Polishing of Sapphire with Colloidal Silica Henry W. Gutsche and Jerry W. Moody

Monsanto Company, E~ec~ronie Products Division, St. Peters, Missouri 63376

ABSTRACT

The polishing of sapphire with colloidal silica has been studied. Stock r e - m o v a l rates have been found to vary with the solids content in the polishing fluid and the temperature on the polishing pad. The pH of the polishing fluid has also been found to be a factor influencing removal rate. Because concen- trated silica solutions are unstable at high temperatures, a compromise be tween tempera ture and concentrat ion must be made. Still, practical removal rates over 25 ~/hr have been achieved. Work damage, an impor tant factor in polishing operations, has been found to extend about 1.0-1.5 mils into the surface of commercially available sawn or ground sapphire blanks. When all embedded diamond from the slicing and grinding operations is removed prior to polishing, a scratch-free, featureless surface is produced on which high quality, epitaxial silicon films can be deposited. C-MOS devices bui l t into the films without the use of any specific processing technique to minimize the effect of the AlzO3-Si interface showed n-channe l leakage of I • 10 -10 A / mi l with negligible wafer - to-wafer variations (1). The polishing of sapphire by colloidal silica is believed to follow a chemical reaction leading to a luminum silicate dihydrate as described in previous l i terature. For the reaction

A1203 + 2SIO2 + 2H20-> A12Si20~ �9 2H20 [1]

an activation energy of 14.6 kcal /mole has been calculated from the tempera- ture dependency oK the removal rate.

Sapphire, i .e., aA12Oz, is used to advantage as a dielectric substrate under th in silicon films for inte- grated circuits and microwave devices with highly desirable features which are difficult or impossible to achieve on bulk silicon. Nonetheless, high mate- rial cost and difficulties with leakage currents have l imited s i l icon-on-sapphire electronics to special ap- plications only. The high material cost is related par t ly to the present low level of production and need not concern us here. The leakage currents, however, are our concern. One source of leakage currents is the Si/A12Os interface where lattice mismatch is often aggravated by the use of substrate mater ial with me- chanically polished surfaces.

Mechanical polishing produces mir ror surfaces on the macroscopic scale only. "Semiconductor" polishing, i.e., preparing mater ial for electronic device work, must produce a surface which is whole, flat, and clean on the microscopic scale, better yet, on the atomic scale. Ideally, semiconductor surface preparat ion should produce a substrate that is enveloped by a natural , low energy, low index crystal plane that contains no rem- nan t work damage and is covered by a well-defined usable or easily removable protective film.

In the case of silicon, polishing with colloidal SiO2 has gained a reputat ion for producing a near ly per- fect substrate surface for even the most critical in te- grated circuit application. Polishing with colloidal SiO2 was patented in 1963 by Walsh and Herzog (2) as a mechanical process, and in 1966 as mechanical- chemical by Lachapelle (3), who found that the alka- l in i ty of the polishing fluid drastically affected the re- moval rate.

Silicon surfaces polished with colloidal SiO2 have been investigated by several authors (4-7). Techniques ranging from Sirtl etching before or after oxidation and epitaxial film growth to x - ray diffraction and ion backscattering have been applied. In all cases SiO2 polished surfaces have been found to be free of dam- age, free of contour, and free of contamination. As a consequence semiconductor polishing with colloidal SiO2 is now the worldwide accepted s tandard method for silicon substrate preparation. Colloidal SiO2 also works on many other substances: metals and glasses, for example. It is par t icular ly useful for polishing garnets (8).

Because of the success with garnets and silicon, it was therefore tempting to investigate whether col- loidal SiO2 could produce a similar surface on sapphire substrate. At first look such an endeavor seems ob- viously futile. When polishing silicon, with colloidal SiO2 the particles may crystallize and reach a hardness sufficient to provide some degree of gentle molecular abrasion (9). Also, the a lkal ini ty of the solution may be sufficient to chemically dissolve the substrate where friction generates enough heat on the surface of silicon. Surfaces of s - a l u m i n u m oxide, on the other hand, are extremely hard (Mohs' > 9) and allow even reactive melts at temperatures over 1000~ to dissolve only a few microns per hour (10). It was therefore rather surpris ing to find that colloidal SiO2, in the complete absence of diamond particles, does indeed ~oiish sapphire and produce a damage-free surface which is suitable for the growth of th in sili- con epitaxial films for device work.

