J. Phys. Chem. Solids Pergamon Press 1960. Vol. 14. pp. 104-110. Printed in Great Britain.

STAIN FILMS ON SILICON

R. J. ARCHER

Bell Telephone Laboratories, Inc., Murray Hill, New Jersey

Abstract-A surface reaction resulting in film growth occurs on silicon immersed in 10-1-10-* molar solutions of nitric acid, sodium nitrite or nitrogen dioxide in hydrofluoric acid. The films also form as the result of a standard etching procedure. Three types of films are obtained depending on the resistivity and type of the subphase silicon. On the basis of silicon content, refractive index and other properties, it is tentatively concluded that one type of film is elemental silicon. The other types may be silicon hydrides. The rate of film growth is proportional to the reciprocal of film thickness and to the 3/2 power of the initial concentration of nitric acid or sodium nitrite. The rate of growth is nearly independent of subphase resistivity except for p-type silicon in the resistivity range from 0.1 n-cm to 0.001 Q-cm which encompasses a maximum in a plot of rate versus resistivity. The ele- mental silicon type films grow on subphases in this resistivity range while all other p-type material and all n-type subphases grow the hydride type films.

1. INTRODUCTION THE occasional formation of colored stain films on silicon following etching in HF-HNOs solutions is commonly observed. FULLER and DITZEN- BERGER(~) established that such films can be consis- tently grown in dilute solutions of nitric acid in concentrated hydrofluoric acid ; TUFWER@) and UHLIR@) report similar film growth during the anodization of silicon in hydrofluoric acid, and the film growth on germanium in HF-Ha02 solutions observed by ELLIS@) is probably a related phenomenon. This paper reports a study of the properties of stain films on silicon and of the staining process.

Before presenting the results, an instance of the occurrence of these films, of significance to those concerned with the preparation or study of silicon surfaces, will be described. Ellipsometer(s) measure- ments reveal that films 100-200 A thick consist- ently form on silicon surfaces in HF-HNOs etch- ing solutions when the dissolution action of the etch is quenched by dilution with water. Fig. 1 plots data showing this effect. Curve (1) is the result of a cleaning and etching procedure, de- veloped to give surfaces with minimal film thick- nesses, the final step of which requires quenching the HNOs-HF etch by adding a, large excess of nitric acid and then rinsing with water. If, instead, the etching action is stopped by diluting the etch

with copious amounts of running distilled water, curve (2) results. The thicknesses of the films following water quenching are an order of magni- tude larger than for nitric acid quenching, and

25

FIG. 1. Films on silicon surfaces following etching quenched by HNOs, curve (1) and by water, curve (2).

1 Q-cm p-type.

their chemical properties establish that they are the same substance as the thicker interference color range stain films.

The following sections summarize the physical and chemical properties of the stain films and of the staining process. The results include measure- ments of the composition, refractive index, density

104

STAIN FILMS

and electron diffraction patterns of the films and determination of the kinetics of the staining reac- tion and the dependence of all of these properties on the composition and concentration of the stain- ing solution and on the resistivity and type of the subphase silicon.

ON SILICON 10.5

Table 1. Weight per cent silicon content of jilms

2. PROPERTIES OF STAIN FILMS

Film type

I II III

Film oxidation Silicon analysis (wt.%) (wt. %)

88+8 91+3 97*1

102+5 100*3

The appearance of the films depends on the re- sistivity and type of the subphase silicon. The films may be classified into three types on this basis. Type I films exhibit normal interference colors. They form on n-type material of any re- sistivity and on all p-type material except for that in the resistivity range from 0.25 Q-cm to 0.001 Q-cm. Type II films show no interference colors beyond a certain thickness but are black. These films grow only on p-type silicon in a narrow resistivity range including 0.25 Q-cm and O-1 Q-cm. For these two materials, respectively, the films are black for thicknesses greater than those corresponding to the second order blue and first order brown interference colors. Type III films grow on p-type silicon in the approximate resis- tivity range from 0.1 Q-cm to 0.001 Q-cm. For subphases near the upper extreme of this range the films are invisible. They exhibit neither inter- ference nor intrinsic colors at any thickness. As the subphase resistivity decreases in the type III range, very pale interference colors are observed which increase in intensity with decreasing re- sistivity. For subphases lower in resistivity than O*OOl Q-cm, normal type I colors are observed.

