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Monosilane adsorption and initial growth stages of silicon layers on the (100) and oxidized silicon surfaces
i
Surface Science North-Holland
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“surface science
275 (1992) 433-442
Monosilane adsorption and initial growth stages of silicon layers on the (100) and oxidized silicon surfaces Ellipsometric V.N.
investigation
Kruchinin,
S.M.
Repinsky
and
A.A.
Shklyaev
Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk 90, Russian Federation Received
18 February
1991; accepted
for publication
5 February
1992
Monosilane adsorption kinetics on (100) and oxidized silicon surfaces has been studied by means of a fast-response ellipsometer. Monosilane adsorption on the (100) silicon surface is irreversible and has no activation barrier in the temperature range 20-500°C. Condensation coefficients of SiH, for the closed and the flowing adsorption technique are rr = 7 x 10-s and 5 x lo-*, respectively. The thickness of the adsorption layer increases in the temperature range 121%350°C which is evidence for the formation of a surface supermonolayer coverage. At T > 350°C decomposition of adsorption complexes takes place, accompanied by hydrogen desorption. This process is characterized by an activation energy of E = 44 * 2 kcal/mol. Monosilane is adsorbed reversibly on the oxidized silicon surface in the temperature range 350-490°C and at a pressure of P < 5 X lo-’ Torr. At pressures P > 5 X lo-* Torr adsorption complexes decompose and amorphous silicon grows. The growth rate is directly proportional to the monosilane pressure in the gas phase and increases with increasing temperature; the activation energy of this process is E = 56 * 6 kcal/mol. The analysis of the kinetic curves indicates that the growth of amorphous silicon is a chain process at the initial stage.
1. Introduction Monosilane is one of the most widely used reagents in the gas-phase processes of the growth of various layers. Many authors treat monosilane adsorption as one of the obligatory initial stages [l-3]. The investigation of monosilane interaction with silicon is of great interest, but for a long time the experimental data concerning the adsorption ability of monosilane on the semiconductor surface were causing contradictory comments [4,5]. The number of works devoted to monosilane adsorption on silicon is rather small [6,7], and this problem is still actual. The present paper deals with the investigation of silane adsorption kinetics on the silicon (100) surface in the temperature range 20-500°C by means of automatic ellipsometry in a high-vacuum 003Y-6028/92/$05.00
0 1992 - Elsevier
Science
Publishers
system. Taking into account that the comparison of the adsorption abilities of the pure silicon surface with the surface coated with a thin layer of natural oxide may be of special interest, we studied also the regularities of monosilane adsorption on the oxidized silicon surface in the temperature range 350-490°C and the initial stages of the growth of amorphous silicon layers. The results obtained in the present paper, together with the data on silicon oxidation by 0, and N,O published earlier [8,9], determine the conditions of SiO, layer formation by the interchange of oxygen and monosilane exposure. This method of the synthesis of SiO, layers is valuable for obtaining a thin dielectric layer, its thickness being defined by the number of gas exposures. Molecule deposition can be used as a controlled method for the semiconductor-dielectric inter-
B.V. All rights reserved
434
V.N. Kruchinin
face formation while obtaining on A3B5 compounds [lo].
et al. / Ellipsometric
MOS
inuestigation
structures
2. Experimental Experimental procedure and high-vacuum ellipsometric technique have been described earlier Ill]. p-type samples with the resistivity of 10 Q. cm, dimensions of 20 X 5 X 0.45 mm3 and (100) surface orientation were used in the experiments with a clean silicon surface. The samples were subjected to chemical-mechanic polishing and treated with peroxide-ammonia etchant. Final purification of the surface was conducted in high vacuum (P = lo-” Torr) by annealing the sample at T = 1200°C. Sample heating was accomplished by transmitting a current. The necessary sample temperature was set after annealing with the help of the stabilized current source. Temperature was controlled ellipsometrically, with an accuracy of i3”C, using the known temperature dependences of the optical characteristics of silicon [12,13]. Two series of experiments were carried out on the (100) silicon surface, the first in the flowing system with the opened ion pump under the constant monosilane pressure P = lop6 Torr, and the second in the closed system with a closed pump and turned-off ionization manometer, the gas pressure constantly increasing. The pressure was increased at a rate of 1.7 X 10e6--3.1 X lop5 Torr . s-l. At the end of the gas exposure the = 10-2 maximum monosilane pressures were Torr. The pressure in the chamber was determined by means of a thermocouple pressure gauge PMT-2. When the ion pump was opened the monosilane was pumped out down to a pressure of lo-” Torr for a few seconds. A oxidized silicon surface was prepared by oxygen chemisorption on the clean surface at T = 350-490°C and 0, exposure of 104-10” L. The observed changes of the ellipsometric angles showed that the oxide layer formed on the silicon surface is not thicker than 7 A. The surface was ready for the experiments after oxygen evacuation. The whole experimental cycle consisted of: sample annealing at T = 12OO”C, preparation of
of monosilane adsorption on silicon
oxidized surface, monosilane exposure and the evacuation of the system. The whole experimental cycle was repeated before each new monosilane exposure. Monosilane exposure in the experiments with the oxidized surface was carried out under the following conditions: the pulsed regime of the exposure; the closed pump valve; the absence of the ionization sources until the necessary pressure was reached. Then the pressure in the chamber was kept constant. The maximum pressure was 1.6 X 10-l Torr. Commercial monosilane, 98% purety, was used. The impurities were controlled by a mass-spectrometer IPDO-2A. The main impurity was argon; no other impurities were found in monosilane. In order to treat the results of the ellipsometric measurements on clean silicon surfaces we used the approximate linearized shapes of the basic equation of the ellipsometry. The linear dependence of the variations of polarization angles on the effective thickness of the adsorption layer has been applied 1141: SA = d - A = [d, ST = q - q, where 3, F and A, W are the initial and current values of the polarization angles, 5 is a coefficient depending on the reflective system properties, d is the effective thickness of the layer. The decrease of T during gas chemisorption on the atomically-pure semiconductor surfaces cannot be explained within the simple model of the monolayer reflective system and seems to be connected with the variation of the optical characteristics of the adsorbed surface layer [14,15]. That is why the futher quantitative treatment was based only on the changes of the angle A. The calcuiation of the coefficient 5 was done at A = 6328 A, 4(, = 70” and Nsi = 3.865 O.O23i, where 4(, is the angle of incidence of the light beam on the sample, A is the working wave length of the ellipsometer and Nsi is the complex index of reflection of silicon at room temperature for the current A value. The calculation of the adsorbed layer thickness was done based on the Lorentz-Lorenz equation. Table values were used for SiH, polarizability. The molecular volume was taken equal to the volume of the molecule in the liquid phase. With the calculated value of the effective refractive index 1.49, 5 was found to be 0.3”/A.
435
KN. Kruchinin et al. / Ellipsometric inuestigation of monosilane adsorption on silicon
In order to estimate the effective thickness of the adsorbed layer of monosilane molecules on the oxidized silicon surface we used the data obtained in accordance with the optical model of the double layer refractive system. The complex refractive coefficient of the substrate was taken Nsi = 4.079 - 0.06, as for the silicon sample heated up to 350°C. The thick?ess of the lower oxidized layer being equal to 7 A, and the refractive index of the oxidized layer was taken 1.46 as for the SiO, layer [ill.
