 N/m 2 (10 psi) for Ceradyne Si3N4 is about afactor of 2.5 higher, and at a pressure of 2.8 x 101 N/m 2 (40 psi) for reaction-bondedSi3N4 is about a factor of 4 higher, than the maximum removal rate of - 1.6sm/hrmeasured for Wesgo SNW-1000 Si3N4 (Figure 6). The reasons for these variationsare not understood at this time.
2.2.2 Thin Film Adhesion
Integrated circuit electronics packaging for future systems with increased circuitdensities will need decreasing substrate feature sizes and larger chip dimensions. Inorder to minimize reliability problems, the substrate should contain minimum de-fects (such as scratches, pores, pits and pullouts) and its roughness should be muchsmaller than the width of the metallization lines. Employment of a soft abrasive inmechanochemical polishing yields scratch-free surfaces containing minimum ex-trinsic defects and, in this respect, mechanochemically polished surfaces are ex-pected to perform better in electronics packaging than the surfaces polished conven-tionally with hard abrasives. However, the maximum surface smoothness that canbe permitted in electronics packaging is also limited by the adhesion of the metalfilms to the substrate. This is because the adhesion of metal films to a ceramicsubstrate is mainly due to two factors: (1) mechanical keying, or interlocking, into ir-regularities of the substrate surface, and (2) chemical bonding. The contribution ofthe mechanical keying to thin film adhesion decreases with the decreasing surfaceroughness of the sample, and this can lead to poor thin film adhesion in the case ofsmooth surfaces.
Measurements of the adhesion of titanium metallization (- 10001thick) to therough and smooth surfaces of hot-pressed Si3N4 have been made. Si3N4 was selectedbecause it is a potential substrate material for future generations of integratedcircuits. Si3 N4 can be made by hot-pressing, as well as by tape casting, with thermalconductivity comparable to that of alumina and with a coefficient of thermal ex-
"4 pansion that matches that of silicon.
Adhesion of Ti metallization to Si3N4 was determined using a uniaxial tension pulltest. As shown in Figure 13, a presoldered wedge-shaped brass stud is attached to apresoldered square on the substrate. For connecting the stud and the metallizedsubstrate, silver solder (95 w/o Sn, 5 w/o Ag) was used. The bonded stud was pulled
14f14
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POLISHING TIME (hr)
Figure 11. Weight Loss During Wet Mechanochemical Polishing with Fe2O3 Ofa 19mm Diameter Disc of Ceradyne Hot-Pressed Si3 N4
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POLISHING TIME (hr)
Figure 12. Weight Loss of Reaction-Bonded SijN4 During WetMechanochemical Polishing with Fe2O3. Sample cross-section =9
x 6.3mm.
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95SN5AG 95SN5A6
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Figure 13. Uniaxial Pulltest for Measurement of Thin Film Adhesion
16
4
in an Instron testing machine, using a crosshead speed of 0.13 cm/mnt. In this test,the failures in most cases were observed to occur at the substrate/metallization in-terface. Square pads of two different sizes, 500 x 500pm and 1250 x "1250pm, wereused. Wesgo SNW-100 Si3N4 was used as the substrate and the adhesion measure-ments were made both before and after mechanochemical polishing. Mecha-nochemical polishing reduced the surface roughness by about a factor of five to apeak-to-valley value of -2001, as noted earlier in Figure 8.
The results of pull test measurements of Ti-Si3 N4 metallizations are summarized inTable 1, where it is noted that the reduction in the surface roughness of Si3N4 bymechanochemical polishing reduces thin film adhesion. The fact that adhesion oc-curs even on smooth surfaces produced by mechanochemical polishing indicates thatsome chemical bonding occurs between Ti and Si 3N4. To verify this, a 200A thicklayer of Ti was RF sputter deposited on the mechanochemically polished surface ofWesgo SNW-100 Si3N4. The Ti-Si3N4 interface was then analyzed using both Augerelectron spectroscopy and X-ray photoelectron spectroscopy (XPS).
Figure 14(a) shows a hypothetical Auger profile that would be expected fromTi-Si 3N4 system if no reaction occurred at the interface. Sputtering through the Tilayer revealed distribution profiles for various elements which were significantlydifferent. The actual profiles are shown in Figure 14(b), where it is noted that Ti canbe detected well below the original Ti-Si 3N4 interface and that the Si concentrationin the interface region is lower compared to that in the bulk Si3N4 . These data in-dicate that there is a diffusion of Ti into a thin (- 200A) surface layer of Si3N4 .
