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Low temperature oxidation of crystalline silicon using excimer laser irradiation
134
Applied Surface Science 36 (1989) 134-1~ North-Holland, Amsterdam
L O W T E I V ~ E R A T U R E O X I D A T I O N O F CRYSTALLENE S I L I C O N USING EXC[MER LASER IRRADIATION Vishal NA'YAR, Ian W. B O Y D Department of Elec;¢onie and Electrical Engineering, University College London, Torrington Place, London WCIE 7dE, UK
F.N. G O O D A L L a n d G. A R T H U R Rutherford Appleton Laboratory, Didcot, Oxon, UK
Received 1 June 1988; accepted for publication 25 July 1988
In this paper we present a study of ultra-violet laser oxidation of silicon at low temperature ( < 650 o C), using both 249 and 193 nm radiation. Calculation of the surface temperature rise during the laser pulses suggests that non-thermal oxidation mechanisms are present. In addition to the growth of planar thin oxides over macroscopic areas, a new technique for selectivelyoxidising silicon by direct image projection, i.e., direct growth lithography (DGL) is also preliminzxily presented.
L Induction Laser induced oxidation of crystalline silicon has been studied by several groups [1-4]. tIowever, the area of low temperature excimer oxidation is still n o t well understood and the data available is rather limited. Low temperature silicon oxidation is a very i m p o r t a n t step in the production of metal oxide semiconductor devices, especially because the decrease in their lateral dimensions requires a corresponding reduction in the vertical oxide thickness. T h e controlled thermal growth of thin oxide layers has proved difficult. However, it ar~pears that for laser energies where the irradiated silicon remains in the solid phase, the oxidation proceeds initially at a n e n h a n c e d rate of growth a n d then becomes limited. In this investigation we present oxide growth curves for 249 a n d 193 n m radiation produced using a L a m b d a Physik EM210 laser. We have taken advantage of the n o n - t h e r m a l oxide growth to rapidly produce very thin layers controllably at low temperatures which have distinct uses in device fabrication. W h e n very low temperatures are traditionally induced the purely thermal oxidation of Si proceeds only very slowly [5-7]. We also report a recent development in which we induced oxide patterns on Si with the excimer 0 1 6 9 - 4 3 3 2 / 8 9 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
V. Nayar et al. / Oxidation of c-Si using excimer laser irradiation
135
laser using a multi-element lens imaging method. This direct growth lighography (DGL) and the results obtained are described.
2. Large
area o x i ~ f i o n
2.1. E x p e r i m e n t a l s y s t e m
The large area oxides were grown using the conceptionally simple experimental configuration shown in fig. 1. Increased energy density was achieved by inserting 2 spectrosil lenses into the beam path. Si with a surface orientation of (111), a resistivity of 6 - 1 0 ~ cm and p-type (boron) doping was used in this study for both wavelengths. The Si samples were prepared with 10% hydrofluoric acid dip and a 15 rain flush in deionised water. A stable native oxide (of 19 ,~ for 249 run and 15 ,~ for 193 nm) was measured on the silicon prior to oxidation. The samples were loaded into the oxidation chamber in a clean room environment. Electrograde oxygen was flushed into the chamber and the Si oxidised at atmospheric pressure. All of the oxidations were carded out at a laser repetition rate of 10 Hz. With the Lambda Physik model EM210 the pulse energy drops slightly both as a function of the repetition rate and the total number of laser shots. For example, the low repetition rate pulse energy after each oxidation run was measured and found to decrease by less than 5~ even after the longest runs at 249 nm. However, at 193 nm the pulse energy was very much more difficult to maintain at a constant value. The effect of this is discussed below. The oxide thicknesses were measured by ellipsometry using a single wavelength Rudolph AutoELII at a fixed oxide refractive index of 1A62. The data obtained were mean values calculated from oxide measurements repeated at the same point and at over 4 points over the oxidised area. The stendard deviation of the data points was calculated to be less than 2 A.