3. Experimental
1 x00
~0
T°C
Fig. 2. Dependence of the peak of the variation of polarization angle A on the sample temperature in the closed system corresponding to the saturated monosilane coverage.
results and discussion
3.1. Adsorption kinetics of SiH, on the (100) silicon surface Typical kinetic dependences of the variations of the polarization angle A in the adsorption process are given in fig. 1. Polarization angles A and V were decreasing, which is characteristic for chemisorption on atomically clean semiconductor surfaces [14]. When the adsorption is completed the kinetic curves of the variations of the polarization angle reach constant values depending on the silicon temperature and the way it was exposed to monosiIane. In the case of monosilane exposure in the closed system the variation of the angle A reached
‘6A fHiN)
0
0
3
400
200
300
400
t fS) Fig. 1. Typical dependences of the variations of the polarization angle d on time during monosilane adsorption. l-3: experiments are carried out in a closed system at temperature 20, 250 and 350°C respectively. The rate of increasing SiH, pressure (Torr.s-I): 1 - 2.8~10-~, 2 - 2.2~10-~, 3 2.6 X 10-s, 4 experiments carried out in a flowing system at 2O”C, P = 1 X 10c6 Torr.
its maximum value at a certain temperature (fig. 2). In the temperature range between 20-125°C the angle peak values do not change. This corresponds to the constant thickness of the adsorption layer. According to the approximate ellipsometric estimations the effective thickness of the layer is 2 A. We shall assume now that this value corresponds to the monosilane monolayer surfa$e coverage. The length of the Si-H bond is 1.48 A, so this assumption seems quite reasonable. With further temperature increase the thickness of the adsorption layer increases until a maximum value of 4 A at 350°C is reached and then it decreases rapidly. When the adsorption process was completed and the curves reached saturation, the admittance valve was closed and the gate of the ion pump was opened. In the experiments carried out at temperatures below 350°C no distinct changes on the curves were observed which is evidence for the irreversibili~ of the adsorption process. At temperatures higher than 350°C the polarization angles relaxed to some final values 6A = 11’ and SF = 6’ (fig. 31, these two values being independent of temperature and staying constant, after the relaxation was completed, for all the further silane exposures (curve 2, fig. 3). With increasing the temperature the relaxation rate increased too. No influence of the ion pump on the relaxation rate was observed. The temperature dependence of the relaxation time of the polarization angle to the value equal to 0.5 of its maximum change was obtained during the relaxation. Using this depen-
436
V N. Kruchinin
6AI
‘~~ 0
et al. / Ellipsomefric
inl~estigafion
,
a00 400 600 t(s)
Fig. 3. Relaxation of the polarization angle d. 1 - the first exposure. 2 - repeated monosilane exposure. The moments the admittance valves are switched off and the moments of the beginning of monasilane exposure are marked with arrows. measurements were made in the flowing system, at 41WC.
dence we calculated the activation energy of the relaxation process to be 44 + 2 kcal/mol. The analysis of the kinetic curves shows that monosilane adsorption on SitlOO) in the region of small surface coverage (6 < 0.5) has no activation barrier and follows the kinetic laws of ideal adsorption. The rate of the adsorption process can be written as: d0 UP = K,( 1 - O)PSiH, = ,,2(1 dt N\( 27TmkT)
-O)Y
(1) where I(, is the adsorption rate constant; Psi,+, is the monosilane pressure, rr is the condensation coefficient, N, is the density of the surface atoms. The calculated values of the condensation coefficients (T for the closed and the flowing adsorption techniques are 7 X lo-’ and 5 X lo-‘, respectively. The difference between the two (7 values seems to be caused by the presence of atomic hydrogen formed in the flowing system due to ionization because the condensation coefficient of atomic hydrogen on Si is nearly unity for the initial stages of adsorption [13]. The chemisorbed SiH, molecule is expected to decompose [31. There is no common opinion on the monosilane decomposition products on Si, but it should be mentioned that the formation of the =SiH z and -SiH type complexes was observed in ref. 1171 where the adsorption of atomic hydrogen on Si(ll1) was studied by an EELS method. The formation of similar complexes seems to be probable also during monosilane
of mono.silane
adsorption
on ,siliwn
adsorption. Thus, the adsorption process of monosilane on the Si(100) surface at temperatures between 20 and 125°C is stopped when the surface becomes coated with a monolayer of SiH, dissociation products. The presence of atomic hydrogen in the flowing system leads to the preferred adsorption of hydrogen on the active surface centres which is, in turn, the reason of the decrease of the effective thickness of the adsorption layer (fig. 1, curve 4). At temperatures higher than 125°C the hydrogen atom mobility increases so that the reaction between monosilane molecules from the gas phase and the adsorption complex becomes possible, accompanied by hydrogen desorption. The effective thickness of the adsorption layer increases. The decomposition of adsorption complexes starts only at T> 350°C and the experimental curves show the relaxation of the polarization angles (fig. 3) which seem to correspond to hydrogen desorption. The desorption rate can be written as foilows: d8 = K,,B dt = rIV,o exp[ ( -44
i 2 kcal/mol)/RT]
,
(2)
where K,, is the decomposition rate constant and y is the vibration frequency of the chemisorbed hydrogen atoms. It was shown [7,16,18] that there arc two forms of hydrogen desorption from a silicon surface. The first form reaches a maximum rate at 375400°C with the activation energy of 44 kcal/mol. The maximum rate of the second des~~rption form is reached at 500-54u”C. Its activation energy was measured to be 57 [16] and 50 [IX] kcal/mol. Two adsorption forms were also observed during the decomposition of amorphous hydrogenated silicon layers. The authors [16,1X] assumed that the low temperature desorption form is connected with the decomposition of =SiH 2 type centres and their transformation into the =SiH type centrcs. while the high temperature desorption employs futher decomposition of centres and complctc removal of hydrogen from the silicon surface. The polarization angle relaxation (fig. 3) is likely to represent the process of hydrogen removal
l/N. Kruchinin
et al. / Ellipsometric
investigation
from the surface which corresponds to low temperature desorption. The close value of the activation energy of relaxation and the activation energy of desorption [16,18] testify to this assumption. The values of the polarization angles SA, and ST,. remaining after complete relaxation show that hydrogen remains in those centres on the silicon surface which decomposes giving rise to high-temperature desorption. This proposal is confirmed by the proximity of 6A, and Sqr values to 6A = 12’ and 6V = 7.2’, obtained in ref. [14] for monolayer coverage on a Si(ll1) surface. Hydrogen removal corresponding to high-temperature desorption form starts at temperature above 490°C; this leads to a noticeable decrease of 6A, and Sqr values. For 0 > 0.3 a deviation from the laws of ideal adsorption was observed on the kinetic curves. This deviation may be the result of the interaction in the adsorption layer and the shifts of the substrate atoms. That the deviation from the ideal adsorption laws is observed only during the first monosilane exposure (fig. 3, curve l), while during futher gas exposures after the relaxation the adsorption follows the laws of the ideal adsorption kinetics (fig. 3, curve 2). The value of u is 8 X 10e2. Such a change of the adsorption ability is associated with a change of the surface. The initial clean Si(100) surface is reconstructed due to surface interactions. After the monosilane evacuation and desorption at 350-500°C the surface remains partly covered with hydrogen, having no superstructure and is able to contain a considerable amount of unsaturated silicon bonds which act as the adsorption centres. In this case (T increases and the adsorption follows the laws of the ideal adsorption kinetics, because there is not the necessity of surface reconstruction conditioned by destroying a superstructure. The process of monosilane adsorption up to 200°C can be described by the following stages: SiH,(g)
-
-SiH,(a)
of monosilane
adsorption
on silicon
437
200-350°C supermonolayer coverage is observed which can be linked with the formation of complexes: SiH,( g) + =SiH,( a) + =SiH - SiH,(a) =SiH,(a)
+ =SiH,(a)
or
-+ =SiH *(a) + H(a),
SiH 4( g) + =SiH - (a) + =SiH - SiH,( a). At temperatures higher than 350°C the decomposition of complexes and the desorption of hydrogen molecules occur. The kinetic analysis of the suggested mechanism shows [20] that under the stationary conditions of the process when a silicon layer should grow on the K, ’ KaPSiH,~ surface. Under our experimental conditions this is the case for T > 350°C. It sould be mentioned that the optical constants of the growing silicon layer are close to the substrate constants because the accumulation of the layer is not fixed by the ellipsometer after adsorption and further keeping of the sample in the adsorbate atmosphere for a long period of time. 3.2. Adsorption silicon surface
kinetics
of SiH,
on the oxidized
A typical dependence of the variation of the polarization angle A with time during monosilane adsorption on the oxidized silicon surface is shown in fig. 4. Both angles d and P decrease during SiH, adsorption on oxidized and clean Si(100) surfaces. The angle variation reaches some peak value depending on the pressure (fig. 4, curve 1).
+ H(a), I , I
-SiH,(a)
+ H(a)
+ H(a).
