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Titanium diffusion into LiNbO3 using excimer laser beam
408
TITANIUM
Applied Surface Science 36 (1989) 408-412 North-Holland, Amsterdam
DIFFUSION
S.A.M. AL-CHALABI
INTO LiNbO 3 USING EXCIMER LASER BEAM
*
Department of Electronic and Electrical Engineering, University College London, Londen WCIE 7JE. UK and F. G O O D A L L Rutherford Appleton Laboratory, (?hilton, Didcot, Oxfordshire OXI I OQX, UK Received 2 June 1988; accepted for publication 12 July 1988
The use of focused excimer laser beam irradiatic a of Ti films on LiNbO3 substrates has been studied. Secondary ion mass spectrometry (SIMS) and electron microprobe analysis (EMPA) of the rapidly annealed material indicates the diffusion of Ti into the bulk material. In this work, we report the diffusion of titanium using a focused excimer laser, the power density of the beam was varied by focusing and defocusing the beam. The process time used is < 1 s compared to many hours m the furnace. The results shows that the Ti diffusion depth varies with the beam power density. The present method has many advantages over previous rapid annealing work and with this procedure direct writing on LiNbO~ might be possible for the production of integrated optical devices.
1. l~ntroducfion Ti i n d i f f u s i o n is o n e o f t h e m o s t c o m m o n l y u s e d t e c h n i q u e s for f a b r i c a t i n g o p t i c a l w a v e g u i d e s in L i N b O j , w h e r e b y a T i - d o p e d h i g h e r refractive i n d e x a c t i n g as t h e g u i d i n g l a y e r is p r o d u c e d in t h e s u r f a c e r e g i o n o f t h e s a m p l e . T h e r m a l e v a p o r a t i o n o r s p u t t e r i n g is n o r m a l l y u s e d for d e p o s i t i n g T i o n L i N b O 3, t h e n s a m p l e s a r e a n n e a l e d in a f u r n a c e for 6 - 9 h in either d r y o r w e t o x y g e n at 9 0 0 - 1 1 0 0 ° C in o r d e r t h a t t h e Ti d i f f u s i o n t a k e s place. I n this work, we r e p o r t t h e r a p i d d i f f u s i o n o f Ti i n t o L i N b O 3 , u s i n g a f o c u s e d e x c i m e r laser b e a m . T h e p r o c e s s t i m e u s e d is < 1 s c o m p a r e d to m a n y h o u r s in t h e f u r n a c e . Rapid annealing of LiNbO 3 has been used recently [1-4] but the present * Present address: 16 Romney Road, New Malden, Surrey, KT3 5NN, UK. 0 1 6 9 - 4 3 3 2 / 8 9 / $ 0 3 . 5 0 © Elsevier Science P u b l i s h e r s B.V. (North-Holland Physics Publishing Division)
S.A.M. AI-Chalabi, 1~. Goodall / Titanium diffusion into LiNbOj
409
Table 1 Laser beam annealingconditions Energy dens[t7 per pulse (n~/cm 2) 1033 1033 664 372 1860
No. of pulses 10 5 5 5 5
method ha:~ many advantages over the previous work. With this method direct writing on LiNbO 3 is possible for the production of integrated optical devices.
2. Sample preparation and annealing process Y-cut LiNbO 3 samples were coated with Ti films 400/~, tl~ck by thermal evaporation. Wafers were then cut into 5 × 5 mm squares for annealing and analysis. Annealing was carried out in air using a Lambda Physik excimer laser at 249 nm, with a pulse width - 15 ns and bandwidth of 0.3 nm. A number of samples were annealed with different power densities and different numbers of shots. This is shown in table 1.
3. Material anaRysls 3.1. Electron microprobe analysis
Electron microprobe analysis (EMPA) was used to obtain a relative measure of the Ti content of the samples. The EMPA examination was restricted to verifying that Ti was still present in the annealed region of the sample, At high probe voltages the Ti .,;ignal was similar in both the annealed and the unexposed areas indicating that the total Ti was similar in the two regions. At low probe voltages analysing only to less than 1 /~m the Ti signal was considerably reduced in the annealed part of the samples indicating that Ti had been driven into the sample. 3.2. Secondary ion mass spec~'rometry
Five samples were chosen and exanfined using dynamic secondary ion mass spectrometry (SIMS). A 10.5 keV argon beam was used to stimulate secondary ion emission. Positive secondary ions were used to monitor the depth profiles of various dements in the samples.
