The optimization process of laser scribing of back contacts is carried out by varying different parameters of laser and thickness of Molybdenum (Mo) thin-films. Mo thin films were deposited by RF magnetron sputtering on the organically cleaned soda lime glass substrate. The thickness of Mo was in the range of 60 nm to 800 nm. For the scribing process the laser power and the laser pulse frequency were varied. Different thickness of Mo shows the different scribe behavior. The optimized process provides a successful isolative laser scribing, having a minimum scribe line width, of Mo layer on glass substrate without any presence of walls, ridges, or collars in scribed areas.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/929

Desai, M.S.

Padhiar, V.B.

Panchal, C.J.

Patel, A.L.

Ray, J.R.

SocioEconomic Challenges Journal (Sumy State University)

 
 
 
J. Nano- Electron. Phys. 
3 (2011) No1, P.9-16 
 2011 SumDU 
(Sumy State University) 
 
 
9 
PACS numbers: 42.62.Cf, 73.61.At 
 
LASER SCRIBING OPTIMIZATION OF RF MAGNETRON SPUTTERED 
MOLYBDENUM THIN FILMS 
 
V.B. Padhiar1, A.L. Patel2#, J.R. Ray1, M.S. Desai1, C.J. Panchal1* 
 
1 Department of Applied Physics, M.S. University of Baroda, 
 Vadodara – 390001, Gujarat India 
* E-mail: cjpanchal_msu@yahoo.com 
2 Director, Sahjanand Laser Technology Ltd.,  
 E-30, G. I.D.C. Electronics Estate, Sector 26,  
 Ghandhinagar-382 028, Gujarat, India 
# E-mail: arvindp@sahajanandlaser.com  
 
The optimization process of laser scribing of back contacts is carried out by varying 
different parameters of laser and thickness of Molybdenum (Mo) thin-films. Mo thin 
films were deposited by RF magnetron sputtering on the organically cleaned soda lime 
glass substrate. The thickness of Mo was in the range of 60 nm to 800 nm. For the 
scribing process the laser power and the laser pulse frequency were varied. Different 
thickness of Mo shows the different scribe behavior. The optimized process provides a 
successful isolative laser scribing, having a minimum scribe line width, of Mo layer 
on glass substrate without any presence of walls, ridges, or collars in scribed areas.  
 
Keywords: LASER SCRIBING, MOLYBDENUM THIN FILM, LASER POWER, 
PULSE FREQUENCY, SCRIBE LINE WIDTH. 
 
(Received 04 February 2011) 
 
1. INTRODUCTION 
 
Thin  film  solar  cell  technology  provides  a  best  solution  towards  more  
economic solar cell energy. The first step in manufacture of thin film solar 
cell  is  the deposition of  molybdenum (Mo)  back contact  layer  on soda lime 
glass substrate and the next step is the isolative laser scribing of this layer 
for making a metal back contacts between the layers. However, the problems 
are arising with a heat-affected zone around the scribed area due to the 
laser pulse beam [1]. The scribing should be done in such a way so that the 
scribed line should not contain bridges or process residue in the grooves that 
could cause an electrical connection and short-circuit the cell [2]. To increase 
the solar cell efficiency, the scribing of Mo layers desires minimum scribed 
area to have maximum active solar cell area [3] and so, minimize the dead 
area in the cell. Optimum operation for thin-film PV materials has been 
investigated by several PV manufacturers [4, 5, 6]. 
 In this study, we present the results regarding the optimization of 
isolative laser scribing of sputtered-Mo thin-film deposited on soda lime 
glass substrate. The optimization process is performed by varying different 
laser parameters and thickness of the Mo thin-film in order to achieve 
lowest resistivity films. The result of successful scribing yields reliable, 
reproducible clean scribes without any presence of the buckling, ridges, or 
collars in the scribed areas. 
 