Hydrogen does not desorb till 200°C [16] and, judging from ref. [191, -SiH,(a) particles can be present on the surface. In the temperature range
0
500
4000
, t
6)
Fig. 4. Variation of the polarization angle A with time during monosilane adsorption on the oxidized silicon surface. PSIH4 = 8.9X 10-I Torr, T = 430°C. The moments of the beginning of the evacuation are marked with arrows.
438
VN. Kruchinin
et ul. / Ellipsomrtric
iwestigation
Fig. 5. Dependence of maximum variations of the polarization angle 3 on monosilane pressure in the system for sample temperatures: 350. 410. 470°C. A dashed line marks the region of the isotherm where the deviation from the Langmuir law is observed.
The dependence of the maximum of the variation of angle A on the gas pressure in the system is represented in fig. 5. The adsorption isotherm so obtained can be described by the Langmuir isotherm up to the pressure of 5 X lop2 Torr:
Oeff =
aPSitl
1+
“ps:Ha ’
(3)
where Psi), is monosilane pressure in the system, Oerf is the iffective degree of surface coating by SiH, molecules. When the monosilane pressure is higher than 5 X lo-* Torr the isotherm is observed to deviate sharply from the Langmuir law. This can be evidence to the change of adsorption type and formation of a supermonolayer coverage on the surface. One can estimate the value of Orff corresponding to this transition using the data obtained with the help of the double layer reflective system model. Assuming that oxidized surface had no active adsorption centres, Bet, was then taken to be equal to the effective thickness of the adsorbed layer defr divided by the diameter of monosilane molecule d,: Oeff = deff/d,,. Since the value of de,.,. corresponding to the transition is = 1 A and d, = 1.97 A, O,,,-= 0.5. The coefficient a from the Langmuir and law was calculated to be 35 Torr- ’ between 350 and 470°C. For an initial monosilane pressure less than 5 x 1OY’ Torr no appreciable changes were observed on curve 1 (fig. 4) after the angles reached their maximum values. After the evacuation of the system, the angles relaxed to values close to
of monosilanr
adsorption on silicon
the initial ones (see curve 2, fig. 4) which is likely to be evidence for reversible adsorption. The thickness of the adsorbed layer (1 A at P < 5 X lo-’ Torr) is = 0.5 of the monolayer thickness formed by monosilane adsorption on the clean silicon surface. On the other hand, our experiments showed that monosilane adsorption is not observed on the oxidized surface at room temperature. This leads to the assumption that at temperatures higher than 350°C monosilane is partly decomposed on the oxide-coated surface: SiH,(g)
+ -SiH,(a)
SiH,(g)
+ -SiH,(
+ H(a), a) --) -SiH
- SiH,( a) + H,(g). (4)
The
first
stage
provides a coating d = 1 A for Torr. At higher pressures a stage of the second type appears and the thickness of the coverage increases. For pressures higher than 5 x lo-’ Torr, further change of the polarization angle A and q was observed (see, fig. 4, curve 3). The change rate of the polarization angle was permanently growing up to a maximum depending on the monosilane pressure and the sample temperature. The length of the acceleration period till the change rate of the angle reached its maximum was varied in our experimental conditions from several tenths of seconds to tens of minutes and generally decreased with monosilane pressure increasing as t - PGiif’“-2. At higher temperature the change rate of the polarization angle increased, which resulted in the shortening of the acceleration period. The maximum change rate of A was proportional to monosilane pressure (fig. 6). The activation energy was determined from the temperature dependence of the maximum rate of the angle variation; its value E:IC,.n,llX.=56 + 6 kcal/mol. When the reaction system was evacuated during the acceleration period or after it, the final values of the polarization angles were not equal to the initial ones which testifies to the growth of a silicon layer formed by monosilane decomposition on the oxidized silicon surface (fig. 4, curve 4).
PSiH, < 5 X 10-l
pctorr) x40 Fig. 6. Dependence of the deviation of the maximum rate of polarization angle A on the monosilane pressure in the system for different sample temperatures: 1 - 490, 2 - 470, 3 - 430, 4 - 410°C.
In the case when monosilane (at a pressure equal to that during the growth) was again admitted into the chamber, after the growth was interrupted and the chamber was evacuated, the whole experimental story was reproduced in the same way, i.e., reversible adsorption (fig. 4, curve 1) and the period of acceleration (fig. 4, curve 3) were observed. The length of the acceleration period and the maximum rate of A variations were approximately the same as for the initial monosilane exposure. To measure the hydrogen content of the formed thick silicon layer (- 1 lrn), IR transmission spectra were recorded with a Fourier IKS113F spectrometer in the range 500-4000 cm-‘. No signals characteristic for Si-H groups (2100 cm-‘). No signals characteristic for Si-H groups (2100 cm-‘> [Zl] were observed The layer was shown to be electr~nographi~ally amorphous. The results presented in figs. 5 and 6 suggest that the growth of an amorphous silicon layer is possible onIy when PsihL,> 5 X lo-” Torr which means a su~ermonolayer coverage of the surface. The formation of supermonolayer adsorption complexes seems to be impossible before the initial monolayer is completed, perhaps because of the high mobility of SiH, molecules in the adsorption layer. Further decomposition of adsorption complexes, accompanied by the growth
of amorphous silicon, is characterized by the following. The kinetic curves possess an acceleration period (fig. 4, curve 3) at the end of which the growth rate reaches its maximum value for the given monosilane pressure and sample temperature. Such a behavior may mean that new active cenfres are formed during the decomposition of adsorption complexes in the beginning of the acceleration period. The decomposition of SiH, molecules is much faster on the active centres. As the number of active centres increases, the total monosilane decomposition rate grows which leads to the observed increase of the rate of polarization angle variation. At the end of the acceleration period the whole surface of the sample becomes active, and the monosilane decomposition rate does not grow further. The evacuation during the layer growth leads to the disappearence of the active centres on the surface. This is confirmed by the observation of reversible adsorption stages and the acceleration period after the second gas exposure to the reaction system. This behavior can be described by Kolmogorov’s equation [22] which can be written for the portion r) of activated surface depending on time as follows:
where P(T) is the formation rate of activated regions and S(r - r) is the area of the growing region on the activated surface. The eq. (5) was used in ref. [231 to describe the transformation of a (320) silicon surface during the phase transition from a flat surface structure to a surface limited by facet planes. The value of the variation of the polarization angle (A) at small thicknesses determines the mean thickness of the formed film. The rate of variation dA/dt is proportional to the growth rate of the amorphous silicon W(t) which is directly linked to the portion of the activated surface 77(t) = ~(~)/~~~~ where W,,, corresponds to the maximum growth rate when the whole surface is activated. The experimental dependence q(t) obtained by the differentiation of the segments of the curve A(t) (for example, fig. 4, curve 31, normalized to unity, is represented at fig. 7a.
440
Fig. 7. Dependence of the variation of the polarization angle d v = (dd/dt )/(dA /dl),_ built for region 3 of the kinetic curve at fig. 5 for different pressures: 1 - 0.162,2 - 0.113, 3 - 0.089 Turr: a - experiment. b ~ calculation.