rapid uptake of oxygen followed by a much slower subsequent uptake. The fast and slow uptakes re- sult, respectively, from oxidation of the film and of the subphase. After the fast initial oxidation, the film has the chemical properties of silica and readily dissolves in hydrofluoric acid. By deter- mining the masses of the specimen before oxida- tion, after oxidation and after removing the oxidized film in hydrofluoric acid, the ratio of the mass of the initial stain film to that of the oxidized film may be obtained. On the assumption that the oxidized film is pure silica, this ratio yields the pro- portion of silicon in the initial stain film. The re- sults of such measurements for all three types of films are given in column two of Table 1. The indicated errors are the average deviations of several determinations.

The silicon content of the stain films was deter- mined by two different methods. Firstly, the pro- portion of silicon in the film was measured by dissolving a weighed quantity of film in a dilute alkaline solution and determining gravimetrically the silica content of the solution by the standard analytical procedure. The results for films of types I and III are shown in the third column of Table 1. The indicated error of + 3 per cent was deter- mined by applying the analysis to solutions of known silicon content.

Within experimental error, the two analytical methods agree. It is concluded that type III films may be elemental silicon whereas type II and more certainly type I films are silicon rich compounds or mixtures. At the same time as the present work, but independently, TURNER(~) found that a stain film grown anodically in hydrofluoric acid was about 96 per cent silicon.

The second method for obtaining the silicon content of the films was by measuring the quantity of oxygen taken up by the films at high tempera- tures in an oxygen ambient. When a silicon specimen uniformly covered with a stain film is

The chemical properties of the films are similar to those of bulk silicon except that the films are more reactive. The films dissolve in aqueous alka- line solutions of pH greater than 9 at room tem- perature whereas bulk silicon is only attacked by hydroxide solutions at temperatures above 60- 70°C. They are oxidized at high temperatures in oxygen and in warm nitric acid much more rapidly than bulk silicon. There are considerable differ- ences in the rates of these reactions between the different types of film in the order

I > III > II

The films are insoluble in HF but are rapidly dis- heated in oxygen at 900-1000°C there is an initial solved by silicon etches.

106 R. J. ARCHER

The refractive indices of the film were measured by the method of kbELEd7). The method requires the determination of the angle of incidence at which monochromatic light polarized in the plane of incidence is reflected at the same intensity from the clean subphase and from the film covered surface. This angle is the Brewster angle for the film substance which is, of course, a measure of its refractive index. Experimental results are plotted in Fig. 2 as ,film refractive index at 5461 A versus

FIG. 2. Refractive index of film versus subphase resistivity.

subphase resistivity for both n- and p-type silicon. These optical properties account for the appear- ance differences between films of types I and III. The occurrence of type I films corresponds to the entire resistivity range giving the nearly constant refractive index of about 2*7.* Invisible and pale- colored type III films correspond to the broad maximum of the p-type curve at a refractive index value of about 4.1. The refractive index of single- crystalline silicon is 4.13.(s) Thus, the magnitudes of the optical discontinuities at the film-subphase interface explain the difference in appearance be- tween type I and type III films, and the agree- ment between the type III film refractive index and that of bulk silicon supports the view that these films are elemental silicon.

Knowing the refractive index, iilm thickness can be determined from interference color using Rollet’s table of optical thickness versus color.@) Thickness values thus obtained combined with measurements of surface density (g/cma) give the densities of the various films. For type I films the

* The point for 0.1 n-cm p-type was obtained for a film showing the first order brown color and may be considered of type I rather than type II.

density is 1.5 g/ems, for type III films (0.004 Q-cm p-type subphase) 2-O g/ems.

The results of electron diffraction measurements on film covered surfaces are ambiguous. Two measurements on type III films gave silicon single- crystalline arrays one of which also showed one sharp ring consistent with the pattern from poly- crystalline silicon. A type II film also gave the single-crystalline silicon pattern. One type I film gave a broad diffuse ring indicating an amorphous layer. Another type I film gave two interpenetrat- ing single-crystalline arrays, one of which matches silicon; the other is weak and unidentified. One point of ambiguity is whether the single-crystalline arrays indicate the structure of the films or of the subphase. If the former, film growth is epitaxial.