 
 
10 V.B. PADHIAR, A.L. PATEL, J.R. RAY, ET AL.  
 
2. EXPERIMENTAL 
 
The processes of laser scribing of Mo thin-films were done using commer-
cially available the laser system that has the multi diode pumped fiber laser 
(20 W average maximum power) [Model No. Akhsar Fiber-Pro, Sahajanand 
Laser Technology Ltd., INDIA] Mo thin films used for the scribing have a 
different thickness from 60 nm to 800 nm. Mo thin films were deposited by 
RF magnetron sputtering at 1mTorr working pressure and 100 W RF power.  
The electrical property viz. sheet resistance of the Mo thin films was 
measured using four point probe method. The laser system, which was used 
for the scribing of Mo thin film, has a specification shown in Table 1. 
 
Table 1 – Laser system parameters used for the scribing the Mo thin films 
 
Laser Multi Diode Pump Fiber Laser 
Nominal average power 20 W (optional 10 W) 
Maximum peak power > 7.5 kW 
Power tunability 10 - 100 % 
Pulse repetation rate 20 - 80 kHz 
Wavelength 1060 r 10 nm 
Pulse duration@20 kHz <120 ns 
Class IV 
Power stability > 95 % 
Pulse energy@20 kHz 1 mJ 
Beam quality 1.5 (M2) 
Output beam diameter(1/e2) 9 mm 
Scribe speed  upto 1000 mm/s 
Inbuilt guide (marking) laser He-Ne laser (660 nm & 0.5 mW) 
 
 In our study we have kept the scribing speed constant at 500 mm/sec in 
order to achieve lowest scribed width. Scribing process of Mo thin film was 
optimized by varying both the laser power and the pulse frequency 
simultaneously. First keeping the maximum average power 20 W and 
varying  the  pulse  frequency  from  1  to  80  Hz  to  get  the  optimum  pulse  
frequency and by using that, vary the average power up to minimum of 
1 W. Different thickness of the Mo shows different kind of scribe pattern. 
The smoothness of the scribed was observed using a polarization microscope 
(LABOURLUX 11, Leitz). 
 
3. RESULTS AND DISCUSSIONS  
 
Mo thin films having different thickness (60 nm to 800 nm) shows a 
different electrical conductivity [7]. The measured sheet resistance of the 
Mo  thin  films  as  a  function  of  thickness  of  the  films  is  shown  in  Fig.  1.  
Different thickness of the Mo thin films shows different kind of the scribing 
behavior. Ablation with a train of laser pulses per spot and scribing lines 
were performed on molybdenum back-contact from film side. The laser spot 
overlap along a scanning line was controlled by the translation speed at a 
constant pulse repetition rate. 
 
 
 
 
 LASER SCRIBING OPTIMIZATION OF RF MAGETRON SPUTTERED… 11 
 
 
 
 
Fig. 1 – Variation in the sheet resistance of Mo thin films as a function of thickness 
indicates that higher thickness shows higher conductivity 
 
 Various combinations of laser power and pulse frequency were used for 
selective ablation of the films. Optimal regimes for laser processing of each 
layer were estimated depending on the Mo film thickness. The beam overlap 
or  the  number  of  pulses  played  an  important  role  in  the  processing  
selectivity because the ablation threshold was sensitive to accumulation of 
the irradiation dose. 
 
3.1 Effect of laser power and pulse frequency  
 
However, at certain scribe parameters, namely the laser power together with 
laser scribe speed and pulse frequency, the scribes were superior to the high-
energy scribes in two immediate respects: 1) The scribe profile had sharply 
defined edges without significant formation of walls, and 2) the glass 
remained undamaged, glass damage invariably appearing at higher energy 
densities. 
 