Table 1. Pulltest Results of Ti Metallization on Si3 N4 Substrate
Substrate Failure Load, kg
Condition 500 pm x 500 pm Pad 1250 rn x 1250 pm Pad
As received 1.4 ± 0.5 5.6 ± 1.8
Mechanochemically 0.9 ± 0.5 3.6 ± 1.4polished
17
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80 INTERFACE
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Ti Ti I REACTION REGION604Ti - Si 3 N4 - 4TiN + 3Sia 60
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CONTAMINATION ON SUBSTRATEBEFORE METAL DEPOSITION
Figure 14. Auger Profile of the Ti/Si3 N4 Interface. View (a) is hypothetical,assuming no reaction between Ti and Si3N4. View (b) shows actualprofiles.
18
XPS analysis indicated that different elements have reacted with each other. Figure.K 15(a) shows Si2p XPS peak of pure silicon sample and Figure 15(c) shows that of
Si3N4 . XPS spectrum of the reaction region revealed the presence of both thesepeaks, as shown in Figure 15(b), thereby indicating the likelihood of the occurrenceof the reaction 4Ti +Si 3N4 - 4TiN +3Si. Further evidence of TiN formation was ob-tained by monitoring Ti2P XPS peak. XPS spectrum obtained from within the reac-tion region indicated the presence of two peaks, one originating from pure Ti and theother from TiN.
Figure 14(b) also shows the concentration profiles of 0 and C. The increasing concen-tration of these two elements towards the surface is due to contamination during thedeposition. The relatively high C concentration at the original interface is probablydue to cleaning the substrate with acetone before sputter deposition of the metalliza-tion. The high oxygen level within the substrate is due to the presence of A120 3 andY20 3, which are used as densification aids for Si 3N4 .
2.2.3 Flexural Strength
Hot pressed Si 3N4 is an important candidate material for ceramic heat engine parts.For this reason efforts were made in this program to determine the effect of mecha-nochemical polishing on the strength of hot-pressed Si 3N4.
Four-point-bend specimens, _25cm long, were cut from a billet of Norton NC-132SSi 3N4 such that both the fracture surface and the fracture propagation direction
would be parallel to the hot-pressing direction. The specimens were then ground to a*.final cross-section of 3.2 x 1.6 mm, using a 320 grit diamord wheel, and their edges
were chamfered to minimize fracture initiation at the edges. A total of 26 specimenswas prepared; 13 were tested in the as-ground condition (referred to as standard testbars) and the other 13 were mechanochemically polished with Fe20 3 removing anaverage of _ 26jum from the ground surface before testing. The samples were frac-tured in an Instron testing machine using a crosshead speed of -0.5 mm/mnt. andspacings of 18 mm between the supporting pins and 9 mm between the loading pins.
The results of fracture strength measurements of standard and mechanochemicallypolished test bars of Si 3N4 are summarized in. Table 2. These data were alsoanalyzed using Weibull statistical methods; the results of these analyses are shownin Figures 16 and 17, respectively. It is seen that there is little difference betweenthe strength values of the mechanochemically polished and ground samples ofSi3N4. In an effort to determine the cause of the observed insensitivity of the frac-ture strength of hot-pressed Si3N4 to two significantly different surface finishingtreatments, a fractographic analysis of the fractured 4-point-bend specimens will heperformed in another program. The results of this analysis will be describedelsewhere.
19
I.
Si(2p) XPS PEAK
Si(100)
I (a)
Ti-Si3N4INTERFACE
I . (b)
Si 3N4 IILUBSTRATE (C)
I ISI
108 104 100 96
BINDING ENERGY, eV
Figure 1 5. Si2p XPS Peak of Si Single Crystal, Si3N4 and the Si3 N4 /Ti Interface
6 Table 2. Fracture Strengths of Standard and Mechanochemically Polished TestBars of Hot-Pressed Si 3N4
StandardSamples Mean Fracture Stress Deviation
MPa (Ksi) MPa (Ksi)
a Standard 789 (114.5) 113 (16.3)
Mechanochemically 768 (111.5) 77 (11.2)polished
2
20
6
99999.99.9080.
