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Kinetics of molecular adsorption and laser-stimulated surface oxidation
KINETICS OF i%OLECULARADSORPTION AND LASER-STI:IULATED SURFACE OXIDATION
V.N.Varakin, A.D.Lozovsky, A&I.Panesh, A.P.Simonov Karpov Institute of Physical Chemistry, Moscow, USSR
In past decade, laser-stimulated heterogeneous chemical reactions have been widely investigated. Laser-induced desorption of metal and semiconductor films, etching of materials, and oxidation of metal surfaces have been used for microelectronics materials processing[1,2]. For detailed description of these complex heterogeneous processes, it is necessary to study their elementary steps. One of the essential steps is adsorption of atoms and molecules on a solid surface. In this work, kinetics of SO, and CO molecules adsorption(in(l?- 107) Torr pressure range) on stainless steel surface have been measured by means of laser-induced thermal desorption (LITD) in order to determine adsorption parameters and study the initial stage of metal surface oxidation by adsorbed CO molecules both stimulated by laser irradiation and without it. Based on measurements with a pulsed laser a kinetic approach is presented, which makes possible to obtain from characteristic times of kinetics an information on adsorption and oxidation. In LITD [3,4), a concentration of adsorbed molecules is determined by measuring of a full number of molecules desorbed due to intensive pulsed laser heating of the definite area of the sample surface. The fluence of laser probe pulse is chosen to be large enough to desorb all the adsorbed molecules within the irradiated area by a single pulse and at the same time to be not too large to avoid surface damage. Due to short time of irradiation, small depth of laser beam penetration into metal, and high thermal conductivity of metal, surface temperature returns very quickly (compared with pulse interval) to its equilibrium value 151. Therefore, in LITD experiments, molecular adsorption occurs at constant surface temperature. A focused pulsed Nd:YAG laser beam at 1.06rm (with 10 ns duration and 150 mJ maximum pulse energy) was used as an energy source for desorption of adsorbed molecules, cleaning of metal surface, and stimulation of metal oxidation. Kinetic curve of adsorption (at fixed gas pressure p and surface temperature of the sample Ta ) describes the formation of equilibrium monolayer coverage of molecules with time constantZ(p,T$). This type of kinetics was observed in experiments of SO, adsorption on stainless steel surface. According to Langmuir modelC_'= 't,-'+Ti' where 2,~CN,,,l&,j~pand r,eexp(Ed/RT,) are times of adsorption ani desorption, I?,,, 1s the maximum concentration of adsorption sites, S, is the initial sticking coefficient, Ed is the desorption energy. To determine times z, and Td which hold information about adsorption parameters: N,/S, and Ed , it i: necessary to study pressure or surfa:? temperature dependences of timer, The first one has been measured(in 10 - IO-? Torr pressure range)leading to values: 'd =91 kJ/mol, N, /S, =6.5~10'~cm“ . The adsorption of CO on stainless steel is accompanied by metal surface oxidation by oxygen atoms, which appear as a result of dissociation of adsorbed CO molecules[4]. The oxides blockade adsorption sites and cause the decrease of adsorption capacity of the sample surface. Therefore, the concentration of adsorbed molecules monotoneously decreases at long time exposures. CO adsorption kinetics (at 9x16' Torr pressure and at 301 K surface temperature) was measured (see fig.1) on condition that oxides and adsorbed molecules had been removed from metal surface by intensive laser cleaning pulse before each gas exposure. The exposure times t were determined by intervals between cleaning and probing laser pulses.
214
V.N.VARAKIN etal.
This adsorption process is described by the following equations:
where N, and N, are concentrations of adsorbed molecules and adsorption sites, which are blockaded by oxides respectively,2, is the time characterizing the rate of decrease of adsorption sites density due to surface oxidation. The solution of these equations is given by:
where T,,and r, are time constants. The curve in fig.1 calculated from (2) with 2, -3.2 min, r, ml5.5 min is consistent with the experimental data (points in fig.11 If relation rd-'* 2-'+ P--i holds, then rise and fall times of a kinetic curve determine ad?sorptionand oxidation times: 2,='t,,TL=YQ.