To describe the process according to eq. (59. the following model will be accepted. The formation rate of centres p(r) will be taken proportional to 6’ (the degree of surface coverage by adsorbed molecules). At P > 5 x lo-” Torr, when amorhous silicon grows, 8 is proportional to PSiH, (see fig. 5). For the steady rate formation of centres on the unactivated part of the surface p(t) - PC1 -p(t)). The centre with the activated surface grows activating the adsorption layer at the edges. If we take the lateral centre growth rate proportional to the degree of surface coverage, i.e., i‘, - 8 - P, then S(r) - P*t”, where t is the time interval during which a centre exists. Then eq. (5) will be written as: r(t)
= 1 -exp
-B(7”)P” i
X
t 1 -Y@)](M)’ 00
d7
.! I
(fJ>
where BIT) is a factor depending on temperature. The calculation of q(t) with a fitting parameter B = 0.005 for pressures of 0.162, 0.1 13, and 0.089 Torr (as for the experimental curves shown at fig. 7a) is presented in fig. 7b. The comparison
shows rather close accordance of the calculated values with the experimental results. Other types of p and S dependence on pressure and time in eq. (5) were aIso considered. A possible model of the process encloses the gradual activati~)n of the adsorption layer in the conditions whcrc the activated regions do not grow in size. Another model was based on the assumption that the number of monosilane decomposition centres was constant in time and depe.nded on the properties of the initial surface, while the whole surface became activated only due to the movement of the borders. In these cases a substantial discrepancy? between the theory and experiment was observed. Thus, that the chosen model coincides with the experiment suggests that the process is chain process, the activation of a single region in the adsorption layer causes the activation of the neighbouring regions, the formation rate of centres and the growth rate of centres being dctermined by the degree of surface coverage by adsorbed monosilane. Before discussing the structure of activated surface it should be pointed out that the bcginning of the process, inciudin~ rnol~osilan~ adsorption up to supermonolayer coverage and the growth of amorphous silicon, is similar on the initial, oxidized silicon surface and on the surface formed after amorphous silicon layer has grown. In the second case, when the temperature was in the range 3SO--49O”C, the surface after evacuation remained coated with the adsorbed hydrogen which could only be removed at t~rnp~rat~lr~s higher than 500°C [16,1X]. This fact leads to the conclusion that in this case, when there art no active ~~dsorptioil centres, the structure of tho substrate plays a minor rote, compared to the processes which take place within the adsorbed nionosilanc layer. As it was suggested earlier, the substantial increase in the ~ondensat~~~n coefficient up to 8 x lO”-” Torr during the repeated monosilane adsorption after hydrogen dcsorption (fig. 31 is connected with the formation of unsaturat~~~ silicon bonds on the surface. Also the unsaturated silicon bonds can be the activation centres in the adsorbed layer for the growth of amorphous sili-
VW. Kruchinin et al. / Ellipsometric kaestigation of monvsilane adsorption on silicon
con in the case of the oxidized surface. Then the scheme (4) should be supplemented by the stages: =SiH - SiH,(a) =SiH-SiH,
-+ =SiH-SiH,
-(a) -+ =Si-SiH,
*(a) + H,(g), -(a) + H,(g).
At the first stage, unsaturated silicon bonds are formed. During the amorphous silicon formation at the second stage the surface activity is maintained by the conservation of unsaturated bonds in -SiH, - centres. This scheme describes the shortest of the possible chains. An activation energy E,,t,max.= 56 i- 6 kcaI/ mol, determined in the present study from the temperature dependence of the maximum growth rate, is typical for the growth of amorphous silicon layers during monosilane pyrolysis at T < 650°C [24]. This value of E_,,ax_ is in agreement with ref. [25] dealing with monosilane pyrolysis where it was shown that in the temperature range 271-471°C and pressure within 10-2-100 Torr a complete monosilane decomposition in the pyrolysis chamber is initiated by the stage of the heterogeneous SiH, decomposition on the surface of an amorphous silicon layer formed on the walls of the chamber.
4. Conclusion In situ data on adsorption-desorption processes and initial stages of layer growth can be obtained with the help of a fast-response ellipsometer, posing no rigid limitations on the experimentai conditions and bringing no perturbations to the process under investigation. Monosilane adsorption on a clean (100) and an oxidized silicon surface essentially occurs in two different ways. Adsorption of SiH, is irreversible on the clean surface with active centrcs, while monosilane adsorbes reversibly on the oxidized surface having no active adsorption centres. Another essentiai feature of monosilane adsorption processes is the possibility of the formation of a supermonolayer coverage. This feature can be used as the starting point for the investigation of silicon layer growth conditions because the decomposition of supermonolayer adsorption
441
complexes accompanied by hydrogen desorption to the gas phase leads to the layer growth in the adsorption-desorption equilibrium conditions, On oxidized surfaces, supermonolayer coverage is followed by the growth of amorphous silicon. In the temperature range between 350-490°C the pressure value of P = 5 X lo-’ Torr separates the region of monosilane decomposition with layer growth from the region of reversible adsorption which can be described by the Langmuir isotherm. The transfer from adsorption to the growth of amorphous silicon is characterized by a gradual activation of the adsorbed layer with the formation of centres with unsaturated silicon bonds qSi-SiHz - . The analysis of the kinetic data presented in the form of the degree of activated surface demonstrates a chain character of the amorphous silicon growth process at the initial stage. The results demonstrate peculiarities of the SiO, layer formation by alternating exposures to monosilane and oxygen. Monosilane decomposition on oxidized surfaces with no active adsorption centres does not gain a uniform silicon layer which is not thicker than 1 nm and suitable for further oxidation. At low temperatures an additional stimulation of the monosilane decomposition on the substrate surface is believed to be necessary for this purpose.
References [I] MS. Sukhov, Neorg. Mat. 12 (1982) 1077. 121 ML. ~itchman, J. Kane and A.E. Widmer, Thin Solid Films 59 (1979) 231. [31 J. Bloem and W.A.P. Claassen, Rec. Trav. Chim. Pays-Bas 99 (1980) 217. [41 A.H. Boonstra, Philos. Res. Rep. Suppl. 3 (1968) 1. 151B.Z. Olshanetsky and S.M. Repinsky; Fiz. Tverd. Tela 16 (1974) 1679. IhI B.S. Meyerson and M.L. Yu, J. Electrochem. Sot. 131 (1984) 2366. [71 SM. Gates, Surf. Sci. 1% (1988) 307. [81 M.R. Baklanov, V.N. Kruchinin, S.M. Repinsky and A.A. Shklyaev, Poverkhn. 8 (1986) 101. [91 M.R. Baklanov, V.N. Kruchinin, S.M. Repinsky and A.A. Shklyaev, Poverkhn. 10 (1986) 79. 1101C. Licoppe and J.M. Moison. Surf. Sci. 21 l/212 (lY89) 979.
442 [ll] [12] [13]
[14] [15] [16] [17] [18]
V.N. Kruchinin
et al. / Ellipsometric
incestigation
M.R. Baklanov, V.N. Kruchinin, SM. Repinsky and A.A. Shklyaev, Reactivity of Solids 7 (1989) 1. M.A. Hopper, R.A. Clarke and L. Young, J. Electrochem. Sot. 122 (1975) 1216. Yu.B. Algazin, Yu.A. Blumkina, N.I. Grebntv, K.K. Svitashev, L.V. Semenenko and T.M. Yabloneva, Opt. Spektrosk. 45 (1978) 330. G.A. Bootsma and F. Meyer, Surf. Sci. 14 (1969) 52. R.J. Archer and G.W. Gobeli. J. Phys. Chem. Solids 26 (1965) 343. G. Schulze and M. Henzler, Surf. Sci. 124 (1983) 336. H. Wagner, R. Butz, U. Bastes and D. Bruchmann, Solid State Commun. 38 (1981) 1155. M.N. Brodsky, M.A. Frisch and J.F. Ziegler, Appl. Phys. Lett. 30 (1977) 561.
of monosilane
adsorption
on silicon
[19] J.A. McMillan, R.R. Kunz and C.M. Greenlief, Surf. Sci. 207 (1989) 364. [20] V.N. Kruchinin, SM. Repinsky and A.A. Shklyaev, Poverkhn. 9 (1986) 65. [21] V.N. Kruchinin, S.M. Repinsky and A.A. Shklyaev. Poverkhn. 3 (1987) 60. [22] A.N. Kolmogorov, Izv. Akad. Nauk SSSR, Ser. Mat. 3 (1937) 355. [23] B.Z. Olshanetsky and A.A. Shklyaev, Zh. Eksp. Teor. Fiz. 81 (1981) 361. [24] A.M. Beers and J. Bloem, Appl. Phys. Lett. 41 (1982) 153. [25] R. Robertson, D. Hils and A. Gallagner, Chem. Phys. Lett. 103 (1984) 397.
Surface Science North-Holland
,,,::::.,. . . ,, ::.:
.‘.‘.‘:,
>:.:.
:::.F.‘..... .\...i......,, ..:.:.:.:..:::.:.:::~:.:.:~:+~.:...:~~ .,.,., .\.. . . . . .. . . . . ,..