Before SiO2 polishing of silicon, one physical im- perfection in part icular has been blamed for many de- vice failures: r emnan t surface damage, that is to say, work damage left in mechanical ly shaped slices after inadequate polishing or etching. Especially in epitaxial wafers, surface damage in the substrate has been re- lated to severe crystal disorder in the deposited film, often render ing the material useless for device work. Similar ly r emnan t surface damage in sapphire substrate under thin epitaxial silicon films must make it next to impossible to prepare a good si l icon-on-sapphire device. To date little has been published on processes for the preparat ion of a damage-free, flat, and clean sapphire-polished slices. Authors usual ly acknowledge the existence of "work" damage in the diamond-pol- ished substrate surfaces they work with but rely then on "healing by anneal ing" for a remedy.

In si l icon-on-si l icon epitaxy only a slight lattice mismatch exists between the usual ly heavily doped substrate and the usual ly l ightly doped film. It still leads to measurable tensile s train in n -on -p wafers however. In s i icon-on-sapphire epitaxy, in addition to a considerable lattice mismatch [6% for {100) Si on (1]-02) sapphire] (11), the large difference in their respective coefficients of thermal expansion cause the silicon film to sustain 8 • 109 dyne /cm 2 strain when grown at 1000~ and cooled to room temperature.

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Vol. 125, No. I P O L I S H I N G OF S A P P H I R E 137

Therefore, for the preparat ion of usable epitaxial sili- con films on sapphire, absence of damage in the sub- strate surface becomes an extremely impor tan t factor.

Experimental Results Crystals.--The slices used were sawed from 2 in.

diam sapphire crystals purchased from Union Carbide, San Diego, California, and from Crystal Systems, Salem, Massachusetts. The slice surfaces were oriented to wi th in +_2 ~ of the (1• plane by Laue back- reflection. Sawing was done on a Silicon Technology saw, Silicon Technology Corporation, Oakland, New Jersey, equipped with a rotat ing ingot holder.

Slice preparation.--After sawing, the slices are 20 mils thick. They were fine ground to 14 mils on a Mueller grinder, Kugel-Mueller , Nurenberg, Germany, equipped with a 230 grit diamond wheel. To achieve complete absence of diamond which is necessary for scratch-free polish and to minimize bow after pol- ishing, the ground slices are annealed in 02 at l l00~ for 2-4 hr.

Polishing.--The polishing step was carried out on an Elgin 1-20 machine, Elgin Tool Company, Whitehall , Michigan, equipped with an oscillating pressure arm. A PR-30 polishing cloth, Process Research Products, Pennington, New Jersey, was found suitable, but non- perforated Pel lon and Buehler Microcloth was also useful, albeit quick to wear out. Stock removal rate was calculated by measur ing the thickness with a dial micrometer at 5 points on the wafer periodically dur ing polishing. The fluid Was Syton HT-40, Monsanto Com- pany, St. Louis, Missouri.

It was found that the thin, heavily damaged layer of a freshly ground wafer was removed at stock re- moval rates of well over 50 #/hr. The removal rates listed in the following tables and figures were mea- sured after this heavily damaged layer was removed and when the rate became independent of total stock removal. Therefore, the removal rates given are be- lieved to be characteristic of the undamaged (1~02) sapphire plane under the conditions described.

The influence of various process variables on the speed of polishing expressed as rate of stock removal is shown in Tables I-IIL Table I indicates that in- creasing the solids content of the colloidal SiO2 solu- tion from about 4 to 25% almost triples the removal rate from 6 to 17 ~/hr. Fur the r increase in solids con- tent does not seem to help matters much, probably because SiO2 solutions with solid contents over 16% become ra ther unstable under polishing conditions. The SiO2 particles agglomerate, precipitate, and finally crystallize thereby reducing the effective concentra- tion of the solution. Therefore, practical removal rates do not exceed 12 ~/hr in strongly basic solutions.

Fortunately , as Table II shows, the speed of polish- ing is also influenced by the pH of the fluid which can be adjusted by carefully adding 0.hN HC1. Maximal removal rates occur at pH values between 7 and 8. In this manne r 10% solid solutions which remove 11-12 ~/hr at pH 10.5 will "cut" at 19-20 ~/hr at pH 8.