 
 
Fig. 2 –  Sharp  scribed  line  (140  µm) of Mo thin-film has 2 watt laser power and 
1 Hz pulse frequency 
 
 The  scribing  image  (Fig.  2)  of  Mo  thin  films  that  has  continuous  and  
regular  overlapping  of  the  laser  spots  forms  the  sharp  edge  scribe.  In  
addition there were no collars and ridges left at the scribe edge. The scribed 
line width, which is important parameter in the patterning of the 
 
 
 
12 V.B. PADHIAR, A.L. PATEL, J.R. RAY, ET AL.  
 
semiconductor single or multi layers, is mainly depending on laser power 
and pulse frequency. Laser power varies in this study is from 20 W to 2 W 
and the pulse frequency varies from 1 Hz to 80 Hz. Fig. 3 shows the 
obtained comparative results of the scribed line of Mo thin film (60 nm) by 
varying the laser power and pulse frequency. By increasing the laser power 
and laser pulse frequency, the scribed line width increases. For higher laser 
power  i.e.  20  W,  the  width  of  the  scribed  line  was  240  Pm at  1  Hz  pulse  
frequency and 390 Pm at 80 Hz pulse frequency. While at lower laser power 
i.e. 2 W, the increase in the width of the scribed line was negligible. 
 
 
 
Fig. 3 – The variation of the scribed line width of Mo thin film (60 nm) as a 
function  of  laser  pulse  frequency  shows  that  as  the  laser  power  and  the  pulse  
frequency were set at minimum level, the scribe line width is lower 
 
 The scribed line width should be kept minimum for its future use in the 
patterning the different semiconductor devices. From Fig. 3, we said that at 
2 W laser power and 1 Hz pulse frequency we get the minimum scribe width 
i.e. 210 Pm. The 210 Pm  is  still  a  higher  value.  Therefore,  by  using  the  
optimal laser parameters for 60 nm Mo film thickness i.e. 2 W laser power 
and 1 Hz pulse frequency we scribed the higher thickness of Mo thin film.  
 Fig. 4 shows the variation in the scribe line width as a function of pulse 
frequency. In this scribing, we found that as the pulse frequency reduces the 
scribe line width reduces. The minimum scribing width was 30 Pm for 2 W 
laser power and 1 Hz pulse frequency. We had also scribe the different 
thickness of Mo thin films at 2 W laser power and 1 Hz pulse frequency and 
got  the similar  kind of  variation.  From that  variation,  we can say that  as  
the thickness of the Mo thin films increases the scribed line width decreases, 
too, this is shown in Fig. 5. 
 At  low  power  (2  W)  and  low  frequency  (1  Hz)  we  have  achieve  the  
minimum scribe line width i.e. 40 Pm (Fig. 5a). In order to achieve lowest 
scribe line width without any presence of walls and collars in the scribed 
area, we scribe the higher thickness Mo films at 0.2 w laser power and 1 Hz 
pulse frequency, as shown in Fig. 5b. At 0.2 w laser power and 1 Hz pulse 
frequency the minimum scribe line width is 38 mm. We also try the lower 
thickness of the Mo thin film for the 0.2 w laser power, but due to the lower 
thickness, the films experienced a stress, so, the scribe line was distorted or 
not uniform. Such scribe lines observed using the polarization microscope is 
shown in Fig. 6. 
 
 
 
 LASER SCRIBING OPTIMIZATION OF RF MAGETRON SPUTTERED… 13 
 
 
 
 
 
Fig. 4 – The variation of the scribed line width of Mo thin film (800 nm) as a 
function of laser pulse frequency shows that as the pulse frequency was reduces; the 
scribe line width is decreases 
 
  
  a b 
 
Fig. 5 – Scribed line thickness is decreases as the thickness of the Mo thin film 
increases at 2 W  laser  power  and  1  Hz  pulse  frequency  (a)  while  at  the  same  
frequency and 0.2 w laser power the minimum scribe line width observed (b) 
 
3.2 Scribe quality 
 
However, the great results were extremely rare and seemed to occur within 
a process window of miniscule proportions. During the extensive 
experimentation only a handful of 40 Pm scribes were achieved with good 
results. Nonetheless, the excellent quality of these scribes invite to 
speculation as to whether some alterations could be made to the system in 
order to achieve such scribes. Scribe results often terrible, with big shards 
and cracks. The high energy scribe results that are referred to as good on 
the  other  hand  are  not  square.  Viewed  from  the  top  they  have  a  typical  
pulse-to-pulse appearance of slightly overlapping circular holes. If the work 
piece speed is too high the pulses will loose their overlap and bridges will 
occur. If the work piece slows down the “lips” that protrude into the scribe 
tend to flake up more readily. 
 