-70.
aR60-1 50-CC40-:)30-R 20-
LL SLOPE =8.3
0
2-
0.1. . * *0000 2 0 0000 0N~ c IOC 14t o0 0 000 0 0
MODULUS OF RUPTURE (KSI)
Figure 16. Weibull Plot of Standard Test Bars of Hot-Pressed Si3N4
g 99.9.99.95.90-80.
-70-
S60: SLOPE = 12.11~50S40S30- CHARACTERISTICS20- VALUE =117 KSI
U- +U.10-
0- 5
4 2-
0.5-
0.2-
0.1' 4 I 1 1 U I I I I I I I~l
0 0 0 000 08-~ m t' WLC 0 G o
MODULUS OF RUPTURE (KSI)
Figure 17. Weibull Plot of Mechanochemically Polished Bars of Hot-PressedSi3N4 21
4
2.3 Other Materials
This section describes the results of our preliminary efforts to develop a mecha-nochemical polishing technique for GaAs, Spinel (MgO.A1 20 3), ZrO2 , SiC and B4C.
In the case of GaAs, initial efforts were made to dry mechanochemically polish itwith CaCO3, BaCO3 and MgO on a linen bakellite lap. Excessive chipping occurredat the edges of the samples and scratch-free surfaces were not obtained by dry mech-anochemical polishing. However, scratch-free and highly reflecting surfaces were ob-tained when GaAs single crystals were wet mechanochemically polished on abeeswax lap, using a slurry of MgO in water. Optical micrograph and electron chan-neling pattern of a surface of GaAs crystal wet mechanochemically polished withMgO slurry are shown in Figure 18.
Samples of other materials (B4C, SiC, ZrO2 and spinel) were first lapped with 15gmdiamond on a brass lap and then mechanochemically polished on a linen bakellitelap for one hour with CaCO3, BaCO 3, MgO, Fe20 3 , NiO, Sr2C0 3 and colloidal silica,using a pressure of -2 x 104N/m2 (3 psi). Samples were examined visually and inan optical microscope after mechanochemical polishing. Appearance of highly re-flecting or scratch-free areas on the lapped surface was taken as an evidence formechanochemical polishing. Table 3 lists the abrasives that were found to reactmechanochemically with various materials. While these data are preliminary, theyare indicative of the broad application of mechanochemical polishing.
22
I
C.
Figure 18S. Optical Micrograph and Electron Channeling Patternof Mechanochemically Polished GaAs
23
/b
g 1 h ~~~ d~lm~ ' ' ,- ..-L m -,.-,.loa.-in I .. . • * "•
Table 3. Potential Soft Abrasives for Mechanochemical Polishing of VariousMaterials
Material Abrasives
MgO A120 3 SrC0 3 , MgO, NiO, Fe2 03,
Colloidal Silica
Zr0 2 MgO, NiO, SrCO3 , Fe2 0 3 ,Colloidal Silica
sic Mgo, SrCO3
B4C SrCO 3 , MgO, NiO, Fe 20 3 ,Colloidal Silica
FGas MgO
24
.1 Section 3Discussion
As noted earlier, chemical reactions involved in the surface finishing can broadly beclassified into two categories, namely, solid-solid and solid-liquid. Since, in the past,
- mechanochemical polishing techniques involving solid-liquid. types of chemical reac-tions has been studied in greater detail than that involving solid-solid types ofchemical reactions, the emphasis of most of our efforts was on material removalthrough solid-solid types of chemical reactions, although clear distinction betweenthe two types of reactions was not always made.
The work performed in this program, and the earlier work of Yasunaga et al. 1, in-. dicates that solid-solid type of chemical reactions are common, can be induced at
pressures of the order of a few psi, and can be used for surface finishing. Yasunaga- et al. demonstrated mechanochemical polishing for single crystals of silicon, quartz
* and alumina. In the present work, we have demonstrated the feasibility of mechano-chemical polishing of Si3N4 and identified potential soft abrasives for many morematerials, such as B4C, SiC, and MgO.A120 3 (Table 3).
As compared to mechanochemical polishing of Si3N4 with Fe20 3 (Figures 6, 11 and12) or of Si with CaCO3 or BaCO3
1, the measured removal rates obtained by mecha-nochemically polishing A1203 with Si0 2 were much lower, in the range of 0.3-0.6#m/hr (Figures 3 and 4). The mechanochemical polishing approach pursued in thepresent work is likely to have a limitation of low removal rate for some more materi-als.