3. PROPERTIES OF THE STAINING PROCESS Films of uniform coverage and exhibiting inter-

ference colors through as many as 7 orders grow on freshly etched silicon immersed in 10-1-10-3 molar solutions of nitric acid in concentrated hydrofluoric acid. The rate of growth increases with concentration and depends on subphase re- sistivity and type. Rates of hundreds of A/set down to fractions of Ajsec are observed. Film growth is also obtained in solutions of sodium nitrite in hydrofluoric acid and of nitrogen dioxide in hydrofluoric acid.

Some samples of hydrofluoric acid do not pro- duce staining solutions even for nitric acid con- centrations as high as O-02 molar, and any sample of hydrofluoric acid is made inactive by exposure

TIME IN SECONDS

4 0.19 200 500

,“? ;0.‘0%7 ’

I I 0 100 200

TIME IN SECO%S 400 500

FIG. 3. Film growth rates on 0.004 R-cm p-type silicon as a function of nitric acid concentration in vol. Oh.

Note two time scales.

STAIN FILMS ON SILICON 107

to a large area of powdered silicon. It is assumed, that some impurity in the hydrofluoric acid is necessary to activate the staining reaction and that powdered silicon “getters” this impurity. Active staining solutions are very slightly brownish or blue-gray in color depending on concentration. Inactive solutions are colorless. “Gettering” has no effect on the staining action of sodium nitrite or nitrogen dioxide solutions.

Vigorous agitation or stirring stops film growth in nitric acid solutions but increases the growth rate in sodium nitrite solutions and is without effect on nitrogen dioxide solutions. The addition of urea to the staining solutions inhibits film growth. Intense illumination increases the growth rate on n-type silicon and decreases that on p-type. Normal room light does not affect the rate. Open beakers of the staining solutions decrease in strength with time.

The rate of film formation in room light at 25-O+ 0~2°C and the dependence on nitric acid concentration, expressed in volume per cent of concentrated nitric acid (70.6 per cent) in concen- trated hydrofluoric acid (48 per cent), is shown in Fig. 3. The silicon is O-004 Q-cm p-type which gives films more or less intermediate between types I and III with pale interference colors through 7 orders. Thickness is obtained from color using the refractive index 3 -96. The data obey a parabolic rate law reasonably well as shown by the linearity of the thickness-square plot of Fig. 4. The dependence of the parabolic rate con- stant on nitric acid concentration is obtained from the log-log plot of the slopes of Fig. 4 against

0 TIME IN SECONDS

FIG. 4. Film growth rates, thickness square versus time, as function of nitric acid concentration. Note two time

scales. 25°C.

10)ID.L 10-e 10-l CONCENTRATION- MOLES/LITER

FIG. 5. Thickness square versus time slopes as a function of concentration. (1) 0.004 C-cm p-type in I-IN08 solu- tions. (2) 10 lkm p-type in HNOs solutions. (3) 0.004

Q-cm P-type in NaNOa solutions.

concentration, curve (l), Fig. 5. The slope of the curve is 1.48 &- O-02, and all of the data of Fig. 3 may be expressed by

dL 3.73 x loyHNOa]‘-48

iii--= L (1

where L is thickness in A, time is in set, concen- tration* in moles/l. and the boundary condition t = ts at L = 0 obtains. Time ts is the induction period seen in Fig. 4.

Solutions of sodium nitrite in hydrofluoric acid give data with the same characteristics as those of Fig. 3 and 4. The corresponding log-log plot of parabolic slope versus concentration for such data gives curve (3) of Fig. 5. The data obey relation 1 when the coefficient of that expression is multiplied by (l/3)1*48, i.e. to produce staining solutions of equal strength the molar concentration of sodium nitrite must be 3 times greater than that of nitric acid.

Data for film growth (type I) on 10 Q-cmp-type silicon in HF-HNOs solutions is shown in Fig. 6. The lower curves exhibit a curious step-like

* Here and elsewhere “concentration” is used loosely to designate the number of moles of solute initially added to a liter of solvent.