 
 
 
14 V.B. PADHIAR, A.L. PATEL, J.R. RAY, ET AL.  
 
   
 
Fig. 6 – Stress introduce in the lower thickness of Mo thin films while scribing so, 
the scribe line becomes non-uniform 
 
 In order to achieve the better quality of the scribe line we vary the laser 
power and pulse frequency for the different thickness of the Mo thin films. 
Ablation  with  a  train  of  laser  pulses  per  spot  defines  the  scribe  quality.  
Theoretically, the pulse train of the laser scribe the material is shown in 
Fig. 7. Some calculations were made in order to establish the geometrical 
situation [8, 9] during the laser pulse. 
 
 
 
Fig. 7 – Geometrical representation of exposed area 
 
 Given  that  the  laser  spot  has  a  radius,  R, and  is  repeated  with  a  
frequency onto a sample that is moving with the speed, v, one can express 
the surface area A of each consecutive pulse as the spot size minus the 
overlap of pulses (see Fig. 7). This area can be considered as the area of film 
removed per pulse. 
 
 
2
2 2 22 arccos
2 2 4
y y y
A R R R
R
S § ·' ' '§ ·   ¨ ¸¨ ¸¨ ¸© ¹© ¹
 (1) 
 
where 'y is the distance between consecutive pulse centers. 
 
 
1
y v
f
'  u  (2) 
 
 The obtained exposed area (scribe line width) of the scribe line of 
different thickness of the Mo thin films is shown in Fig. 8. 
 
 
 
 
 LASER SCRIBING OPTIMIZATION OF RF MAGETRON SPUTTERED… 15 
 
 
 
 
Fig. 8 – Scribe line width of Mo thin films of different thickness i.e. from (a) to (h) 
is 60 nm to 800 nm, results of an average laser power is 2 W and 5 Hz laser pulse 
frequency 
 
 As increasing the thickness of the Mo thin films, we saw from the Fig. 8, 
that the scribe line width decreases. At the higher thickness of the Mo thin 
films, we got the better scribe area as well as the minimum side edges. For 
800 nm thin Mo films, the minimum scribe line width i.e. 80 ȝm was obtain 
(Fig. 8h). The maximum scribe line width, i.e. 220 ȝm, with poor scribe area 
is observed at 60 nm thin Mo films (Fig. 8a). 
 
4. CONCLUSIONS 
 
The presented work shows a process of laser scribing for Mo thin-film of 
different thickness (60 nm to 800 nm) has been considered. Laser 
parameters have been determined that provide reproducible, good scribes, 
that do not present any unwanted bridges. Scribing is performed at energy 
 
 
 
16 V.B. PADHIAR, A.L. PATEL, J.R. RAY, ET AL.  
 
densities of around 1 mJ at 20 kHz, with a pulse-to-pulse overlap by the 
relation between laser power and laser pulse frequency. Considering the 
laser scribing step, molybdenum having a higher thickness yield better 
results compare to the lower thickness. Lower thickness of Mo shows the 
non uniform distribution or dull scribe edges of the scribe line. The scribe 
line width is observed minimum (80 Pm) at higher thickness (800 nm) and 
maximum (390 Pm) at lower thickness (60 nm). 
 
 The authors are thankful to UGC-DRS (file no. 530/2/DRS/2007(SAP-1)) 
for providing an electrical measurement facility.  
 
