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A dual sticking coefficient chemical vapor deposition model
Microelectronic Engineering 19 (1992) 503-506 Elsevier
503
A dual Sticking Coefficient Chemical Vapor Deposition Model Holger Wille and Edmund P. Burte
Fraunhofer Arbeitsgruppe ffir Integrierte Schaltungen, Artilleriestrasse 12, 8520 Erlangen, Germany
Abstract
A quantitatively exact simulation of the step coverage of thin solid films prepared by low pressure chemical vapor deposition (LPCVD) is helpful to the computer aided development of the fabrication process of microelectronic devices. In addition, conclusions regarding the molecular microscopic reactions can be drawn due to our most simple physico-chemical model with only three well comprehensible parameters.
I. Introduction
The coverage of steps and edges on a structured substrate is usually calculated by Monte-Carlo-simulation or by modelling the Knudsen-diffusion near the surface. However, these methods do need very much computation time or possess limited accuracy caused by theoretical approximations. A third way is the calculation of the topography-dependent local adsorption equilibrium of the reactive molecules. This is done by balancing the particle-delivering and -consuming mass flows in every point on the structured substrate. In every segment four different particle flows 1 - 4 can be distinguished, as shown in Fig. 1.
1: Adsorption flow from the process gas On the condition that the mean free path length in the process gas is larger than the substrate feature size, this particle stream is a function of the pressure and of the accessibility of segment AB only. Due to the high temperature (430"C) and the low pressure (< 1,0 Torr) during the deposition the possible depletion of free adsorption sites can be disregarded (Langmuir's isotherm in the low pressure region). 0167-9317/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved.
504
H. Wille, E.P. Burte / Chemical vapor deposition model
Schematic presentation of the four different particle flows 1 - 4 near the surface of a structured substrate
Fig. 1
2: Desorption flow Desorbing molecules or radicals are leaving the surface with a cosine-distribution postulated and measured by numerous authors [1]. The desorption flow is assumed to be proportional to the local concentration of the adsorbed precursors.
3: Transfer flow Instead of leaving the cavity in Fig. 1 desorbing molecules may also reach another point of the structure. Every segment like AB will get a contribution from each visible segment. This contribution is a function of their lengths and of the distance. A rather simple expression for the transfer flow was derived [2]. A particle somewhere on CD reaches segment AB with a probability
T ~
=
AC
-
BC
2
-
AD
"A--~
+
BD
--
0...i
XY is the distance between the points X and Y.
4: Reaction flow Alternatively to the desorption, adsorbed precursor molecules can react to the solid film. The reaction flow is set proportional to the adsorbed precursor concentration (first order reaction kinetics). The relative strength of reaction and desorption corresponds to the sticking coefficient. This coefficient is the only adjustable parameter here.
Because of the mass conservation the sum of the four different particle flows 1 - 4 is equivalent to zero. Such a mass conservation equation must be set up for all segments, resulting in a large linear equation system. Its solution is the steady state precursor concentration on the surface at a certain time during film growth and is used to calculate the growth step.
H. Wille, E.P. Burte / Chemical vapor deposition model
505
2. Simulation
Trying to simulate the conformity of LTO (low temperature oxide) deposited into trenches with different widths between 0.5 #m and 10 #m leads to unacceptable deviations. The step coverage is too low in small trenches while it is too high in broader ones. Therefore, as a most simple extension, the parallel deposition of two different precursor molecules with two different sticking coefficients sc! and sc n is introduced. The third p a r a m e t e r is the contribution anteil I of the first of the two precursor molecules to layer growth. Of course, the sum of the contributions of the two different precursors to layer growth is 1.
3. Examples
Fig. 2 and Fig. 3 show narrow trenches with 0.5/~m-LTO layers deposited at 370°C and at pressures of 0.1 Torr and 1.0 Torr. To simulate this structures independently of the width of the trench, the two sticking coefficients have been adjusted to sc I = 0.1 and scii = 1.0 while the contribution of the first precursor to the growth rate outside the trench was anteil I = 0.3 and anteil I = 1.0, respectively. A variety of experiments showed that the two sticking coefficients are a function of the temperature only and do not depend on wafer spacing, pressure, gas flow nor even on the silane/oxygen ratio.
Fig. 2
Deposition of SiO 2 at 370"C and 0.1 Torr into a trench 1/~m wide. Simulation with scI = 0.1, SCli = 1.0 and anteil I = 0.3
506
H. Wille, E.P. Burte / Chemical vapor deposition model ~ ' =
.... , ,
~T~
~
i
~
¸
i~ ~ii
Fig. 3
Deposition of SiO 2 at 3700C and 1.0 Torr into a trench 1.6 #m wide. Simulation with sq = 0.1, sqi = 1.0 and anteil I = 1.0
4. Conclusions
The behavior of the sticking coefficient is consistent with the model, wherein the processes on the substrate and in the gas phase are separated by the adsorption step. The micro-processes reaction and desorption, described by the sticking coefficient, are therefore not influenced by the gas phase. Furthermore, the concentration of precursor molecules with sticking coefficients in the range of 10-1 causing a layer growth of = 1/~m/h at a pressure below 1.0 Torr cannot be higher than 10 -4. We assume that this concentration is generated by homogeneous reactions of Sill 4 and that the precursor molecules react with oxygen on the surface. Consequently, the oxygen is in excess under all circumstances, what justifies the assumed first order reaction kinetics a posteriori.
H. C. Wulu, K. C. Saraswat, and J. P. McVittie, J. Electrochem. Soc., 138 (1991) 1831, and references herein. H. Wille, E. P. Burte, and H. Ryssel, J. Appl. Phys., 71 (1992) 3532.