Oxygenin
EM2101' ~ ~ Laser
t_______l'[=
Bea
Condensing Lenses
To pump
Fig. 1. Excimerlaser oxidation system, the beam condensing arrangement was only necessary for the 193 nm runs because of the low energy/pulse emitted.
136
V. Neyar et al. / Oxidation of c-Si using excimer laser irradiation
2.2. k e s u l t s
Since the beam energy density strongly influences the oxide growth rate, we have ensured that energy densities greater than 0.45 J / e r a 2, which may induce surface meIfing leading to rapid oxidation, were not approached [8-10]. Fig. 2 shows the oxide thickness variation as a func,ion of the number of shots delivered to the surface. At an enelgy density of approximately 0.23 J / c m 2 the oxide thickness increased to 30 A after 40000 shots at 249 rim. The pulse width at both wavelengths is around 23 _-.s and thus the total exposure time during 4 0 ~ shots is less than 1 ms. The estimated temperature rise at this energy density is estimated to be less than 5(X)°C [4]. At the repetition frequency used,, at 240 ,am, during oxidation the pulse energy was approximately 20% l e g than the pulse energy measured at low repetition (less than 1 Hz). From a purely thermal standpoint one would not expect an oxide thickness increase up to 30 A during a millisecond at the elevated temperature estimated above. In fact, even by extrapolating the known temperature dependence of thermal oxidation of solid Si [5] one cannot achieve the result observed at an oxidation temperature of 600 ° C in less than 500 nfin. Therefore, there must be a non-thermal oxidation process inducing the growth. F r o m the form of the curve the oxidation appears to be self-limiting. Using a s ~ l a r energy density, a series of oxides were grown with 193 nm radiation for comparison of the growth rate. Preliminary 193 nm data are also plotted in fig. 2. At 193 rim, growth appears to proceed at the same rate as at 249 rim. however, the smrAl increase after 20 000 shots is more likely a result of the fall of the p ~ = c,ergy rather than a fundamental limitation on the oxide growth. 3.1
'''l
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I ....
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¢
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o
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2.1
-
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~..7
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....
i ....
0.0 o.5
i ....
1.0
i ....
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Humber Fig.
2. Oxide
growth
curves
for
both
i ....
2.0 of
193 nm
i ....
i ....
2.5 3.0
Shots
(@) and
193 nm.
n .... 3.5
i ....
J,',
4.0 4.S 5.0
xlO000 249
nm
(O). (n)
Preheated
oxidation
at
V. Nayar et at. / Oxidation of c-Si using excimer laser irradiation
137
In addition to the low temperature laser oxidation some preliminary experiments were carried out on silicon preheated to a background temperature of 350°C. It is interesting to note that the oxide grew rapidly in just 10000 shots to about 28 A. Tiffs suggests that by using preheating one can improve the growth rate substantially and possibly raise the limiting thickness. More data is being collected on this processing regime.