REFERENCES 
1. A.D. Compaan, I. Matulionis, and S. Nakade, Final Technical Progress Report, 
NREL/SR-520-24842 (1998). 
2. K.L. Chopra, Thin Film Phenomenon (McGraw-hill: 1969). 
3. C. Solanki, Solar Photovoltaics: Fundamentals, Technologies and Applications 
(PHI Learning Pvt. Ltd: 2009). 
4. K.L. Chopra, I. Kaur, Thin Film Device Applications (NewYork: Plenum Press: 
1983). 
5. V. Ramanathan, L.A. Russell, C.H. Liu, and P.V. Meyers, Solar Cells 28, 129 
(1990). 
6. J.G. O'Dowd, N.R. Johnson, R.S. Oswald, and F. Willing, Proceeding 23rd IEEE 
PVSC, 926 (1993). 
7. O.S. Heavens, Thin Film Physics (Edinburgh: T. & A. Constable Ltd: 1970). 
8. R. Arya, Solarex Final Technical Report. NREL, contract ZN-I-19019-4 (1995). 
9. W.M. Steen, J. Mazumder, Laser Material Processing, (Springer-Verlag London 
Limited: 2010). 
 


Laser scribing optimization of RF magnetron sputtered molybdenum thin films

Electronic Sumy State University Institutional Repository

   J. Nano- Electron. Phys. 3 (2011) No1, P.9-16  2011 SumDU (Sumy State University)   9 PACS numbers: 42.62.Cf, 73.61.At  LASER SCRIBING OPTIMIZATION OF RF MAGNETRON SPUTTERED MOLYBDENUM THIN FILMS  V.B. Padhiar1, A.L. Patel2#, J.R. Ray1, M.S. Desai1, C.J. Panchal1*  1 Department of Applied Physics, M.S. University of Baroda,  Vadodara – 390001, Gujarat India * E-mail: cjpanchal_msu@yahoo.com 2 Director, Sahjanand Laser Technology Ltd.,   E-30, G. I.D.C. Electronics Estate, Sector 26,   Ghandhinagar-382 028, Gujarat, India # E-mail: arvindp@sahajanandlaser.com   The optimization process of laser scribing of back contacts is carried out by varying different parameters of laser and thickness of Molybdenum (Mo) thin-films. Mo thin films were deposited by RF magnetron sputtering on the organically cleaned soda lime glass substrate. The thickness of Mo was in the range of 60 nm to 800 nm. For the scribing process the laser power and the laser pulse frequency were varied. Different thickness of Mo shows the different scribe behavior. The optimized process provides a successful isolative laser scribing, having a minimum scribe line width, of Mo layer on glass substrate without any presence of walls, ridges, or collars in scribed areas.   Keywords: LASER SCRIBING, MOLYBDENUM THIN FILM, LASER POWER, PULSE FREQUENCY, SCRIBE LINE WIDTH.  (Received 04 February 2011)  1. INTRODUCTION  Thin  film  solar  cell  technology  provides  a  best  solution  towards  more  economic solar cell energy. The first step in manufacture of thin film solar cell  is  the deposition of  molybdenum (Mo)  back contact  layer  on soda lime glass substrate and the next step is the isolative laser scribing of this layer for making a metal back contacts between the layers. However, the problems are arising with a heat-affected zone around the scribed area due to the laser pulse beam [1]. The scribing should be done in such a way so that the scribed line should not contain bridges or process residue in the grooves that could cause an electrical connection and short-circuit the cell [2]. To increase the solar cell efficiency, the scribing of Mo layers desires minimum scribed area to have maximum active solar cell area [3] and so, minimize the dead area in the cell. Optimum operation for thin-film PV materials has been investigated by several PV manufacturers [4, 5, 6].  In this study, we present the results regarding the optimization of isolative laser scribing of sputtered-Mo thin-film deposited on soda lime glass substrate. The optimization process is performed by varying different laser parameters and thickness of the Mo thin-film in order to achieve lowest resistivity films. The result of successful scribing yields reliable, reproducible clean scribes without any presence of the buckling, ridges, or collars in the scribed areas.    