This work was partly supported by the European Community within the ESPRIT project STORM.
503
A dual Sticking Coefficient Chemical Vapor Deposition Model Holger Wille and Edmund P. Burte
Fraunhofer Arbeitsgruppe ffir Integrierte Schaltungen, Artilleriestrasse 12, 8520 Erlangen, Germany
Abstract
A quantitatively exact simulation of the step coverage of thin solid films prepared by low pressure chemical vapor deposition (LPCVD) is helpful to the computer aided development of the fabrication process of microelectronic devices. In addition, conclusions regarding the molecular microscopic reactions can be drawn due to our most simple physico-chemical model with only three well comprehensible parameters.
I. Introduction
The coverage of steps and edges on a structured substrate is usually calculated by Monte-Carlo-simulation or by modelling the Knudsen-diffusion near the surface. However, these methods do need very much computation time or possess limited accuracy caused by theoretical approximations. A third way is the calculation of the topography-dependent local adsorption equilibrium of the reactive molecules. This is done by balancing the particle-delivering and -consuming mass flows in every point on the structured substrate. In every segment four different particle flows 1 - 4 can be distinguished, as shown in Fig. 1.
1: Adsorption flow from the process gas On the condition that the mean free path length in the process gas is larger than the substrate feature size, this particle stream is a function of the pressure and of the accessibility of segment AB only. Due to the high temperature (430"C) and the low pressure (< 1,0 Torr) during the deposition the possible depletion of free adsorption sites can be disregarded (Langmuir's isotherm in the low pressure region). 0167-9317/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved.
504
H. Wille, E.P. Burte / Chemical vapor deposition model
Schematic presentation of the four different particle flows 1 - 4 near the surface of a structured substrate
Fig. 1
2: Desorption flow Desorbing molecules or radicals are leaving the surface with a cosine-distribution postulated and measured by numerous authors [1]. The desorption flow is assumed to be proportional to the local concentration of the adsorbed precursors.
3: Transfer flow Instead of leaving the cavity in Fig. 1 desorbing molecules may also reach another point of the structure. Every segment like AB will get a contribution from each visible segment. This contribution is a function of their lengths and of the distance. A rather simple expression for the transfer flow was derived [2]. A particle somewhere on CD reaches segment AB with a probability
T ~
=
AC
-
BC
2
-
AD
"A--~
+
BD
--
0...i
XY is the distance between the points X and Y.
4: Reaction flow Alternatively to the desorption, adsorbed precursor molecules can react to the solid film. The reaction flow is set proportional to the adsorbed precursor concentration (first order reaction kinetics). The relative strength of reaction and desorption corresponds to the sticking coefficient. This coefficient is the only adjustable parameter here.
Because of the mass conservation the sum of the four different particle flows 1 - 4 is equivalent to zero. Such a mass conservation equation must be set up for all segments, resulting in a large linear equation system. Its solution is the steady state precursor concentration on the surface at a certain time during film growth and is used to calculate the growth step.
H. Wille, E.P. Burte / Chemical vapor deposition model
505
2. Simulation
Trying to simulate the conformity of LTO (low temperature oxide) deposited into trenches with different widths between 0.5 #m and 10 #m leads to unacceptable deviations. The step coverage is too low in small trenches while it is too high in broader ones. Therefore, as a most simple extension, the parallel deposition of two different precursor molecules with two different sticking coefficients sc! and sc n is introduced. The third p a r a m e t e r is the contribution anteil I of the first of the two precursor molecules to layer growth. Of course, the sum of the contributions of the two different precursors to layer growth is 1.
3. Examples
Fig. 2 and Fig. 3 show narrow trenches with 0.5/~m-LTO layers deposited at 370°C and at pressures of 0.1 Torr and 1.0 Torr. To simulate this structures independently of the width of the trench, the two sticking coefficients have been adjusted to sc I = 0.1 and scii = 1.0 while the contribution of the first precursor to the growth rate outside the trench was anteil I = 0.3 and anteil I = 1.0, respectively. A variety of experiments showed that the two sticking coefficients are a function of the temperature only and do not depend on wafer spacing, pressure, gas flow nor even on the silane/oxygen ratio.
Fig. 2
Deposition of SiO 2 at 370"C and 0.1 Torr into a trench 1/~m wide. Simulation with scI = 0.1, SCli = 1.0 and anteil I = 0.3
506
H. Wille, E.P. Burte / Chemical vapor deposition model ~ ' =
.... , ,
~T~
~
i
~
¸
i~ ~ii
Fig. 3
Deposition of SiO 2 at 3700C and 1.0 Torr into a trench 1.6 #m wide. Simulation with sq = 0.1, sqi = 1.0 and anteil I = 1.0
4. Conclusions
The behavior of the sticking coefficient is consistent with the model, wherein the processes on the substrate and in the gas phase are separated by the adsorption step. The micro-processes reaction and desorption, described by the sticking coefficient, are therefore not influenced by the gas phase. Furthermore, the concentration of precursor molecules with sticking coefficients in the range of 10-1 causing a layer growth of = 1/~m/h at a pressure below 1.0 Torr cannot be higher than 10 -4. We assume that this concentration is generated by homogeneous reactions of Sill 4 and that the precursor molecules react with oxygen on the surface. Consequently, the oxygen is in excess under all circumstances, what justifies the assumed first order reaction kinetics a posteriori.
H. C. Wulu, K. C. Saraswat, and J. P. McVittie, J. Electrochem. Soc., 138 (1991) 1831, and references herein. H. Wille, E. P. Burte, and H. Ryssel, J. Appl. Phys., 71 (1992) 3532.
This work was partly supported by the European Community within the ESPRIT project STORM.