3. Pa~emedoxldafion 3.1. Experimental arrangement
D G L was performed on several samples as a function of the number of shots. Surface patterning was visible after just one shot in the case of high energy density pulses. The optical arrangement is described dsewhere [11]. In/tially high energy density pulses caused surface damage. However at reduced fluences, and by improving the uniformity of illumination, patterns without any damage could he grown. Scanning electron microscopy (SEM) was used to identify the oxide patterns. D G L was performed in air on Si surfaces prepared as above. 3.2. Results
We have taken advantage of this reduced thermal rapid oxidation process to produce oxide patterns using the reduction lens noted above. By controlling the beam density at the mask we could regulate the energy density imaged on the Si surface. For straight-through illumination without any beam expansion very high densities were achieved. When the beam was expanded to illuminate the whole mask area the energy density on the image plane decreased to approximately 0.25 .l/cm 2, very close to the value used in our planar growth study. Patterns were readily achieved in this set-up even for very low exposure times. Even single shot patterns could be achieved, but only by increasing the energy density. SEM micrographs were taken on uncoated oxidised surfaces (except for the high energy density sample which was coated with a thin layer of gold). Insulating oxide features were highlighted due to differential charge collection. The series of photographs 3-5 show the patterns at various magnifications. Fig. 3 indicates the effect of very high ( > 5 J / e r a 2) fluences. Severe surface darr~age resulted from non-uniform melting a~d recrystallisation and possibly local ablation. In the other photographs (figs. 4 and 5) patterns from a sample uniformly illuminated by 2000 shots are shown, where, both positive and negative regions of oxide are reproduced. Fig. ,~ shows oxide lin~s which were originally 70, 50, 40, 30, 20 and 10 g m wide on the mask. Since the lens
138
V. Nayar et al. / Oxidation of c-Si using excimer laser irradiation
Fig. 3. SEM micrograph showing the effect of high energy density upon the Si surface induced by the reduction lens system. reduces the mask pattern by a factor of 10, the actual lir,~widths on the Si are 7, 5, 4, 3, 2 and 1/Lm. These dknensions are verified l:y a comparison of the onJde lines and the scale at the top of the SEM rrficrographs. Fig. 5 d e m o n strafes the ability of the technique to grow repeated p a t t e r n s in a single 3 m m field exposure, i.e. there were n o movements of the sample or the images. With the present a r r a n g e m e n t D G L was a t : e m p t e d only in air. T h e ordde thickness m the patterns is estimated to vary between 25 a n d 28 A across the image. This was determined by removing the mask plate a n d imaging a d o t o n the surface. Although only 2000 shots were required to produce these selective
Fig. 4. Oxide patterns generated by reduced surface energy densities. The oxide lines are visible down to linewJdthsof almost 1/tm.
[~ Nayar et al. / Oxidation of c-Si using exciraer laser irradiation
139
Fig. 5. Many pattern units grown simultaneously with just 2000 shots. oxide patterns, increasing the number of shots only increased the oxide thick~..ess until it became self-limited without any notable variation in the lateral oimensions. 4. Summary and conclusions We have studied the growth of ultra-thin SiO z on Si at low temperatures using exchner laser radiation. The surface temperature rise cannot account for the growth of the oxide on a purely thermal basis and in comparison with visible laser oxidation U V fight has a much greater enhancement in the low temperature regime. Previous studies have linked this to the generation of a highly non-thermalised carrier plasma in or near the interfacial region and laser induced bond breaking in the interfacial region. The laser energy both exceeds the energy necessary to photoen'fit carriers from the Si into the oxide conduction band and also to break Si-Si bonds. One can increase the oxidation rate substantially by preheating to low temperatures. A recent study of low temperature oxidation [7] suggests that growth is controlled by the availability of atomic oxygen for temperatures below 750 o C. It is possible that the oxide growth observed in this study may be due to a process which creates atomic oxygen which can diffuse quickly through the ultrathin oxide at low temperatures, until itself diffusion-limits the reaction. One should therefore expect that at 193 nm the oxide would grow faster because it can more efficiently cause molecular oxygen to dissociate into atomic species. However, this effect may be small at atmospheric pressure because the lifetime of atomic oxygen is less than a millisecond.
14o
v. Naya~ el ~ ~ / Oxidation of c-Si using excimer laser irradiation
We have demonstra,,ed preliminary results of direct growth lithography of ultra-fifth oxide on Si using an imaging technique capable of producing linewidths down to r~.bout 1 /~m. Our patterned growth did not require the motion of either the sample or the beam, r_ei~her does it require traditional photoresist-based technology, thereby having a t i s t i n c t time advantage over this method. One ,::ould clearly apply the same optical system to etch and deposit material selectively. This method of surface processing is particularly suited to step and repeat type technology. It has an h~trinsic advantage in that the mask used to generate the imase has linewidths a factor of 10 greater than the image. It is clearly much easier to define a mask with linewidths o f 10 # m than 1 /~m, since producing, high defirfition (sub-nficron) masks in itself requires substantial effort. Custom device fabrication would benefit from this technology because the m',mber of device processing ,. teps are reduced drastically allowing reduced turnzround times.