10 V.B. PADHIAR, A.L. PATEL, J.R. RAY, ET AL.   2. EXPERIMENTAL  The processes of laser scribing of Mo thin-films were done using commer-cially available the laser system that has the multi diode pumped fiber laser (20 W average maximum power) [Model No. Akhsar Fiber-Pro, Sahajanand Laser Technology Ltd., INDIA] Mo thin films used for the scribing have a different thickness from 60 nm to 800 nm. Mo thin films were deposited by RF magnetron sputtering at 1mTorr working pressure and 100 W RF power.  The electrical property viz. sheet resistance of the Mo thin films was measured using four point probe method. The laser system, which was used for the scribing of Mo thin film, has a specification shown in Table 1.  Table 1 – Laser system parameters used for the scribing the Mo thin films  Laser Multi Diode Pump Fiber Laser Nominal average power 20 W (optional 10 W) Maximum peak power > 7.5 kW Power tunability 10 - 100 % Pulse repetation rate 20 - 80 kHz Wavelength 1060 r 10 nm Pulse duration@20 kHz <120 ns Class IV Power stability > 95 % Pulse energy@20 kHz 1 mJ Beam quality 1.5 (M2) Output beam diameter(1/e2) 9 mm Scribe speed  upto 1000 mm/s Inbuilt guide (marking) laser He-Ne laser (660 nm & 0.5 mW)   In our study we have kept the scribing speed constant at 500 mm/sec in order to achieve lowest scribed width. Scribing process of Mo thin film was optimized by varying both the laser power and the pulse frequency simultaneously. First keeping the maximum average power 20 W and varying  the  pulse  frequency  from  1  to  80  Hz  to  get  the  optimum  pulse  frequency and by using that, vary the average power up to minimum of 1 W. Different thickness of the Mo shows different kind of scribe pattern. The smoothness of the scribed was observed using a polarization microscope (LABOURLUX 11, Leitz).  3. RESULTS AND DISCUSSIONS   Mo thin films having different thickness (60 nm to 800 nm) shows a different electrical conductivity [7]. The measured sheet resistance of the Mo  thin  films  as  a  function  of  thickness  of  the  films  is  shown  in  Fig.  1.  Different thickness of the Mo thin films shows different kind of the scribing behavior. Ablation with a train of laser pulses per spot and scribing lines were performed on molybdenum back-contact from film side. The laser spot overlap along a scanning line was controlled by the translation speed at a constant pulse repetition rate.      LASER SCRIBING OPTIMIZATION OF RF MAGETRON SPUTTERED… 11     Fig. 1 – Variation in the sheet resistance of Mo thin films as a function of thickness indicates that higher thickness shows higher conductivity   Various combinations of laser power and pulse frequency were used for selective ablation of the films. Optimal regimes for laser processing of each layer were estimated depending on the Mo film thickness. The beam overlap or  the  number  of  pulses  played  an  important  role  in  the  processing  selectivity because the ablation threshold was sensitive to accumulation of the irradiation dose.  3.1 Effect of laser power and pulse frequency   However, at certain scribe parameters, namely the laser power together with laser scribe speed and pulse frequency, the scribes were superior to the high-energy scribes in two immediate respects: 1) The scribe profile had sharply defined edges without significant formation of walls, and 2) the glass remained undamaged, glass damage invariably appearing at higher energy densities.    Fig. 2 –  Sharp  scribed  line  (140  µm) of Mo thin-film has 2 watt laser power and 1 Hz pulse frequency   The  scribing  image  (Fig.  2)  of  Mo  thin  films  that  has  continuous  and  regular  overlapping  of  the  laser  spots  forms  the  sharp  edge  scribe.  