,,.( ~,,,...., ,,
,: ,.,.,..,\,,
~.~.‘.‘.:-:i’.‘.‘.:.::.:.:.:‘.:‘.’.’.:-~ .... . .... ....,:,:.::.:: :::...: i:i,:,i:i;: :;i:$‘:i:g:::.:.: y .. .,.,
“surface science
275 (1992) 433-442
Monosilane adsorption and initial growth stages of silicon layers on the (100) and oxidized silicon surfaces Ellipsometric V.N.
investigation
Kruchinin,
S.M.
Repinsky
and
A.A.
Shklyaev
Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk 90, Russian Federation Received
18 February
1991; accepted
for publication
5 February
1992
Monosilane adsorption kinetics on (100) and oxidized silicon surfaces has been studied by means of a fast-response ellipsometer. Monosilane adsorption on the (100) silicon surface is irreversible and has no activation barrier in the temperature range 20-500°C. Condensation coefficients of SiH, for the closed and the flowing adsorption technique are rr = 7 x 10-s and 5 x lo-*, respectively. The thickness of the adsorption layer increases in the temperature range 121%350°C which is evidence for the formation of a surface supermonolayer coverage. At T > 350°C decomposition of adsorption complexes takes place, accompanied by hydrogen desorption. This process is characterized by an activation energy of E = 44 * 2 kcal/mol. Monosilane is adsorbed reversibly on the oxidized silicon surface in the temperature range 350-490°C and at a pressure of P < 5 X lo-’ Torr. At pressures P > 5 X lo-* Torr adsorption complexes decompose and amorphous silicon grows. The growth rate is directly proportional to the monosilane pressure in the gas phase and increases with increasing temperature; the activation energy of this process is E = 56 * 6 kcal/mol. The analysis of the kinetic curves indicates that the growth of amorphous silicon is a chain process at the initial stage.
1. Introduction Monosilane is one of the most widely used reagents in the gas-phase processes of the growth of various layers. Many authors treat monosilane adsorption as one of the obligatory initial stages [l-3]. The investigation of monosilane interaction with silicon is of great interest, but for a long time the experimental data concerning the adsorption ability of monosilane on the semiconductor surface were causing contradictory comments [4,5]. The number of works devoted to monosilane adsorption on silicon is rather small [6,7], and this problem is still actual. The present paper deals with the investigation of silane adsorption kinetics on the silicon (100) surface in the temperature range 20-500°C by means of automatic ellipsometry in a high-vacuum 003Y-6028/92/$05.00
0 1992 - Elsevier
Science
Publishers
system. Taking into account that the comparison of the adsorption abilities of the pure silicon surface with the surface coated with a thin layer of natural oxide may be of special interest, we studied also the regularities of monosilane adsorption on the oxidized silicon surface in the temperature range 350-490°C and the initial stages of the growth of amorphous silicon layers. The results obtained in the present paper, together with the data on silicon oxidation by 0, and N,O published earlier [8,9], determine the conditions of SiO, layer formation by the interchange of oxygen and monosilane exposure. This method of the synthesis of SiO, layers is valuable for obtaining a thin dielectric layer, its thickness being defined by the number of gas exposures. Molecule deposition can be used as a controlled method for the semiconductor-dielectric inter-
B.V. All rights reserved
434
V.N. Kruchinin
face formation while obtaining on A3B5 compounds [lo].
et al. / Ellipsometric
MOS
inuestigation
structures
2. Experimental Experimental procedure and high-vacuum ellipsometric technique have been described earlier Ill]. p-type samples with the resistivity of 10 Q. cm, dimensions of 20 X 5 X 0.45 mm3 and (100) surface orientation were used in the experiments with a clean silicon surface. The samples were subjected to chemical-mechanic polishing and treated with peroxide-ammonia etchant. Final purification of the surface was conducted in high vacuum (P = lo-” Torr) by annealing the sample at T = 1200°C. Sample heating was accomplished by transmitting a current. The necessary sample temperature was set after annealing with the help of the stabilized current source. Temperature was controlled ellipsometrically, with an accuracy of i3”C, using the known temperature dependences of the optical characteristics of silicon [12,13]. Two series of experiments were carried out on the (100) silicon surface, the first in the flowing system with the opened ion pump under the constant monosilane pressure P = lop6 Torr, and the second in the closed system with a closed pump and turned-off ionization manometer, the gas pressure constantly increasing. The pressure was increased at a rate of 1.7 X 10e6--3.1 X lop5 Torr . s-l. At the end of the gas exposure the = 10-2 maximum monosilane pressures were Torr. The pressure in the chamber was determined by means of a thermocouple pressure gauge PMT-2. When the ion pump was opened the monosilane was pumped out down to a pressure of lo-” Torr for a few seconds. A oxidized silicon surface was prepared by oxygen chemisorption on the clean surface at T = 350-490°C and 0, exposure of 104-10” L. The observed changes of the ellipsometric angles showed that the oxide layer formed on the silicon surface is not thicker than 7 A. The surface was ready for the experiments after oxygen evacuation. The whole experimental cycle consisted of: sample annealing at T = 12OO”C, preparation of
of monosilane adsorption on silicon
oxidized surface, monosilane exposure and the evacuation of the system. The whole experimental cycle was repeated before each new monosilane exposure. Monosilane exposure in the experiments with the oxidized surface was carried out under the following conditions: the pulsed regime of the exposure; the closed pump valve; the absence of the ionization sources until the necessary pressure was reached. Then the pressure in the chamber was kept constant. The maximum pressure was 1.6 X 10-l Torr. Commercial monosilane, 98% purety, was used. The impurities were controlled by a mass-spectrometer IPDO-2A. The main impurity was argon; no other impurities were found in monosilane. In order to treat the results of the ellipsometric measurements on clean silicon surfaces we used the approximate linearized shapes of the basic equation of the ellipsometry. The linear dependence of the variations of polarization angles on the effective thickness of the adsorption layer has been applied 1141: SA = d - A = [d, ST = q - q, where 3, F and A, W are the initial and current values of the polarization angles, 5 is a coefficient depending on the reflective system properties, d is the effective thickness of the layer. The decrease of T during gas chemisorption on the atomically-pure semiconductor surfaces cannot be explained within the simple model of the monolayer reflective system and seems to be connected with the variation of the optical characteristics of the adsorbed surface layer [14,15]. That is why the futher quantitative treatment was based only on the changes of the angle A. The calcuiation of the coefficient 5 was done at A = 6328 A, 4(, = 70” and Nsi = 3.865 O.O23i, where 4(, is the angle of incidence of the light beam on the sample, A is the working wave length of the ellipsometer and Nsi is the complex index of reflection of silicon at room temperature for the current A value. The calculation of the adsorbed layer thickness was done based on the Lorentz-Lorenz equation. Table values were used for SiH, polarizability. The molecular volume was taken equal to the volume of the molecule in the liquid phase. With the calculated value of the effective refractive index 1.49, 5 was found to be 0.3”/A.
435
KN. Kruchinin et al. / Ellipsometric inuestigation of monosilane adsorption on silicon
In order to estimate the effective thickness of the adsorbed layer of monosilane molecules on the oxidized silicon surface we used the data obtained in accordance with the optical model of the double layer refractive system. The complex refractive coefficient of the substrate was taken Nsi = 4.079 - 0.06, as for the silicon sample heated up to 350°C. The thick?ess of the lower oxidized layer being equal to 7 A, and the refractive index of the oxidized layer was taken 1.46 as for the SiO, layer [ill.