410
S.A.M. AI-Chalabi, F. Goodall / Titanium diffusion into LiNbO~
For the analysis of T i : L i N b O 3 dopant profile, the concentration of titanium within the lattice has been considered to be proportional to the ratio of Ti-to-Nb peak heights This in effect normalises the titanium "dopant peak" to the Nb "lattice peak". Fig. 1 shows the T i / N b ratio for sample i l plotted as a function of depth. A high concentration of Ti is shown to a depth of 0.4 /~m after with a rapid depletion in Ti concentration occurs which disappears completely at a depth of 2 #m. These results suggest the formation of L i - T i - O compounds described by Bums et al. [6]. Betts et al. [5] found that they are a dominant feature for shallow structures which is the case in tile present diffusion process. They suggest a titanium layer (,n the surface region (to a depth of about 0.3 #m) with a concentration approaching a step function. This subsurface layer ~hen acts as a thicker, modifier source for further titanium diffusion into ti-: bulk of the crystal but with a reduced diffusion coefficient due to enha~aced bond stability. Rutherford backscatteEng analysis on samples annealed in the furnace [7] and those rapidly annealed [3] by electron beam, confirrr,ed the formal;ions of these compounds. We suggest that here the same diffusion steps have taken place except the time is less than 1 s. Fig. 2 shows the titanium depth profiles at 10% concel.ltration of the peak of samples described in table 1, against laser power density. The extent of diffusion of titanium into the bulk material clearly varies with power density and shows an increase in diffusion depth as the power density is increased.
1-0 E 0"8
o 0'6
i2- 0"4 ~ O'a
0"1
0"5
1"0
1"5
Depth in ( ~ m )
Fig. ]. Ti/Nb ratio as a measureof titaniumconcentrationwithinthe latticeplotted againstdepth measuredby SIMS.
S.A.M. AI.Chalabi, F. Gooda# / Titanium diffusion into LiNbO 3
[]
1750t
411
/
•E 1500
,_ 1250 100(]
,,5 7so 50C 25C
o.'2~
o'.so
O'.TS 1:0o
1"25
Depthin (~m) Fig. 2. Depth of titanium at which the concentration has fallen to 10% of its peak value against laser energy density. This suggests that the diffusion process can be controlled by varying the power density and can be optimised by more experimental work. Fig. 3 shows an optical micrograph of a sample annealed with the excimer laser. Clearly shown in this micrograph is a structure on the surface similar to those found by previous workers [7,3] where their n o n - u n i f o r m shapes proved to be due to titanium-rich islands formed after diffusion [3].
Fig. 3. Optical micrograph of sample after irradiation with laser.
412
S.A.M. AI-Chalabi, F. Goodall / Titanium diffusion into LiNbO 3
4. Discussiion and conclusions By direct writing with the laser beam, we expect that saznple decomposition a n d Li outdiffusion could be further reduced, since the time required for this process is less than 1 s. The process of laser b e a m annealing simplifies the problem of decomposition by confining the majority of the transient heating to the waveguide channels. Lateral diffusion of Ti has been a p r o b l e m for Ti in diffused waveguide. W e hope that with this fast annealing process lateral diffusion v~ill be much reduced a n d result in the production of more sharply defined liaes. This should reduce the crosstalk which occurs in devices processed ~,.v conventional thermal diffusion. Work is currently in progress to investigate this effect.
References [1] S.A.M. AI-Chalabi, Appl. Phys. Letters 47 (1985) 564. [21 S.A.M. AI-Chalabi, Radiation Effects 98 (1986) 39. [3] S.A.M. AI-CIhalabi,K.M. Barfoot and B.L. Weiss, Mater. Letters 4 (1986) 93. [4] S.A.M. Al-CLhalabi,B.L. Weiss and R.P. Homewood, Nucl. Instr. Methods Phys. Res. B 28 (1987) 255. [5] R.A. Betts, C.W. Pitts, K.R. Riddle and L.M. Walpita, Appl. Phys. A 31 (1983) 29. [6] W.K. Bums, P.H. Klein and E.J. West, J. Appl. Phys. 50 (1979) 6175. [7] M. Nicola Annenise, Clan Dio Canali, M. De Savio, A. Camera, P. Mazzoldi and G. Celoni, IEEE Trans. Components, Hybrid Manuf. Technol. CHMT 5 (1982) 212.