108 R. J. ARCHER

TIME IN SECONDS

FIG. 6. Film growth rates on 10 Q-cmp-type silicon as a function of nitric acid concentration in vol. %.

growth. This phenomenon is consistently observed for type I films in dilute staining solutions but not for type III fi!ms. Another unusual aspect of the data is the approach to a limiting thickness shown by the upper two curves. Beyond a certain thick- ness (the starred points) the slopes decrease much more rapidly than parabolically. This behavior which is attributed to dissolution of the film by the solution is always obtained with p-type subphases of resistivity greater than about O-1 Q-cm but has not been observed for any other case. The data of Fig. 6 give reasonably linear plots on a thickness- square versus time scale up to and including the starred points and for all points on the lower curve. The results of such a plot give curve (2) of Fig. 5. The data are consistent with equation (1) if the constant (3.73 x 10s) of that equation is multiplied by l/3.

The dependence of the growth kinetics on sub- phase resistivity for p-type* silicon in 0.015 vol.% nitric acid solutions is shown in Fig. 7. The points on the four upper curves for invisible type III and black type II films were obtained by weighing the silicon specimen before and after dissolving the films in 20 per cent sodium hydroxide solutions. Surface density is then converted to thickness by means of the film densities given above. For type II films the density is unknown and the thick- nesses of the black films on the 0.1 Q-cm speci- men were obtained using 1.5 g/cm3 the value for type I films. Consequently these data may be in error by a constant factor.

* For allresistivities, the silicon is boron doped.

a 8 * 1

0 200 400 600 800 1ooo l2W 1400

TIME -SECONDS

FIG. 7. Film growth rates on P-type silicon as a function of subphase resistivity, noted on curves in Q-cm, in

0.015 vol. y0 HNOs.

The dependence of growth rate on subphase resistivity is shown in Fig. 8 where thickness, from Fig. 7, after 500 set in the staining solution is plotted against resistivity. Corresponding results for n-type subphases are also shown but the de- tailed data will not be given. The n-type thickness versus time data are very similar to the 1 and 10 Q-cmp-type curves.

The resistivity range encompassing the maxi- mum in the p-type curve of Fig. 8 corresponds to the range of type III film formation (except for the 0.1 R-cm point which is for a type II film). Very approximately the relative parabolic rate constants are obtained by squaring the thickness ordinate of Fig. 8. It is thus seen that at the maximum, type III films grow about 16 times faster than type I films. The relative differences in the growth rates on n-type and p-type materials for resistivities

04

= 10 IL.. I,., I.,, *... I..a

I 0.1 0.01 0.00 I

RESISTIVITY OF SUBPHASE OHM-CM

FIG. 8. Dependence of kinetics on subphase resistivity and type.

STAIN FILMS ON SILICON 109

greater than 0.25 &cm is real but is larger here than for similar experiments performed using more concentrated nitric acid staining solutions where the rate differences are almost within experimental error. The minimal rate for the 0.0007 SZ-cm p-type sample shown in Fig. 8 is observed for staining solutions of any concentration as is the small increase in rate with decreasing n-type sub- phase resistivity. The similarity of the character- istics of the refractive index curves, Fig. 2, and the rate curves of Fig. 8 is noteworthy.

4. DISCUSSION

There has been speculation that the stain film substance might be silicon monoxide, but the silicon content of the films precludes that possi- bility. A more likely hypothesis is that type I and perhaps type II films are silicon hydride and that type III films are elemental silicon or silicon rich mixtures of elemental silicon and silicon hydride. Solid, polymeric silicon hydrides with composi- tions varying from SiH to SiHs are described in the literature.@* 1s) Little work has been’ done on the properties of these substances, but the follow- ing pertinent observations are reported. Like stain films, the material dissolves in aqueous alka- line solutions. The compounds decompose to elemental silicon and hydrogen when heated to 1000°C. Consistent with this hydride property, type I films heated to 1000°C in vacuum dis- appear, that is, the subphase surface no longer shows an interference color. Analysis by mass spectrometry of the evolved gases from a type I film following such a heating gave about equal quantities of Hs and HsO. The total quantity of hydrogen evolved, both as Hs and as HsO, would indicate a formula SiHl,s for the particular film mass heated. However, this one run is not believed to be quantitatively valid because of the possibility that the film was partially oxidized (hence the evolved water) and that the film substance was not completely decomposed. Qualitatively, though, the result supports the hydride hypothesis. No Hs or HsO (beyond normal background) was ob- tained when a silicon specimen without a film was .heated. Finally, no fluorine, nitrogen or oxygen was evolved from the film. Further measurements are planned to establish quantitatively the hydro- gen content of the films. Within experimental error the proportion of silicon in type I films is