3. Experimental
1 x00
~0
T°C
Fig. 2. Dependence of the peak of the variation of polarization angle A on the sample temperature in the closed system corresponding to the saturated monosilane coverage.
results and discussion
3.1. Adsorption kinetics of SiH, on the (100) silicon surface Typical kinetic dependences of the variations of the polarization angle A in the adsorption process are given in fig. 1. Polarization angles A and V were decreasing, which is characteristic for chemisorption on atomically clean semiconductor surfaces [14]. When the adsorption is completed the kinetic curves of the variations of the polarization angle reach constant values depending on the silicon temperature and the way it was exposed to monosiIane. In the case of monosilane exposure in the closed system the variation of the angle A reached
‘6A fHiN)
0
0
3
400
200
300
400
t fS) Fig. 1. Typical dependences of the variations of the polarization angle d on time during monosilane adsorption. l-3: experiments are carried out in a closed system at temperature 20, 250 and 350°C respectively. The rate of increasing SiH, pressure (Torr.s-I): 1 - 2.8~10-~, 2 - 2.2~10-~, 3 2.6 X 10-s, 4 experiments carried out in a flowing system at 2O”C, P = 1 X 10c6 Torr.
its maximum value at a certain temperature (fig. 2). In the temperature range between 20-125°C the angle peak values do not change. This corresponds to the constant thickness of the adsorption layer. According to the approximate ellipsometric estimations the effective thickness of the layer is 2 A. We shall assume now that this value corresponds to the monosilane monolayer surfa$e coverage. The length of the Si-H bond is 1.48 A, so this assumption seems quite reasonable. With further temperature increase the thickness of the adsorption layer increases until a maximum value of 4 A at 350°C is reached and then it decreases rapidly. When the adsorption process was completed and the curves reached saturation, the admittance valve was closed and the gate of the ion pump was opened. In the experiments carried out at temperatures below 350°C no distinct changes on the curves were observed which is evidence for the irreversibili~ of the adsorption process. At temperatures higher than 350°C the polarization angles relaxed to some final values 6A = 11’ and SF = 6’ (fig. 31, these two values being independent of temperature and staying constant, after the relaxation was completed, for all the further silane exposures (curve 2, fig. 3). With increasing the temperature the relaxation rate increased too. No influence of the ion pump on the relaxation rate was observed. The temperature dependence of the relaxation time of the polarization angle to the value equal to 0.5 of its maximum change was obtained during the relaxation. Using this depen-
436
V N. Kruchinin
6AI
‘~~ 0
et al. / Ellipsomefric
inl~estigafion
,
a00 400 600 t(s)
Fig. 3. Relaxation of the polarization angle d. 1 - the first exposure. 2 - repeated monosilane exposure. The moments the admittance valves are switched off and the moments of the beginning of monasilane exposure are marked with arrows. measurements were made in the flowing system, at 41WC.
dence we calculated the activation energy of the relaxation process to be 44 + 2 kcal/mol. The analysis of the kinetic curves shows that monosilane adsorption on SitlOO) in the region of small surface coverage (6 < 0.5) has no activation barrier and follows the kinetic laws of ideal adsorption. The rate of the adsorption process can be written as: d0 UP = K,( 1 - O)PSiH, = ,,2(1 dt N\( 27TmkT)
-O)Y
(1) where I(, is the adsorption rate constant; Psi,+, is the monosilane pressure, rr is the condensation coefficient, N, is the density of the surface atoms. The calculated values of the condensation coefficients (T for the closed and the flowing adsorption techniques are 7 X lo-’ and 5 X lo-‘, respectively. The difference between the two (7 values seems to be caused by the presence of atomic hydrogen formed in the flowing system due to ionization because the condensation coefficient of atomic hydrogen on Si is nearly unity for the initial stages of adsorption [13]. The chemisorbed SiH, molecule is expected to decompose [31. There is no common opinion on the monosilane decomposition products on Si, but it should be mentioned that the formation of the =SiH z and -SiH type complexes was observed in ref. 1171 where the adsorption of atomic hydrogen on Si(ll1) was studied by an EELS method. The formation of similar complexes seems to be probable also during monosilane
of mono.silane
adsorption
on ,siliwn
adsorption. Thus, the adsorption process of monosilane on the Si(100) surface at temperatures between 20 and 125°C is stopped when the surface becomes coated with a monolayer of SiH, dissociation products. The presence of atomic hydrogen in the flowing system leads to the preferred adsorption of hydrogen on the active surface centres which is, in turn, the reason of the decrease of the effective thickness of the adsorption layer (fig. 1, curve 4). At temperatures higher than 125°C the hydrogen atom mobility increases so that the reaction between monosilane molecules from the gas phase and the adsorption complex becomes possible, accompanied by hydrogen desorption. The effective thickness of the adsorption layer increases. The decomposition of adsorption complexes starts only at T> 350°C and the experimental curves show the relaxation of the polarization angles (fig. 3) which seem to correspond to hydrogen desorption. The desorption rate can be written as foilows: d8 = K,,B dt = rIV,o exp[ ( -44
i 2 kcal/mol)/RT]
,
(2)
where K,, is the decomposition rate constant and y is the vibration frequency of the chemisorbed hydrogen atoms. It was shown [7,16,18] that there arc two forms of hydrogen desorption from a silicon surface. The first form reaches a maximum rate at 375400°C with the activation energy of 44 kcal/mol. The maximum rate of the second des~~rption form is reached at 500-54u”C. Its activation energy was measured to be 57 [16] and 50 [IX] kcal/mol. Two adsorption forms were also observed during the decomposition of amorphous hydrogenated silicon layers. The authors [16,1X] assumed that the low temperature desorption form is connected with the decomposition of =SiH 2 type centres and their transformation into the =SiH type centrcs. while the high temperature desorption employs futher decomposition of centres and complctc removal of hydrogen from the silicon surface. The polarization angle relaxation (fig. 3) is likely to represent the process of hydrogen removal
l/N. Kruchinin
et al. / Ellipsometric
investigation
from the surface which corresponds to low temperature desorption. The close value of the activation energy of relaxation and the activation energy of desorption [16,18] testify to this assumption. The values of the polarization angles SA, and ST,. remaining after complete relaxation show that hydrogen remains in those centres on the silicon surface which decomposes giving rise to high-temperature desorption. This proposal is confirmed by the proximity of 6A, and Sqr values to 6A = 12’ and 6V = 7.2’, obtained in ref. [14] for monolayer coverage on a Si(ll1) surface. Hydrogen removal corresponding to high-temperature desorption form starts at temperature above 490°C; this leads to a noticeable decrease of 6A, and Sqr values. For 0 > 0.3 a deviation from the laws of ideal adsorption was observed on the kinetic curves. This deviation may be the result of the interaction in the adsorption layer and the shifts of the substrate atoms. That the deviation from the ideal adsorption laws is observed only during the first monosilane exposure (fig. 3, curve l), while during futher gas exposures after the relaxation the adsorption follows the laws of the ideal adsorption kinetics (fig. 3, curve 2). The value of u is 8 X 10e2. Such a change of the adsorption ability is associated with a change of the surface. The initial clean Si(100) surface is reconstructed due to surface interactions. After the monosilane evacuation and desorption at 350-500°C the surface remains partly covered with hydrogen, having no superstructure and is able to contain a considerable amount of unsaturated silicon bonds which act as the adsorption centres. In this case (T increases and the adsorption follows the laws of the ideal adsorption kinetics, because there is not the necessity of surface reconstruction conditioned by destroying a superstructure. The process of monosilane adsorption up to 200°C can be described by the following stages: SiH,(g)
-
-SiH,(a)
of monosilane
adsorption
on silicon
437
200-350°C supermonolayer coverage is observed which can be linked with the formation of complexes: SiH,( g) + =SiH,( a) + =SiH - SiH,(a) =SiH,(a)
+ =SiH,(a)
or
-+ =SiH *(a) + H(a),
SiH 4( g) + =SiH - (a) + =SiH - SiH,( a). At temperatures higher than 350°C the decomposition of complexes and the desorption of hydrogen molecules occur. The kinetic analysis of the suggested mechanism shows [20] that under the stationary conditions of the process when a silicon layer should grow on the K, ’ KaPSiH,~ surface. Under our experimental conditions this is the case for T > 350°C. It sould be mentioned that the optical constants of the growing silicon layer are close to the substrate constants because the accumulation of the layer is not fixed by the ellipsometer after adsorption and further keeping of the sample in the adsorbate atmosphere for a long period of time. 3.2. Adsorption silicon surface
kinetics
of SiH,
on the oxidized
A typical dependence of the variation of the polarization angle A with time during monosilane adsorption on the oxidized silicon surface is shown in fig. 4. Both angles d and P decrease during SiH, adsorption on oxidized and clean Si(100) surfaces. The angle variation reaches some peak value depending on the pressure (fig. 4, curve 1).