TITANIUM
Applied Surface Science 36 (1989) 408-412 North-Holland, Amsterdam
DIFFUSION
S.A.M. AL-CHALABI
INTO LiNbO 3 USING EXCIMER LASER BEAM
*
Department of Electronic and Electrical Engineering, University College London, Londen WCIE 7JE. UK and F. G O O D A L L Rutherford Appleton Laboratory, (?hilton, Didcot, Oxfordshire OXI I OQX, UK Received 2 June 1988; accepted for publication 12 July 1988
The use of focused excimer laser beam irradiatic a of Ti films on LiNbO3 substrates has been studied. Secondary ion mass spectrometry (SIMS) and electron microprobe analysis (EMPA) of the rapidly annealed material indicates the diffusion of Ti into the bulk material. In this work, we report the diffusion of titanium using a focused excimer laser, the power density of the beam was varied by focusing and defocusing the beam. The process time used is < 1 s compared to many hours m the furnace. The results shows that the Ti diffusion depth varies with the beam power density. The present method has many advantages over previous rapid annealing work and with this procedure direct writing on LiNbO~ might be possible for the production of integrated optical devices.
1. l~ntroducfion Ti i n d i f f u s i o n is o n e o f t h e m o s t c o m m o n l y u s e d t e c h n i q u e s for f a b r i c a t i n g o p t i c a l w a v e g u i d e s in L i N b O j , w h e r e b y a T i - d o p e d h i g h e r refractive i n d e x a c t i n g as t h e g u i d i n g l a y e r is p r o d u c e d in t h e s u r f a c e r e g i o n o f t h e s a m p l e . T h e r m a l e v a p o r a t i o n o r s p u t t e r i n g is n o r m a l l y u s e d for d e p o s i t i n g T i o n L i N b O 3, t h e n s a m p l e s a r e a n n e a l e d in a f u r n a c e for 6 - 9 h in either d r y o r w e t o x y g e n at 9 0 0 - 1 1 0 0 ° C in o r d e r t h a t t h e Ti d i f f u s i o n t a k e s place. I n this work, we r e p o r t t h e r a p i d d i f f u s i o n o f Ti i n t o L i N b O 3 , u s i n g a f o c u s e d e x c i m e r laser b e a m . T h e p r o c e s s t i m e u s e d is < 1 s c o m p a r e d to m a n y h o u r s in t h e f u r n a c e . Rapid annealing of LiNbO 3 has been used recently [1-4] but the present * Present address: 16 Romney Road, New Malden, Surrey, KT3 5NN, UK. 0 1 6 9 - 4 3 3 2 / 8 9 / $ 0 3 . 5 0 © Elsevier Science P u b l i s h e r s B.V. (North-Holland Physics Publishing Division)
S.A.M. AI-Chalabi, 1~. Goodall / Titanium diffusion into LiNbOj
409
Table 1 Laser beam annealingconditions Energy dens[t7 per pulse (n~/cm 2) 1033 1033 664 372 1860
No. of pulses 10 5 5 5 5
method ha:~ many advantages over the previous work. With this method direct writing on LiNbO 3 is possible for the production of integrated optical devices.
2. Sample preparation and annealing process Y-cut LiNbO 3 samples were coated with Ti films 400/~, tl~ck by thermal evaporation. Wafers were then cut into 5 × 5 mm squares for annealing and analysis. Annealing was carried out in air using a Lambda Physik excimer laser at 249 nm, with a pulse width - 15 ns and bandwidth of 0.3 nm. A number of samples were annealed with different power densities and different numbers of shots. This is shown in table 1.
3. Material anaRysls 3.1. Electron microprobe analysis
Electron microprobe analysis (EMPA) was used to obtain a relative measure of the Ti content of the samples. The EMPA examination was restricted to verifying that Ti was still present in the annealed region of the sample, At high probe voltages the Ti .,;ignal was similar in both the annealed and the unexposed areas indicating that the total Ti was similar in the two regions. At low probe voltages analysing only to less than 1 /~m the Ti signal was considerably reduced in the annealed part of the samples indicating that Ti had been driven into the sample. 3.2. Secondary ion mass spec~'rometry
Five samples were chosen and exanfined using dynamic secondary ion mass spectrometry (SIMS). A 10.5 keV argon beam was used to stimulate secondary ion emission. Positive secondary ions were used to monitor the depth profiles of various dements in the samples.