In  addition there were no collars and ridges left at the scribe edge. The scribed line width, which is important parameter in the patterning of the    12 V.B. PADHIAR, A.L. PATEL, J.R. RAY, ET AL.   semiconductor single or multi layers, is mainly depending on laser power and pulse frequency. Laser power varies in this study is from 20 W to 2 W and the pulse frequency varies from 1 Hz to 80 Hz. Fig. 3 shows the obtained comparative results of the scribed line of Mo thin film (60 nm) by varying the laser power and pulse frequency. By increasing the laser power and laser pulse frequency, the scribed line width increases. For higher laser power  i.e.  20  W,  the  width  of  the  scribed  line  was  240  Pm at  1  Hz  pulse  frequency and 390 Pm at 80 Hz pulse frequency. While at lower laser power i.e. 2 W, the increase in the width of the scribed line was negligible.    Fig. 3 – The variation of the scribed line width of Mo thin film (60 nm) as a function  of  laser  pulse  frequency  shows  that  as  the  laser  power  and  the  pulse  frequency were set at minimum level, the scribe line width is lower   The scribed line width should be kept minimum for its future use in the patterning the different semiconductor devices. From Fig. 3, we said that at 2 W laser power and 1 Hz pulse frequency we get the minimum scribe width i.e. 210 Pm. The 210 Pm  is  still  a  higher  value.  Therefore,  by  using  the  optimal laser parameters for 60 nm Mo film thickness i.e. 2 W laser power and 1 Hz pulse frequency we scribed the higher thickness of Mo thin film.   Fig. 4 shows the variation in the scribe line width as a function of pulse frequency. In this scribing, we found that as the pulse frequency reduces the scribe line width reduces. The minimum scribing width was 30 Pm for 2 W laser power and 1 Hz pulse frequency. We had also scribe the different thickness of Mo thin films at 2 W laser power and 1 Hz pulse frequency and got  the similar  kind of  variation.  From that  variation,  we can say that  as  the thickness of the Mo thin films increases the scribed line width decreases, too, this is shown in Fig. 5.  At  low  power  (2  W)  and  low  frequency  (1  Hz)  we  have  achieve  the  minimum scribe line width i.e. 40 Pm (Fig. 5a). In order to achieve lowest scribe line width without any presence of walls and collars in the scribed area, we scribe the higher thickness Mo films at 0.2 w laser power and 1 Hz pulse frequency, as shown in Fig. 5b. At 0.2 w laser power and 1 Hz pulse frequency the minimum scribe line width is 38 mm. We also try the lower thickness of the Mo thin film for the 0.2 w laser power, but due to the lower thickness, the films experienced a stress, so, the scribe line was distorted or not uniform. Such scribe lines observed using the polarization microscope is shown in Fig. 6.     LASER SCRIBING OPTIMIZATION OF RF MAGETRON SPUTTERED… 13      Fig. 4 – The variation of the scribed line width of Mo thin film (800 nm) as a function of laser pulse frequency shows that as the pulse frequency was reduces; the scribe line width is decreases      a b  Fig. 5 – Scribed line thickness is decreases as the thickness of the Mo thin film increases at 2 W  laser  power  and  1  Hz  pulse  frequency  (a)  while  at  the  same  frequency and 0.2 w laser power the minimum scribe line width observed (b)  3.2 Scribe quality  However, the great results were extremely rare and seemed to occur within a process window of miniscule proportions. During the extensive experimentation only a handful of 40 Pm scribes were achieved with good results. Nonetheless, the excellent quality of these scribes invite to speculation as to whether some alterations could be made to the system in order to achieve such scribes. Scribe results often terrible, with big shards and cracks. The high energy scribe results that are referred to as good on the  other  hand  are  not  square.  Viewed  from  the  top  they  have  a  typical  pulse-to-pulse appearance of slightly overlapping circular holes. If the work piece speed is too high the pulses will loose their overlap and bridges will occur. If the work piece slows down the “lips” that protrude into the scribe tend to flake up more readily.     