Acknowledgements W e would like to acknowledge the Science and Engineering Research Council (SERC) and the Royal Signals and Radar Establishment for funding V. Nayar under a CASE studentship. We are grateful to A.M. Hodge and F. Micheli for many useful discussions about silicon oxidation and laser pr~cess:m.g. We also thank Professor Mino Green at Imperial College L o n d o n for use of the ellipsometer. This work was partially funded u n d e r SERC contra.:°~ G R / E 16090, and initiated by a Nuffield F o u n d a t i o n award.
Re[em::nces [1] !,.W.Boy& Laser Processingof Thin Films and Microstructures(Springer, Berlin, 1987). [21 S A. Schafer and S.A. Lyon, J. Vacuum Sci. Technol. 21 (1982) 423. [3] ILM. Young and W.A. Tiller, Appl. P|~ys.Letters 50 (1987) 80. [4J E. Fogarassy,S. Unamuno, J.L. Regoli~i and C. Fuchs, Phil. Mag. B 55 (1987) 253. [5"J E. Taft, J. Electrechem.So,:. 131 (1984) 2460. [~| E. Irene. P~fil.Mag. B 55 (1987) 131. 171 A.M. Hoff and 3. Ryzyno, Appl. Phys. Letters 52 (1988) 1264. [8t C.K. Ong, E.H. Sin and H.S. Tan, J. Opt. Soc. Am. B 3 (1986) 812. 't.¢] M. Berti, L.F. Dona &',ilia Rose, A.V. Drigo, C. Cohen, J. Siejka, G.G. Bentini and E. Jannitfi, Phys. Rev. B 34 (1986) 2346. [10] F. Foulon, E. Fogarassy,A. Saloui, C. Fuchs, S. Unamano and P. Siffert, Appl. Phys. A 45 (1988) 361. ['tl] F.N. Goodall, in: Advanced Lithography,Proe. Short Meeting Seri~ 10, Feb. 1988, Inst. of Phys.
Applied Surface Science 36 (1989) 134-1~ North-Holland, Amsterdam
L O W T E I V ~ E R A T U R E O X I D A T I O N O F CRYSTALLENE S I L I C O N USING EXC[MER LASER IRRADIATION Vishal NA'YAR, Ian W. B O Y D Department of Elec;¢onie and Electrical Engineering, University College London, Torrington Place, London WCIE 7dE, UK
F.N. G O O D A L L a n d G. A R T H U R Rutherford Appleton Laboratory, Didcot, Oxon, UK
Received 1 June 1988; accepted for publication 25 July 1988
In this paper we present a study of ultra-violet laser oxidation of silicon at low temperature ( < 650 o C), using both 249 and 193 nm radiation. Calculation of the surface temperature rise during the laser pulses suggests that non-thermal oxidation mechanisms are present. In addition to the growth of planar thin oxides over macroscopic areas, a new technique for selectivelyoxidising silicon by direct image projection, i.e., direct growth lithography (DGL) is also preliminzxily presented.
L Induction Laser induced oxidation of crystalline silicon has been studied by several groups [1-4]. tIowever, the area of low temperature excimer oxidation is still n o t well understood and the data available is rather limited. Low temperature silicon oxidation is a very i m p o r t a n t step in the production of metal oxide semiconductor devices, especially because the decrease in their lateral dimensions requires a corresponding reduction in the vertical oxide thickness. T h e controlled thermal growth of thin oxide layers has proved difficult. However, it ar~pears that for laser energies where the irradiated silicon remains in the solid phase, the oxidation proceeds initially at a n e n h a n c e d rate of growth a n d then becomes limited. In this investigation we present oxide growth curves for 249 a n d 193 n m radiation produced using a L a m b d a Physik EM210 laser. We have taken advantage of the n o n - t h e r m a l oxide growth to rapidly produce very thin layers controllably at low temperatures which have distinct uses in device fabrication. W h e n very low temperatures are traditionally induced the purely thermal oxidation of Si proceeds only very slowly [5-7]. We also report a recent development in which we induced oxide patterns on Si with the excimer 0 1 6 9 - 4 3 3 2 / 8 9 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
V. Nayar et al. / Oxidation of c-Si using excimer laser irradiation
135
laser using a multi-element lens imaging method. This direct growth lighography (DGL) and the results obtained are described.