+ H(a), I , I
-SiH,(a)
+ H(a)
+ H(a).
Hydrogen does not desorb till 200°C [16] and, judging from ref. [191, -SiH,(a) particles can be present on the surface. In the temperature range
0
500
4000
, t
6)
Fig. 4. Variation of the polarization angle A with time during monosilane adsorption on the oxidized silicon surface. PSIH4 = 8.9X 10-I Torr, T = 430°C. The moments of the beginning of the evacuation are marked with arrows.
438
VN. Kruchinin
et ul. / Ellipsomrtric
iwestigation
Fig. 5. Dependence of maximum variations of the polarization angle 3 on monosilane pressure in the system for sample temperatures: 350. 410. 470°C. A dashed line marks the region of the isotherm where the deviation from the Langmuir law is observed.
The dependence of the maximum of the variation of angle A on the gas pressure in the system is represented in fig. 5. The adsorption isotherm so obtained can be described by the Langmuir isotherm up to the pressure of 5 X lop2 Torr:
Oeff =
aPSitl
1+
“ps:Ha ’
(3)
where Psi), is monosilane pressure in the system, Oerf is the iffective degree of surface coating by SiH, molecules. When the monosilane pressure is higher than 5 X lo-* Torr the isotherm is observed to deviate sharply from the Langmuir law. This can be evidence to the change of adsorption type and formation of a supermonolayer coverage on the surface. One can estimate the value of Orff corresponding to this transition using the data obtained with the help of the double layer reflective system model. Assuming that oxidized surface had no active adsorption centres, Bet, was then taken to be equal to the effective thickness of the adsorbed layer defr divided by the diameter of monosilane molecule d,: Oeff = deff/d,,. Since the value of de,.,. corresponding to the transition is = 1 A and d, = 1.97 A, O,,,-= 0.5. The coefficient a from the Langmuir and law was calculated to be 35 Torr- ’ between 350 and 470°C. For an initial monosilane pressure less than 5 x 1OY’ Torr no appreciable changes were observed on curve 1 (fig. 4) after the angles reached their maximum values. After the evacuation of the system, the angles relaxed to values close to
of monosilanr
adsorption on silicon
the initial ones (see curve 2, fig. 4) which is likely to be evidence for reversible adsorption. The thickness of the adsorbed layer (1 A at P < 5 X lo-’ Torr) is = 0.5 of the monolayer thickness formed by monosilane adsorption on the clean silicon surface. On the other hand, our experiments showed that monosilane adsorption is not observed on the oxidized surface at room temperature. This leads to the assumption that at temperatures higher than 350°C monosilane is partly decomposed on the oxide-coated surface: SiH,(g)
+ -SiH,(a)
SiH,(g)
+ -SiH,(
+ H(a), a) --) -SiH
- SiH,( a) + H,(g). (4)
The
first
stage
provides a coating d = 1 A for Torr. At higher pressures a stage of the second type appears and the thickness of the coverage increases. For pressures higher than 5 x lo-’ Torr, further change of the polarization angle A and q was observed (see, fig. 4, curve 3). The change rate of the polarization angle was permanently growing up to a maximum depending on the monosilane pressure and the sample temperature. The length of the acceleration period till the change rate of the angle reached its maximum was varied in our experimental conditions from several tenths of seconds to tens of minutes and generally decreased with monosilane pressure increasing as t - PGiif’“-2. At higher temperature the change rate of the polarization angle increased, which resulted in the shortening of the acceleration period. The maximum change rate of A was proportional to monosilane pressure (fig. 6). The activation energy was determined from the temperature dependence of the maximum rate of the angle variation; its value E:IC,.n,llX.=56 + 6 kcal/mol. When the reaction system was evacuated during the acceleration period or after it, the final values of the polarization angles were not equal to the initial ones which testifies to the growth of a silicon layer formed by monosilane decomposition on the oxidized silicon surface (fig. 4, curve 4).
PSiH, < 5 X 10-l
pctorr) x40 Fig. 6. Dependence of the deviation of the maximum rate of polarization angle A on the monosilane pressure in the system for different sample temperatures: 1 - 490, 2 - 470, 3 - 430, 4 - 410°C.
In the case when monosilane (at a pressure equal to that during the growth) was again admitted into the chamber, after the growth was interrupted and the chamber was evacuated, the whole experimental story was reproduced in the same way, i.e., reversible adsorption (fig. 4, curve 1) and the period of acceleration (fig. 4, curve 3) were observed. The length of the acceleration period and the maximum rate of A variations were approximately the same as for the initial monosilane exposure. To measure the hydrogen content of the formed thick silicon layer (- 1 lrn), IR transmission spectra were recorded with a Fourier IKS113F spectrometer in the range 500-4000 cm-‘. No signals characteristic for Si-H groups (2100 cm-‘). No signals characteristic for Si-H groups (2100 cm-‘> [Zl] were observed The layer was shown to be electr~nographi~ally amorphous. The results presented in figs. 5 and 6 suggest that the growth of an amorphous silicon layer is possible onIy when PsihL,> 5 X lo-” Torr which means a su~ermonolayer coverage of the surface. The formation of supermonolayer adsorption complexes seems to be impossible before the initial monolayer is completed, perhaps because of the high mobility of SiH, molecules in the adsorption layer. Further decomposition of adsorption complexes, accompanied by the growth
of amorphous silicon, is characterized by the following. The kinetic curves possess an acceleration period (fig. 4, curve 3) at the end of which the growth rate reaches its maximum value for the given monosilane pressure and sample temperature. Such a behavior may mean that new active cenfres are formed during the decomposition of adsorption complexes in the beginning of the acceleration period. The decomposition of SiH, molecules is much faster on the active centres. As the number of active centres increases, the total monosilane decomposition rate grows which leads to the observed increase of the rate of polarization angle variation. At the end of the acceleration period the whole surface of the sample becomes active, and the monosilane decomposition rate does not grow further. The evacuation during the layer growth leads to the disappearence of the active centres on the surface. This is confirmed by the observation of reversible adsorption stages and the acceleration period after the second gas exposure to the reaction system. This behavior can be described by Kolmogorov’s equation [22] which can be written for the portion r) of activated surface depending on time as follows:
where P(T) is the formation rate of activated regions and S(r - r) is the area of the growing region on the activated surface. The eq. (5) was used in ref. [231 to describe the transformation of a (320) silicon surface during the phase transition from a flat surface structure to a surface limited by facet planes. The value of the variation of the polarization angle (A) at small thicknesses determines the mean thickness of the formed film. The rate of variation dA/dt is proportional to the growth rate of the amorphous silicon W(t) which is directly linked to the portion of the activated surface 77(t) = ~(~)/~~~~ where W,,, corresponds to the maximum growth rate when the whole surface is activated. The experimental dependence q(t) obtained by the differentiation of the segments of the curve A(t) (for example, fig. 4, curve 31, normalized to unity, is represented at fig. 7a.
440
Fig. 7. Dependence of the variation of the polarization angle d v = (dd/dt )/(dA /dl),_ built for region 3 of the kinetic curve at fig. 5 for different pressures: 1 - 0.162,2 - 0.113, 3 - 0.089 Turr: a - experiment. b ~ calculation.