14 V.B. PADHIAR, A.L. PATEL, J.R. RAY, ET AL.       Fig. 6 – Stress introduce in the lower thickness of Mo thin films while scribing so, the scribe line becomes non-uniform   In order to achieve the better quality of the scribe line we vary the laser power and pulse frequency for the different thickness of the Mo thin films. Ablation  with  a  train  of  laser  pulses  per  spot  defines  the  scribe  quality.  Theoretically, the pulse train of the laser scribe the material is shown in Fig. 7. Some calculations were made in order to establish the geometrical situation [8, 9] during the laser pulse.    Fig. 7 – Geometrical representation of exposed area   Given  that  the  laser  spot  has  a  radius,  R, and  is  repeated  with  a  frequency onto a sample that is moving with the speed, v, one can express the surface area A of each consecutive pulse as the spot size minus the overlap of pulses (see Fig. 7). This area can be considered as the area of film removed per pulse.   22 2 22 arccos2 2 4y y yA R R RRS § ·' ' '§ ·   ¨ ¸¨ ¸¨ ¸© ¹© ¹ (1)  where 'y is the distance between consecutive pulse centers.   1y vf'  u  (2)   The obtained exposed area (scribe line width) of the scribe line of different thickness of the Mo thin films is shown in Fig. 8.      LASER SCRIBING OPTIMIZATION OF RF MAGETRON SPUTTERED… 15     Fig. 8 – Scribe line width of Mo thin films of different thickness i.e. from (a) to (h) is 60 nm to 800 nm, results of an average laser power is 2 W and 5 Hz laser pulse frequency   As increasing the thickness of the Mo thin films, we saw from the Fig. 8, that the scribe line width decreases. At the higher thickness of the Mo thin films, we got the better scribe area as well as the minimum side edges. For 800 nm thin Mo films, the minimum scribe line width i.e. 80 ȝm was obtain (Fig. 8h). The maximum scribe line width, i.e. 220 ȝm, with poor scribe area is observed at 60 nm thin Mo films (Fig. 8a).  4. CONCLUSIONS  The presented work shows a process of laser scribing for Mo thin-film of different thickness (60 nm to 800 nm) has been considered. Laser parameters have been determined that provide reproducible, good scribes, that do not present any unwanted bridges. Scribing is performed at energy    16 V.B. PADHIAR, A.L. PATEL, J.R. RAY, ET AL.   densities of around 1 mJ at 20 kHz, with a pulse-to-pulse overlap by the relation between laser power and laser pulse frequency. Considering the laser scribing step, molybdenum having a higher thickness yield better results compare to the lower thickness. Lower thickness of Mo shows the non uniform distribution or dull scribe edges of the scribe line. The scribe line width is observed minimum (80 Pm) at higher thickness (800 nm) and maximum (390 Pm) at lower thickness (60 nm).   The authors are thankful to UGC-DRS (file no. 530/2/DRS/2007(SAP-1)) for providing an electrical measurement facility.   REFERENCES 1. A.D. Compaan, I. Matulionis, and S. Nakade, Final Technical Progress Report, NREL/SR-520-24842 (1998). 2. K.L. Chopra, Thin Film Phenomenon (McGraw-hill: 1969). 3. C. Solanki, Solar Photovoltaics: Fundamentals, Technologies and Applications (PHI Learning Pvt. Ltd: 2009). 4. K.L. Chopra, I. Kaur, Thin Film Device Applications (NewYork: Plenum Press: 1983). 5. V. Ramanathan, L.A. Russell, C.H. Liu, and P.V. Meyers, Solar Cells 28, 129 (1990). 6. J.G. O'Dowd, N.R. Johnson, R.S. Oswald, and F. Willing, Proceeding 23rd IEEE PVSC, 926 (1993). 7. O.S. Heavens, Thin Film Physics (Edinburgh: T. & A. Constable Ltd: 1970). 8. R. Arya, Solarex Final Technical Report. NREL, contract ZN-I-19019-4 (1995). 9. W.M. Steen, J. Mazumder, Laser Material Processing, (Springer-Verlag London Limited: 2010).  

Laser scribing optimization of RF magnetron sputtered molybdenum thin films

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