2. Large
area o x i ~ f i o n
2.1. E x p e r i m e n t a l s y s t e m
The large area oxides were grown using the conceptionally simple experimental configuration shown in fig. 1. Increased energy density was achieved by inserting 2 spectrosil lenses into the beam path. Si with a surface orientation of (111), a resistivity of 6 - 1 0 ~ cm and p-type (boron) doping was used in this study for both wavelengths. The Si samples were prepared with 10% hydrofluoric acid dip and a 15 rain flush in deionised water. A stable native oxide (of 19 ,~ for 249 run and 15 ,~ for 193 nm) was measured on the silicon prior to oxidation. The samples were loaded into the oxidation chamber in a clean room environment. Electrograde oxygen was flushed into the chamber and the Si oxidised at atmospheric pressure. All of the oxidations were carded out at a laser repetition rate of 10 Hz. With the Lambda Physik model EM210 the pulse energy drops slightly both as a function of the repetition rate and the total number of laser shots. For example, the low repetition rate pulse energy after each oxidation run was measured and found to decrease by less than 5~ even after the longest runs at 249 nm. However, at 193 nm the pulse energy was very much more difficult to maintain at a constant value. The effect of this is discussed below. The oxide thicknesses were measured by ellipsometry using a single wavelength Rudolph AutoELII at a fixed oxide refractive index of 1A62. The data obtained were mean values calculated from oxide measurements repeated at the same point and at over 4 points over the oxidised area. The stendard deviation of the data points was calculated to be less than 2 A.
Oxygenin
EM2101' ~ ~ Laser
t_______l'[=
Bea
Condensing Lenses
To pump
Fig. 1. Excimerlaser oxidation system, the beam condensing arrangement was only necessary for the 193 nm runs because of the low energy/pulse emitted.
136
V. Neyar et al. / Oxidation of c-Si using excimer laser irradiation
2.2. k e s u l t s
Since the beam energy density strongly influences the oxide growth rate, we have ensured that energy densities greater than 0.45 J / e r a 2, which may induce surface meIfing leading to rapid oxidation, were not approached [8-10]. Fig. 2 shows the oxide thickness variation as a func,ion of the number of shots delivered to the surface. At an enelgy density of approximately 0.23 J / c m 2 the oxide thickness increased to 30 A after 40000 shots at 249 rim. The pulse width at both wavelengths is around 23 _-.s and thus the total exposure time during 4 0 ~ shots is less than 1 ms. The estimated temperature rise at this energy density is estimated to be less than 5(X)°C [4]. At the repetition frequency used,, at 240 ,am, during oxidation the pulse energy was approximately 20% l e g than the pulse energy measured at low repetition (less than 1 Hz). From a purely thermal standpoint one would not expect an oxide thickness increase up to 30 A during a millisecond at the elevated temperature estimated above. In fact, even by extrapolating the known temperature dependence of thermal oxidation of solid Si [5] one cannot achieve the result observed at an oxidation temperature of 600 ° C in less than 500 nfin. Therefore, there must be a non-thermal oxidation process inducing the growth. F r o m the form of the curve the oxidation appears to be self-limiting. Using a s ~ l a r energy density, a series of oxides were grown with 193 nm radiation for comparison of the growth rate. Preliminary 193 nm data are also plotted in fig. 2. At 193 rim, growth appears to proceed at the same rate as at 249 rim. however, the smrAl increase after 20 000 shots is more likely a result of the fall of the p ~ = c,ergy rather than a fundamental limitation on the oxide growth. 3.1
'''l
....
I ....
I ....
I ....
I ....
I''''l
....
¢
2.S
i ' ' ' ~ ' ~ ' ~ 2 4 9 n
o
D
o 2.S
®
2.1
-
_ 0
~..7
~.~
....
i ....