I
20 30 Fig.1
40
50
60 ‘10 t,min Fig.2
Fig.1. CO adsorption kinetics in the single pulses case. Fig.2. Dependence8 of concentrations of adsorbed CO molecules on time ex osure in the case of series of laser probe pulses with 2 (01, 4 701, 6 (x), 8 (~1 min repetition intervals. In LITD experiments, laser pulses induce intensive (temperature jump ~1000 K) and short time heating of metal surface. Therefore, it is interesting to study an influence of,laser irradiation on oxidation rate. CO adsorption kinetics (at p= 9rlO Torr, Ts~~301K) has been measured also in the case of irradiation of stainless steel surface by series of laser probe pulses separated by delay periods T in 2 - 8 min range (see fig.2). Each laser pulse in series desorbed all the adsorbed molecules in the irradiated area but not the surface oxides. Before each series, en intensive laser pulse cleared the oxides and adsorbed molecules from the sample surface. Repetition rates of pulses in series were chosen such as to allow an appreciable part of a monolayer coverage to be formed during interval between successive probe laser pulses. The main results of these experiments are: 1. The effective oxidation time e, (during which the concentration of adsorbed CO molecules decreases by a factor of e> jndependent of repetition rate in the chosen in this experiment range. 2. The acceleration of oxidation up to B,dlO min compared withCt,~l5.5 min in the single pulse experiment. The first fact is explained by compensation of two actions of laser irradiation. On the one hand, laser pulses destroy the adsorbed monolayer coverage leading to deceleration of oxidation. On the other hand, it is reasonable to assume that laser heating of the surface causes the dissociation of certain part 6 of adsorbed CO molecules and oxygen atoms produced oxidise metal surface.
Kinetics of laser-stimulated
reactions
215
Under this assumption, the effective oxidation time 0, dependence on delay period T takes the form:
A good a reement between calculated(with t‘l33.2 min, ‘i’,= 15.5 min, end $ variablef and excerimental data is found with 6x0.3. The acceieration-of oxidation in the case of the repetitive pulse laser irradiation may be further increased at higher repetition rates and higher laser fluences up to values (z,/$ and 2, respectively) controlled by adsorption and surface damage. In summary, the kinetic approach in LITD has proved to be useful in study of initial stages of spontaneous end laser-stimulated metal surface oxidation. The application of pulsed lasers seems to have great potentials in investigation of elementary steps of heterogeneous processes. REFERENCES 1. D.J.Ehrlich and J.Y.Tsao, J.Vacuum Sci.Technol., v.Bl, 969 (1983). 2. T.J.Chuang, Surface Sci.Rep., v.3, 1 (1983) . 3. K.Christmann, O.Schober, G.Ertl, M.Neumann, J.Chem.Phys., v.60, 4528 (1974). 4. J.A.Tagle and A.Pospieszczyk, Appl. Surface Sci., v.17, 189 (1983). 5. J.H.Bechtel, J.Appl.Phys., v.46, 1585 (1975).
V.N.Varakin, A.D.Lozovsky, A&I.Panesh, A.P.Simonov Karpov Institute of Physical Chemistry, Moscow, USSR
In past decade, laser-stimulated heterogeneous chemical reactions have been widely investigated. Laser-induced desorption of metal and semiconductor films, etching of materials, and oxidation of metal surfaces have been used for microelectronics materials processing[1,2]. For detailed description of these complex heterogeneous processes, it is necessary to study their elementary steps. One of the essential steps is adsorption of atoms and molecules on a solid surface. In this work, kinetics of SO, and CO molecules adsorption(in(l?- 107) Torr pressure range) on stainless steel surface have been measured by means of laser-induced thermal desorption (LITD) in order to determine adsorption parameters and study the initial stage of metal surface oxidation by adsorbed CO molecules both stimulated by laser irradiation and without it. Based on measurements with a pulsed laser a kinetic approach is presented, which makes possible to obtain from characteristic times of kinetics an information on adsorption and oxidation. In LITD [3,4), a concentration of adsorbed molecules is determined by measuring of a full number of molecules desorbed due to intensive pulsed laser heating of the definite area of the sample surface. The fluence of laser probe pulse is chosen to be large enough to desorb all the adsorbed molecules within the irradiated area by a single pulse and at the same time to be not too large to avoid surface damage. Due to short time of irradiation, small depth of laser beam penetration into metal, and high thermal conductivity of metal, surface temperature returns very quickly (compared with pulse interval) to its equilibrium value 151. Therefore, in LITD experiments, molecular adsorption occurs at constant surface temperature. A focused pulsed Nd:YAG laser beam at 1.06rm (with 10 ns duration and 150 mJ maximum pulse energy) was used as an energy source for desorption of adsorbed molecules, cleaning of metal surface, and stimulation of metal oxidation. Kinetic curve of adsorption (at fixed gas pressure p and surface temperature of the sample Ta ) describes the formation of equilibrium monolayer coverage of molecules with time constantZ(p,T$). This type of kinetics was observed in experiments of SO, adsorption on stainless steel surface. According to Langmuir modelC_'= 't,-'+Ti' where 2,~CN,,,l&,j~pand r,eexp(Ed/RT,) are times of adsorption ani desorption, I?,,, 1s the maximum concentration of adsorption sites, S, is the initial sticking coefficient, Ed is the desorption energy. To determine times z, and Td which hold information about adsorption parameters: N,/S, and Ed , it i: necessary to study pressure or surfa:? temperature dependences of timer, The first one has been measured(in 10 - IO-? Torr pressure range)leading to values: 'd =91 kJ/mol, N, /S, =6.5~10'~cm“ . The adsorption of CO on stainless steel is accompanied by metal surface oxidation by oxygen atoms, which appear as a result of dissociation of adsorbed CO molecules[4]. The oxides blockade adsorption sites and cause the decrease of adsorption capacity of the sample surface. Therefore, the concentration of adsorbed molecules monotoneously decreases at long time exposures. CO adsorption kinetics (at 9x16' Torr pressure and at 301 K surface temperature) was measured (see fig.1) on condition that oxides and adsorbed molecules had been removed from metal surface by intensive laser cleaning pulse before each gas exposure. The exposure times t were determined by intervals between cleaning and probing laser pulses.