In addition to high removal rates, high-purity of polished surface and good flatness,I[ there is one more aspect of mechanochemical polishing of Si3N4 with Fe20 3 that
needs to be emphasized: the lack of orange-peel appearance. Reaction bonded Si3N4,and hot-pressed Si3N4 obtained from three different vendors were polished mechano-chemically in the present work. Mechanochemically polished surfaces of none ofthese Si3N4 showed any orange-peel appearance. Surfaces of many materials in thepolycrystalline form show orange-peel appearance when polished mechanochemi-cally using colloidal silica. In many cases, colloidal silica removes materialpredominantly through liquid-solid types of chemical reactions at rates which aresignificantly higher than those achieved by solid-solid type of mechanochemicalpolishing. For example, sapphire of orientation (1102) can be polished at a rate of
- over 25 pm/hr using colloidal silica.6
6H.W. Gutsche and J.W. Moody, J. Elec. Chem. Soc. 125, 136(1978).
25
-
One of the most important motivations for developing mechanochemical polishingtechnique for a given material is to see if the technique can be used for one-steppolishing of that material. For example, from as-ground condition to scratch-free anddamage-free condition in one mechanochemical polishing step. This requires high re-moval rate, whose magnitude varies from material to material. For many hard,polycrystalline ceramics in which the depth of damage introduced by good grindingpractices is of the order of a few tens of micrometers, a process that removes materi-al damage free at a rate of a few micrometers per hour could be used for one-steppolishing. Much higher removal rates are required for one-step polishing of manysemiconductors and glasses in which the grinding or sawing damage extends to adepth of several tens of micrometers.
There are several potential applications for a mechanochemical polishing processthat has a limitation of low removal rate. These applications are essentially the onesin which the objective is to improve the performance of a component by removing asmall amount of material. One example is the post polishing of a conventionallypolished material to remove fine scratches and a thin damaged layer or post
4 polishing of a colloidal silica polished material to remove orange-peel appearance aswell as to improve flatness. Another example is the post polishing of diamond-turnedoptics to remove a defect with record-groove appearance which is frequently ob-served.
Yet another example is in connection with an observation that is frequently maderegarding a contact polishing technique, conventional or mechanochemical. It is thepresence of abrasive grains near the polished surface. This may not be acceptable insome cases. For example, the workpiece may be a laser window which is polishedwith an abrasive A (because it removes material at a high rate) for use in a high-power laser system operating at wavelength X. If the abrasive A strongly absorbslight of wavelength X, then its presence near the polished surface will not be accep-table because it would reduce the damage threshold of the window. Assume that thewindow can be polished mechanochemically with an abrasive B, which removes ma-terial at a low rate but does not absorb light of wavelength X. In such a case, a sig-nificant improvement in the damage threshold of the window could be realized bypolishing it first with abrasive A, to remove the damage introduced by previousgrinding and lapping operations, and then with abrasive B to remove the residue ofpolishing abrasive A.
26
Index of Publications
1. K.D. McHenry, R.J. Stokes and B.G. Keopke, "Mechanochemical Polishing ofCeramics," presented at the American Ceramic Society Annual Meeting (1981).
a 2. J.P. Cummings, L. Weiman, R. Anderson, H. Vora and D. Kompelien, "Pro-cessing and Properties of a High Density 'Chromium/Copper System for Elec-tronics Packaging." Presented at the International Electronics Packaging Con-ference, Cleveland, Ohio, November 9 and 10, 1981.
3. H. Vora, R.H. Anderson and R.J. Stokes, "Surface Finishing Using SoftAbrasives" in laser induced damage in optical materials: 1981, H.E. Bennett,A.J. Glass, A.H. Guenther and B.E. Newman, editors. To be published as an NBSspecial publication.
4. H. Vora and R.J. Stokes, "Study of Mechanochemical Machining of Ceramics andb the Effect on Thin Flm Behavior," Annual Technical Report #N00014-80-
C-0437-1, Contract N00014-80-C-0437, June 1981.
5. H. Vora, T.W. Orent and R.J. Stokes, "Mechanochemical Polishing of SiliconNitride," J. Am. Ceram. Soc. 65, C-140 (1982).
6. H. Vora and R.J. Stokes, "Study of Mechanochemical Machining of Ceramics andthe Effect on Thin Film Behavior," Final Technical Report #N00014-80-C-0437-2,Contract N00014-80-C-0437, January 1983.
2
27
. .. ...".
Distribution List (Basic)(Technical and Summary Reports, November 1979)
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'32
D~IG