0.0 o.5
i ....
1.0
i ....
t.5
Humber Fig.
2. Oxide
growth
curves
for
both
i ....
2.0 of
193 nm
i ....
i ....
2.5 3.0
Shots
(@) and
193 nm.
n .... 3.5
i ....
J,',
4.0 4.S 5.0
xlO000 249
nm
(O). (n)
Preheated
oxidation
at
V. Nayar et at. / Oxidation of c-Si using excimer laser irradiation
137
In addition to the low temperature laser oxidation some preliminary experiments were carried out on silicon preheated to a background temperature of 350°C. It is interesting to note that the oxide grew rapidly in just 10000 shots to about 28 A. Tiffs suggests that by using preheating one can improve the growth rate substantially and possibly raise the limiting thickness. More data is being collected on this processing regime.
3. Pa~emedoxldafion 3.1. Experimental arrangement
D G L was performed on several samples as a function of the number of shots. Surface patterning was visible after just one shot in the case of high energy density pulses. The optical arrangement is described dsewhere [11]. In/tially high energy density pulses caused surface damage. However at reduced fluences, and by improving the uniformity of illumination, patterns without any damage could he grown. Scanning electron microscopy (SEM) was used to identify the oxide patterns. D G L was performed in air on Si surfaces prepared as above. 3.2. Results
We have taken advantage of this reduced thermal rapid oxidation process to produce oxide patterns using the reduction lens noted above. By controlling the beam density at the mask we could regulate the energy density imaged on the Si surface. For straight-through illumination without any beam expansion very high densities were achieved. When the beam was expanded to illuminate the whole mask area the energy density on the image plane decreased to approximately 0.25 .l/cm 2, very close to the value used in our planar growth study. Patterns were readily achieved in this set-up even for very low exposure times. Even single shot patterns could be achieved, but only by increasing the energy density. SEM micrographs were taken on uncoated oxidised surfaces (except for the high energy density sample which was coated with a thin layer of gold). Insulating oxide features were highlighted due to differential charge collection. The series of photographs 3-5 show the patterns at various magnifications. Fig. 3 indicates the effect of very high ( > 5 J / e r a 2) fluences. Severe surface darr~age resulted from non-uniform melting a~d recrystallisation and possibly local ablation. In the other photographs (figs. 4 and 5) patterns from a sample uniformly illuminated by 2000 shots are shown, where, both positive and negative regions of oxide are reproduced. Fig. ,~ shows oxide lin~s which were originally 70, 50, 40, 30, 20 and 10 g m wide on the mask. Since the lens
138
V. Nayar et al. / Oxidation of c-Si using excimer laser irradiation
Fig. 3. SEM micrograph showing the effect of high energy density upon the Si surface induced by the reduction lens system. reduces the mask pattern by a factor of 10, the actual lir,~widths on the Si are 7, 5, 4, 3, 2 and 1/Lm. These dknensions are verified l:y a comparison of the onJde lines and the scale at the top of the SEM rrficrographs. Fig. 5 d e m o n strafes the ability of the technique to grow repeated p a t t e r n s in a single 3 m m field exposure, i.e. there were n o movements of the sample or the images. With the present a r r a n g e m e n t D G L was a t : e m p t e d only in air. T h e ordde thickness m the patterns is estimated to vary between 25 a n d 28 A across the image. This was determined by removing the mask plate a n d imaging a d o t o n the surface. Although only 2000 shots were required to produce these selective
Fig. 4. Oxide patterns generated by reduced surface energy densities. The oxide lines are visible down to linewJdthsof almost 1/tm.