To describe the process according to eq. (59. the following model will be accepted. The formation rate of centres p(r) will be taken proportional to 6’ (the degree of surface coverage by adsorbed molecules). At P > 5 x lo-” Torr, when amorhous silicon grows, 8 is proportional to PSiH, (see fig. 5). For the steady rate formation of centres on the unactivated part of the surface p(t) - PC1 -p(t)). The centre with the activated surface grows activating the adsorption layer at the edges. If we take the lateral centre growth rate proportional to the degree of surface coverage, i.e., i‘, - 8 - P, then S(r) - P*t”, where t is the time interval during which a centre exists. Then eq. (5) will be written as: r(t)
= 1 -exp
-B(7”)P” i
X
t 1 -Y@)](M)’ 00
d7
.! I
(fJ>
where BIT) is a factor depending on temperature. The calculation of q(t) with a fitting parameter B = 0.005 for pressures of 0.162, 0.1 13, and 0.089 Torr (as for the experimental curves shown at fig. 7a) is presented in fig. 7b. The comparison
shows rather close accordance of the calculated values with the experimental results. Other types of p and S dependence on pressure and time in eq. (5) were aIso considered. A possible model of the process encloses the gradual activati~)n of the adsorption layer in the conditions whcrc the activated regions do not grow in size. Another model was based on the assumption that the number of monosilane decomposition centres was constant in time and depe.nded on the properties of the initial surface, while the whole surface became activated only due to the movement of the borders. In these cases a substantial discrepancy? between the theory and experiment was observed. Thus, that the chosen model coincides with the experiment suggests that the process is chain process, the activation of a single region in the adsorption layer causes the activation of the neighbouring regions, the formation rate of centres and the growth rate of centres being dctermined by the degree of surface coverage by adsorbed monosilane. Before discussing the structure of activated surface it should be pointed out that the bcginning of the process, inciudin~ rnol~osilan~ adsorption up to supermonolayer coverage and the growth of amorphous silicon, is similar on the initial, oxidized silicon surface and on the surface formed after amorphous silicon layer has grown. In the second case, when the temperature was in the range 3SO--49O”C, the surface after evacuation remained coated with the adsorbed hydrogen which could only be removed at t~rnp~rat~lr~s higher than 500°C [16,1X]. This fact leads to the conclusion that in this case, when there art no active ~~dsorptioil centres, the structure of tho substrate plays a minor rote, compared to the processes which take place within the adsorbed nionosilanc layer. As it was suggested earlier, the substantial increase in the ~ondensat~~~n coefficient up to 8 x lO”-” Torr during the repeated monosilane adsorption after hydrogen dcsorption (fig. 31 is connected with the formation of unsaturat~~~ silicon bonds on the surface. Also the unsaturated silicon bonds can be the activation centres in the adsorbed layer for the growth of amorphous sili-
VW. Kruchinin et al. / Ellipsometric kaestigation of monvsilane adsorption on silicon
con in the case of the oxidized surface. Then the scheme (4) should be supplemented by the stages: =SiH - SiH,(a) =SiH-SiH,
-+ =SiH-SiH,
-(a) -+ =Si-SiH,
*(a) + H,(g), -(a) + H,(g).
At the first stage, unsaturated silicon bonds are formed. During the amorphous silicon formation at the second stage the surface activity is maintained by the conservation of unsaturated bonds in -SiH, - centres. This scheme describes the shortest of the possible chains. An activation energy E,,t,max.= 56 i- 6 kcaI/ mol, determined in the present study from the temperature dependence of the maximum growth rate, is typical for the growth of amorphous silicon layers during monosilane pyrolysis at T < 650°C [24]. This value of E_,,ax_ is in agreement with ref. [25] dealing with monosilane pyrolysis where it was shown that in the temperature range 271-471°C and pressure within 10-2-100 Torr a complete monosilane decomposition in the pyrolysis chamber is initiated by the stage of the heterogeneous SiH, decomposition on the surface of an amorphous silicon layer formed on the walls of the chamber.
4. Conclusion In situ data on adsorption-desorption processes and initial stages of layer growth can be obtained with the help of a fast-response ellipsometer, posing no rigid limitations on the experimentai conditions and bringing no perturbations to the process under investigation. Monosilane adsorption on a clean (100) and an oxidized silicon surface essentially occurs in two different ways. Adsorption of SiH, is irreversible on the clean surface with active centrcs, while monosilane adsorbes reversibly on the oxidized surface having no active adsorption centres. Another essentiai feature of monosilane adsorption processes is the possibility of the formation of a supermonolayer coverage. This feature can be used as the starting point for the investigation of silicon layer growth conditions because the decomposition of supermonolayer adsorption
441
complexes accompanied by hydrogen desorption to the gas phase leads to the layer growth in the adsorption-desorption equilibrium conditions, On oxidized surfaces, supermonolayer coverage is followed by the growth of amorphous silicon. In the temperature range between 350-490°C the pressure value of P = 5 X lo-’ Torr separates the region of monosilane decomposition with layer growth from the region of reversible adsorption which can be described by the Langmuir isotherm. The transfer from adsorption to the growth of amorphous silicon is characterized by a gradual activation of the adsorbed layer with the formation of centres with unsaturated silicon bonds qSi-SiHz - . The analysis of the kinetic data presented in the form of the degree of activated surface demonstrates a chain character of the amorphous silicon growth process at the initial stage. The results demonstrate peculiarities of the SiO, layer formation by alternating exposures to monosilane and oxygen. Monosilane decomposition on oxidized surfaces with no active adsorption centres does not gain a uniform silicon layer which is not thicker than 1 nm and suitable for further oxidation. At low temperatures an additional stimulation of the monosilane decomposition on the substrate surface is believed to be necessary for this purpose.
References [I] MS. Sukhov, Neorg. Mat. 12 (1982) 1077. 121 ML. ~itchman, J. Kane and A.E. Widmer, Thin Solid Films 59 (1979) 231. [31 J. Bloem and W.A.P. Claassen, Rec. Trav. Chim. Pays-Bas 99 (1980) 217. [41 A.H. Boonstra, Philos. Res. Rep. Suppl. 3 (1968) 1. 151B.Z. Olshanetsky and S.M. Repinsky; Fiz. Tverd. Tela 16 (1974) 1679. IhI B.S. Meyerson and M.L. Yu, J. Electrochem. Sot. 131 (1984) 2366. [71 SM. Gates, Surf. Sci. 1% (1988) 307. [81 M.R. Baklanov, V.N. Kruchinin, S.M. Repinsky and A.A. Shklyaev, Poverkhn. 8 (1986) 101. [91 M.R. Baklanov, V.N. Kruchinin, S.M. Repinsky and A.A. Shklyaev, Poverkhn. 10 (1986) 79. 1101C. Licoppe and J.M. Moison. Surf. Sci. 21 l/212 (lY89) 979.
442 [ll] [12] [13]
[14] [15] [16] [17] [18]
V.N. Kruchinin
et al. / Ellipsometric
incestigation
M.R. Baklanov, V.N. Kruchinin, SM. Repinsky and A.A. Shklyaev, Reactivity of Solids 7 (1989) 1. M.A. Hopper, R.A. Clarke and L. Young, J. Electrochem. Sot. 122 (1975) 1216. Yu.B. Algazin, Yu.A. Blumkina, N.I. Grebntv, K.K. Svitashev, L.V. Semenenko and T.M. Yabloneva, Opt. Spektrosk. 45 (1978) 330. G.A. Bootsma and F. Meyer, Surf. Sci. 14 (1969) 52. R.J. Archer and G.W. Gobeli. J. Phys. Chem. Solids 26 (1965) 343. G. Schulze and M. Henzler, Surf. Sci. 124 (1983) 336. H. Wagner, R. Butz, U. Bastes and D. Bruchmann, Solid State Commun. 38 (1981) 1155. M.N. Brodsky, M.A. Frisch and J.F. Ziegler, Appl. Phys. Lett. 30 (1977) 561.
of monosilane
adsorption
on silicon
[19] J.A. McMillan, R.R. Kunz and C.M. Greenlief, Surf. Sci. 207 (1989) 364. [20] V.N. Kruchinin, SM. Repinsky and A.A. Shklyaev, Poverkhn. 9 (1986) 65. [21] V.N. Kruchinin, S.M. Repinsky and A.A. Shklyaev. Poverkhn. 3 (1987) 60. [22] A.N. Kolmogorov, Izv. Akad. Nauk SSSR, Ser. Mat. 3 (1937) 355. [23] B.Z. Olshanetsky and A.A. Shklyaev, Zh. Eksp. Teor. Fiz. 81 (1981) 361. [24] A.M. Beers and J. Bloem, Appl. Phys. Lett. 41 (1982) 153. [25] R. Robertson, D. Hils and A. Gallagner, Chem. Phys. Lett. 103 (1984) 397.