214
V.N.VARAKIN etal.
This adsorption process is described by the following equations:
where N, and N, are concentrations of adsorbed molecules and adsorption sites, which are blockaded by oxides respectively,2, is the time characterizing the rate of decrease of adsorption sites density due to surface oxidation. The solution of these equations is given by:
where T,,and r, are time constants. The curve in fig.1 calculated from (2) with 2, -3.2 min, r, ml5.5 min is consistent with the experimental data (points in fig.11 If relation rd-'* 2-'+ P--i holds, then rise and fall times of a kinetic curve determine ad?sorptionand oxidation times: 2,='t,,TL=YQ.
I
20 30 Fig.1
40
50
60 ‘10 t,min Fig.2
Fig.1. CO adsorption kinetics in the single pulses case. Fig.2. Dependence8 of concentrations of adsorbed CO molecules on time ex osure in the case of series of laser probe pulses with 2 (01, 4 701, 6 (x), 8 (~1 min repetition intervals. In LITD experiments, laser pulses induce intensive (temperature jump ~1000 K) and short time heating of metal surface. Therefore, it is interesting to study an influence of,laser irradiation on oxidation rate. CO adsorption kinetics (at p= 9rlO Torr, Ts~~301K) has been measured also in the case of irradiation of stainless steel surface by series of laser probe pulses separated by delay periods T in 2 - 8 min range (see fig.2). Each laser pulse in series desorbed all the adsorbed molecules in the irradiated area but not the surface oxides. Before each series, en intensive laser pulse cleared the oxides and adsorbed molecules from the sample surface. Repetition rates of pulses in series were chosen such as to allow an appreciable part of a monolayer coverage to be formed during interval between successive probe laser pulses. The main results of these experiments are: 1. The effective oxidation time e, (during which the concentration of adsorbed CO molecules decreases by a factor of e> jndependent of repetition rate in the chosen in this experiment range. 2. The acceleration of oxidation up to B,dlO min compared withCt,~l5.5 min in the single pulse experiment. The first fact is explained by compensation of two actions of laser irradiation. On the one hand, laser pulses destroy the adsorbed monolayer coverage leading to deceleration of oxidation. On the other hand, it is reasonable to assume that laser heating of the surface causes the dissociation of certain part 6 of adsorbed CO molecules and oxygen atoms produced oxidise metal surface.
Kinetics of laser-stimulated
reactions
215
Under this assumption, the effective oxidation time 0, dependence on delay period T takes the form:
A good a reement between calculated(with t‘l33.2 min, ‘i’,= 15.5 min, end $ variablef and excerimental data is found with 6x0.3. The acceieration-of oxidation in the case of the repetitive pulse laser irradiation may be further increased at higher repetition rates and higher laser fluences up to values (z,/$ and 2, respectively) controlled by adsorption and surface damage. In summary, the kinetic approach in LITD has proved to be useful in study of initial stages of spontaneous end laser-stimulated metal surface oxidation. The application of pulsed lasers seems to have great potentials in investigation of elementary steps of heterogeneous processes. REFERENCES 1. D.J.Ehrlich and J.Y.Tsao, J.Vacuum Sci.Technol., v.Bl, 969 (1983). 2. T.J.Chuang, Surface Sci.Rep., v.3, 1 (1983) . 3. K.Christmann, O.Schober, G.Ertl, M.Neumann, J.Chem.Phys., v.60, 4528 (1974). 4. J.A.Tagle and A.Pospieszczyk, Appl. Surface Sci., v.17, 189 (1983). 5. J.H.Bechtel, J.Appl.Phys., v.46, 1585 (1975).