[~ Nayar et al. / Oxidation of c-Si using exciraer laser irradiation
139
Fig. 5. Many pattern units grown simultaneously with just 2000 shots. oxide patterns, increasing the number of shots only increased the oxide thick~..ess until it became self-limited without any notable variation in the lateral oimensions. 4. Summary and conclusions We have studied the growth of ultra-thin SiO z on Si at low temperatures using exchner laser radiation. The surface temperature rise cannot account for the growth of the oxide on a purely thermal basis and in comparison with visible laser oxidation U V fight has a much greater enhancement in the low temperature regime. Previous studies have linked this to the generation of a highly non-thermalised carrier plasma in or near the interfacial region and laser induced bond breaking in the interfacial region. The laser energy both exceeds the energy necessary to photoen'fit carriers from the Si into the oxide conduction band and also to break Si-Si bonds. One can increase the oxidation rate substantially by preheating to low temperatures. A recent study of low temperature oxidation [7] suggests that growth is controlled by the availability of atomic oxygen for temperatures below 750 o C. It is possible that the oxide growth observed in this study may be due to a process which creates atomic oxygen which can diffuse quickly through the ultrathin oxide at low temperatures, until itself diffusion-limits the reaction. One should therefore expect that at 193 nm the oxide would grow faster because it can more efficiently cause molecular oxygen to dissociate into atomic species. However, this effect may be small at atmospheric pressure because the lifetime of atomic oxygen is less than a millisecond.
14o
v. Naya~ el ~ ~ / Oxidation of c-Si using excimer laser irradiation
We have demonstra,,ed preliminary results of direct growth lithography of ultra-fifth oxide on Si using an imaging technique capable of producing linewidths down to r~.bout 1 /~m. Our patterned growth did not require the motion of either the sample or the beam, r_ei~her does it require traditional photoresist-based technology, thereby having a t i s t i n c t time advantage over this method. One ,::ould clearly apply the same optical system to etch and deposit material selectively. This method of surface processing is particularly suited to step and repeat type technology. It has an h~trinsic advantage in that the mask used to generate the imase has linewidths a factor of 10 greater than the image. It is clearly much easier to define a mask with linewidths o f 10 # m than 1 /~m, since producing, high defirfition (sub-nficron) masks in itself requires substantial effort. Custom device fabrication would benefit from this technology because the m',mber of device processing ,. teps are reduced drastically allowing reduced turnzround times.
Acknowledgements W e would like to acknowledge the Science and Engineering Research Council (SERC) and the Royal Signals and Radar Establishment for funding V. Nayar under a CASE studentship. We are grateful to A.M. Hodge and F. Micheli for many useful discussions about silicon oxidation and laser pr~cess:m.g. We also thank Professor Mino Green at Imperial College L o n d o n for use of the ellipsometer. This work was partially funded u n d e r SERC contra.:°~ G R / E 16090, and initiated by a Nuffield F o u n d a t i o n award.
Re[em::nces [1] !,.W.Boy& Laser Processingof Thin Films and Microstructures(Springer, Berlin, 1987). [21 S A. Schafer and S.A. Lyon, J. Vacuum Sci. Technol. 21 (1982) 423. [3] ILM. Young and W.A. Tiller, Appl. P|~ys.Letters 50 (1987) 80. [4J E. Fogarassy,S. Unamuno, J.L. Regoli~i and C. Fuchs, Phil. Mag. B 55 (1987) 253. [5"J E. Taft, J. Electrechem.So,:. 131 (1984) 2460. [~| E. Irene. P~fil.Mag. B 55 (1987) 131. 171 A.M. Hoff and 3. Ryzyno, Appl. Phys. Letters 52 (1988) 1264. [8t C.K. Ong, E.H. Sin and H.S. Tan, J. Opt. Soc. Am. B 3 (1986) 812. 't.¢] M. Berti, L.F. Dona &',ilia Rose, A.V. Drigo, C. Cohen, J. Siejka, G.G. Bentini and E. Jannitfi, Phys. Rev. B 34 (1986) 2346. [10] F. Foulon, E. Fogarassy,A. Saloui, C. Fuchs, S. Unamano and P. Siffert, Appl. Phys. A 45 (1988) 361. ['tl] F.N. Goodall, in: Advanced Lithography,Proe. Short Meeting Seri~ 10, Feb. 1988, Inst. of Phys.