When polishing sapphire with colloidal SiO2, abra- sion can be ruled out as cause for stock removal be- cause sapphire is harder than the SiO2. Stock removal must occur because of chemical reaction, and it comes

Table 11. Removal rate vs. pH of fluid (Temperature 78~ solids 10%)

Removal rate pH (Mhr)

10.5 11.4 IO 14.0 9 15.2 8 19.2 7 17.8 6 11.4 5 10.2 2 3.6

as no surprise that the removal rate is also dependent on the tempera ture at the surface of the polishing pad as shown in Table III. This temperature, as moni- tored using a Fenwa l -Spa r t an 2040 LTF infrared thermometer, is adjustable by varying the pressure of the polishing arm and the flow and tempera ture of the polishing fluid onto the turntable . We found that 82~C is about the highest operating tempera ture com- patible with the stabil i ty of both the polishing fluid and the pad adhesive. Table I l I shows that the removal rate increased with tempera ture up to the max imum tempera ture employed. For practical, rout ine work a 16% solids, pH 8 fluid is recommended at 82~ pad temperature. This will "cut" the (1102) plane sapphire at rates --~25 #/hr.

Reaction Mechanism The surprising fact that SiO2 reacts at all with t h e

extremely hard and near ly iner t sapphire can be ex- plained in part by the thermodynamics of the t r i - part i te system A12OJSiO2/HeO. In the simplest case A1203 ~- SiO2 --> A12SiO5 such common, na tura l ly oc- curring a luminum silicates as kyanite, andalusite, or mull i te may form. All three reactions are thermody- namical ly possible, the mull i te formation being the most l ikely of the three at 400~ which is about the temperature on the surface of the sapphire dur ing the polishing process. The free energies of these reactions are (T _-- 400~ AF (kyani te) = --18 cal /mole; MF (andalusite) ---- --55 cal/mole; and hF (mull i te) ---- --377 cal /mole calculated from enthalpy and entropy data published by the Bureau of Mines (12). The re- action A1203 (a lpha-a lumina) + SiO2 -~ A12SiO5 (mulli te) is endothermic as might be expected and requires an enthalpy change of -~5100 cal /mole at 400~

Natura l ly these data merely indicate that AluO3 and SiO2 may react at temperatures only slightly above the boiling point of water. However, what actual ly happens dur ing polishing probably has been described by Schwartz and Brenner (13) who found that "in a neutra l A12Os/SiOJH20 system the stable compound A12Si.20~" 2H20 with a s t ructure identical or very ~imilar to na tura l kaolin is capable of forming," and, ' i t is uninfluenced by the presence of an excess of silica. It is formed with par t icular readiness when at least 6 moles of SiO2 are present for each mole of A12Q." Since we are observing in our polishing sys- tem maximal stock removal rates at pH values be- tween 7 and 8, we suggest that the formation of A12Si207 �9 2H20 as described by Schwartz and Brenner can indeed explain the chemistry of polishing s a p -

Table I. Removal rate vs. percent solids (Temperature 78~ pH 10.5)

Table III. Removal rate vs. temperature (Solids 10%, pH 10.5)

SiO2 solids Removal rate Temper- Removal rate (% weight) (~/hr) ature (~ ( ~ h r )

4 6.1 64 5.3 8 10.7 74 9.1

12 11.4 78 11.2 16 11.7 80 13.2 20 14.7 81 15.7 26 17.3 82 16.5 40 17.8 83 26.0 (16% solids)

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138 J. Electrochem. Sot.: S O L I D - S T A T E SCIENCE A N D T E C H N O L O G Y January 1978

phi re wi th colloidal silica at or near the boil ing point of water . Indeed, a s l ight loss of weight can be mea - sured when rough sapphi re b lanks are s imply im- mersed in a boil ing sil ica solut ion of pH 7-8.

To proceed wi th measurab le speed the reac t ion must have avai lab le th ree condit ions: (i) Heat must be sup- pl ied to suppor t the reaction; (ii) the react ion produc t must be cont inual ly removed from the sapphi re sur - face; and (iii) the reactants must be forced into in t i - mate contact at all times. Al l three conditions are met dur ing the pol ishing process. The sapphi re surface is pressed agains t a fas t - ro ta t ing high fr ic t ion polishing cloth. The polishing cloth serves as vehicle and hea te r for the pol ishing fluid and by moving the Syton pa r - ticles over the surface of the sapphi re under pressure, cont inuously "shaves off" the react ion produc t the reby constant ly exposing f resh sapphi re surface to the SiO2 in the polishing fluid. The react ion product can some- t imes be observed under the e lec t ron microscope in the form of t r iangular , appa ren t ly oriented, p rec ip i - tates ,.,IO00A in d iamete r which wipe off wi th a moist cotton swab (Fig. 1).