410
S.A.M. AI-Chalabi, F. Goodall / Titanium diffusion into LiNbO~
For the analysis of T i : L i N b O 3 dopant profile, the concentration of titanium within the lattice has been considered to be proportional to the ratio of Ti-to-Nb peak heights This in effect normalises the titanium "dopant peak" to the Nb "lattice peak". Fig. 1 shows the T i / N b ratio for sample i l plotted as a function of depth. A high concentration of Ti is shown to a depth of 0.4 /~m after with a rapid depletion in Ti concentration occurs which disappears completely at a depth of 2 #m. These results suggest the formation of L i - T i - O compounds described by Bums et al. [6]. Betts et al. [5] found that they are a dominant feature for shallow structures which is the case in tile present diffusion process. They suggest a titanium layer (,n the surface region (to a depth of about 0.3 #m) with a concentration approaching a step function. This subsurface layer ~hen acts as a thicker, modifier source for further titanium diffusion into ti-: bulk of the crystal but with a reduced diffusion coefficient due to enha~aced bond stability. Rutherford backscatteEng analysis on samples annealed in the furnace [7] and those rapidly annealed [3] by electron beam, confirrr,ed the formal;ions of these compounds. We suggest that here the same diffusion steps have taken place except the time is less than 1 s. Fig. 2 shows the titanium depth profiles at 10% concel.ltration of the peak of samples described in table 1, against laser power density. The extent of diffusion of titanium into the bulk material clearly varies with power density and shows an increase in diffusion depth as the power density is increased.
1-0 E 0"8
o 0'6
i2- 0"4 ~ O'a
0"1
0"5
1"0
1"5
Depth in ( ~ m )
Fig. ]. Ti/Nb ratio as a measureof titaniumconcentrationwithinthe latticeplotted againstdepth measuredby SIMS.
S.A.M. AI.Chalabi, F. Gooda# / Titanium diffusion into LiNbO 3
[]
1750t
411
/
•E 1500
,_ 1250 100(]
,,5 7so 50C 25C
o.'2~
o'.so
O'.TS 1:0o
1"25
Depthin (~m) Fig. 2. Depth of titanium at which the concentration has fallen to 10% of its peak value against laser energy density. This suggests that the diffusion process can be controlled by varying the power density and can be optimised by more experimental work. Fig. 3 shows an optical micrograph of a sample annealed with the excimer laser. Clearly shown in this micrograph is a structure on the surface similar to those found by previous workers [7,3] where their n o n - u n i f o r m shapes proved to be due to titanium-rich islands formed after diffusion [3].
Fig. 3. Optical micrograph of sample after irradiation with laser.
412
S.A.M. AI-Chalabi, F. Goodall / Titanium diffusion into LiNbO 3
4. Discussiion and conclusions By direct writing with the laser beam, we expect that saznple decomposition a n d Li outdiffusion could be further reduced, since the time required for this process is less than 1 s. The process of laser b e a m annealing simplifies the problem of decomposition by confining the majority of the transient heating to the waveguide channels. Lateral diffusion of Ti has been a p r o b l e m for Ti in diffused waveguide. W e hope that with this fast annealing process lateral diffusion v~ill be much reduced a n d result in the production of more sharply defined liaes. This should reduce the crosstalk which occurs in devices processed ~,.v conventional thermal diffusion. Work is currently in progress to investigate this effect.
References [1] S.A.M. AI-Chalabi, Appl. Phys. Letters 47 (1985) 564. [21 S.A.M. AI-Chalabi, Radiation Effects 98 (1986) 39. [3] S.A.M. AI-CIhalabi,K.M. Barfoot and B.L. Weiss, Mater. Letters 4 (1986) 93. [4] S.A.M. Al-CLhalabi,B.L. Weiss and R.P. Homewood, Nucl. Instr. Methods Phys. Res. B 28 (1987) 255. [5] R.A. Betts, C.W. Pitts, K.R. Riddle and L.M. Walpita, Appl. Phys. A 31 (1983) 29. [6] W.K. Bums, P.H. Klein and E.J. West, J. Appl. Phys. 50 (1979) 6175. [7] M. Nicola Annenise, Clan Dio Canali, M. De Savio, A. Camera, P. Mazzoldi and G. Celoni, IEEE Trans. Components, Hybrid Manuf. Technol. CHMT 5 (1982) 212.