consistent with the formula SiHs. Finally, the formation of silicon hydride under staining con- ditions would not be a unique phenomenon since the formation of hydride films on a number of metals during dissolution in hydrofluoric acid has been observed. (1s)

The facts that type III films have refractive in- dices very close to that of elemental silicon, that their silicon content is 100 per cent & 3 per cent and that they are considerably less reactive than type I films, e.g. they dissolve much more slowly in alkaline solution, leads to the view that they are predominantly elemental silicon.

Consistent with UHLIR'S@) measurements on the anodic formation of stain films on silicon, the oxidation step in the film growth corrosion process is assumed to be

Si+Si+++2e (2)

The resultant two-plus silicon compound could be SiHs for type I films, or for films which are elemental silicon it can be supposed that an inter- mediate two-plus compound forms and spontane- ously decomposes to elemental silicon and some oxidation product. Most compounds which con- tain silicon in the two-plus oxidation state are thermodynamically unstable at room temperature.

In all of the staining solutions, the species NOs-, HNOs, NO2 and NO coexist. The cathodic or re- duction step of a corrosion reaction in this system is, according to VETTER(~~),

e + NOs + NOs- (3)

and the rate limiting step is normally the formation of NOs by the reaction

HN03 + HNOs a 2N02 + Hz0 (4)

This mechanism explains the fact that nitric acid solutions made with “gettered” hydrofluoric acid do not stain, since the reaction cannot occur until it is initiated by at least a trace of NOs or HNOs, and this trace comes from the reduction of nitric acid by impurities in the hydrofluoric acid which are removed by “gettering”. The reaction is auto- catalytic since once started, it leads to a build up in the concentration of HNOs at the interface. Agitation, in the case of nitric acid solutions, in- hibits film growth by dispersing the large inter- facial concentration of HNOs. The opposite or nil

110 R. J. ARCHER

agitation effects in NOs and NaNOs solutions must be related to the fact that these solutions con- tain relatively large concentrations of HNOa. Urea inhibits film growth by decomposing HNOa to Na.

The 3 :l molar ratio between sodium nitrate and nitric acid staining solutions of equal strength may be explained by the following decomposition of the nitrite

3HNOs --f HNOs + 2N0 + Hz0 (5)

The parabolic kinetics of the staining reaction suggests that the rate limiting step is the diffusion of a reactant through the growing film, but the 312 power dependence on the initial quantity of nitric acid or of sodium nitrite used to make the solutions has no obvious interpretation.

The abrupt change in the properties of the film and in their rates of growth at the onset of the maxima of Figs. 2 and 8, may reflect the electro- chemical nature of the corrosion process that causes film growth. Although a mechanism that explains the maxima has not been worked out, the following speculation may be pertinent. At those parta of the subphase surface which are cathodic- ally polarized, a space charge barrier exists in the silicon which limits cathodic current, and if this current were rate limiting for film growth an abrupt increase in rate would be expected when the breakdown voltage of the barrier was smaller than the voltaic e.m.f. of the corrosion reaction. The breakdown voltage depends on resistivity and for material in the resistivity range of the maxima is about l-2 volts.(13) This value is of the same order of magnitude as the e.m.f. of the reaction. The indicated mechanism could thus account for

the increase in growth rate, but is apparently in- consistent with the subsequent decrease in rate at lower resistivities and with the fact that the kinetics indicate a rate limiting step involving diffusion through the growing film.

Clearly, the formulation of a detailed mechanism consistent with all of the properties of this unusual reaction and of the resulting films will require further experimental effort.

The author wishes to thank R. D. HEIDENREICH and Mrs. M. H. READ for the electron diffraction measurements, J. P. WRIGHT for the silica analysis and E. E. FRANCOIS for the mass spectrometry determination.

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