Stock remova l in Syton polishing of r is therefore en t i re ly chemical. No abrasive, f ractur ing, or o therwise mechanica l act ion removes la t t ice con- s t i tuents in the sys tem described. Therefore, this sys- tem will not produce a flat, featureless surface as long as damage exists in tha t surface, s imply because a damaged surface e lement wil l a lways polish fas ter than any damage- f r ee surface e lement next to it. Since good phase -con t ras t optics make vis ible ad jacen t height differences as shal low as 30-50A, microscopic examina t ion of c lean Sy ton-po l i shed surfaces consti- tutes a sensi t ive test for absence of r emnan t surface damage. In fact, x - r a y rocking curves showed absence of damage, indica t ing s t ra in in all samples tha t were polished unt i l thei r surface appeared featureless at 100• and m a x i m u m phase contras t posi t ion of the Nomarsk i a t t achment to a Reicher t microscope.

C-MOS devices bui l t into the films wi thout the use of any specific processing technique to minimize the effect of the A1203-Si in ter face showed n-channe l leakage of 1 • 10 -10 A / m i l wi th negl igible w a f e r - t o - wafer var ia t ions (11).

The act ivat ion energy for the polishing react ion was de te rmined f rom the t empe ra tu r e dependency of the react ion ra te as shown in Fig. 2. F rom the slope of the l ine the act ivat ion energy was calcula ted as 14.5 k c a l / mole.

Conclusions Silica solut ions have been found to be an effective

agent for pol ishing sapphi re wafers for S - O - S device work. Under p roper condit ions of pH and tempera ture ,

Fig. I. Reaction products on sapphire surface; electronic micro- scope 10,000 X,

.7

61

.5

.4

.3

.2

o \ A = R In kl

\

\

\ O\ 0 \

\

\

\

\ \

= 14,610 cal

T = 60-84 ~ C

2.82 2.90 3.00 T x 10 -3 K

Fig. 2. Removal rate vs. temperature.

stock removal rates ~-25 ~ /hr can be realized. I t is be- l ieved that the polishing resul ts f rom a chemical re - action be tween the sapphi re and the sil ica par t ic les in which a lumina sil icates are formed. Since abras ion can be ru led out comple te ly and a chemical react ion is involved, this pol ishing procedure produces a sur- face which must be free of al l r emnan t work damage.

Acknowledgments The authors wish to thank Mrs. Brenda Schulte for

her pat ience and help wi th car ry ing out the exper i - ments. Thanks also go to Drs. J. Fay r ing and D. Dohm of the Physical Science Center of Monsanto for the i r va luable EM and x - r a y work on the po l i shed-sapphi re surface. We also apprec ia te very much the device work pe r fo rmed by Dr. H. Hawkins of Hughes Ai rc ra f t Company to evalua te the influence of the A1203-Si in terface on the electr ical character is t ics of C-MOS structures.

Manuscr ip t submi t ted Apr i l 25, 1977; revised m a n u - scr ipt received Aug. 19, 1977.

Any discussion of this paper will appear in a Discus- sion Section to be publ ished in the December 1978 JOTJRNAL. All discussions for the December 1978 Discus- sion Section should be submi t ted by Aug. 1, 1978.

Publication costs o] this article were assisted by Monsanto Company.

REFERENCES 1. D. M. Hawkins, P r iva te communicat ion. 2. R. J. Walsh and A. H. Herzog, U.S. Pat. 3,970,273. 3. R. L. Lachapel le , U.S. Pat. 3,323,141. 4. H. W. Gutsche, Pa!oer presented at the 6th IEEE

Microelectronics Symposium, Clayton, Mo., June 21, 1967.

5. E. Mendel, Solid State Technol., 10, 27 (1967). 6. A. Mayer , RCA Rev., 31, 414 (1970). 7. T. M. Buck and G. H. Wheat ley , Pape r presented at

the Amer ican 'Vacuum Society Fa l l Meeting, Seatt le , Washington, October 1969.

8. M. F. Ehman, This Journal, 12, 1240 (1974). 9. E. Rabinowitz, Sci. Am., 218, 91 (1968).

10. P. H. Robinson and C. W. Mueller , Trans. MetalI. Soc. AIME, 236, 268 (1966).

11. P. A. Larssen, Acta Crystallogr., 20, 599 (1966); C. Y. Ang and H. M. Manasevit , Solid-State Elec- tron., 8, 994 (1965).

12. K. K. Kelly, Bull. 584, Bureau of Mines (1966). 13. R. Schwartz and A. Brenner , Chem. Ber., 56, 1933

(1923).

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