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Sensor for temperature measurement of laser heated surfaces
Vacuum/volume
Pergamon
Sensor for temperature heated surfaces
measurement
45/number
12/pages 1187 to 1189/1994 Elsevier Science Ltd Printed in Great Britain 0042-207x/94$7.00+.00
of laser
V Shanov*, P Petkovt, B Ivanov*, C Popov* and C Vodenicharovt, Technological University *Departments of Semiconductors and t Physics, 8 Kliment Ohridski Str, 1756 Sofia, Bulgaria received for publication
of Sofia,
9 August 1993
A small area nickel-based sensor for surface temperature measurement has been developed on different insulating substrates. The dependence of resistance on temperature in the range 50-350°C was studied and the temperature coefficient of resistance was determined, which provided the high sensibility of the device. The sensor was successfully tested in laser irradiation and the promising results obtained made it applicable for surface temperature estimation with an accuracy of + 1°C during laser processing.
Introduction Surface temperature is one of the most important parameters for laser processing. Because the different optical and thermal properties of the substrates used are not known accurately in some cases, theoretical estimates of laser-induced temperature cannot always be made. Reliable measurements of surface temperature in a laser spot are essential for the understanding of laser-induced processes. Furthermore, if temperature is measured experimentally, the values obtained could be compared to theoretically calculated ones, in order to check the validity of a particular model; it shows the necessity for the development of a precise sensor able to measure the surface temperature caused by a laser beam. Because of the peculiarities of the laser processing, these sensors should possess some special features: (i) to be without transit-time effects; (ii) to have small dimensions; (iii) to possess strict data reproduction; and (iv) to be covered on top with material with optical and thermal properties, similar to those of the substrate, on which the process can be carried out. In the literature there is scant information for such sensors’. Another approach exists, connected with the development of thin-film thermocouples for surface temperature measurement 2.3, but production is more difficult from a technological point of view; this directed our efforts towards the development of a sensor able to measure surface temperature directly and precisely during laser processing. Experimental A nickel film was chosen for the working element of the sensor, due to its relatively high temperature coefficient of resistance. Furthermore, it is much cheaper than the precious metal platinum, previously used for this purpose. Nickel films (9&100 nm, target value 100 nm) were deposited by evaporation and sputtering on different substrates: monocrystalline silicon wafers with undoped polysilicon, alumina ceramics, Corning glass and glass ceramics. The variety of substrates used was determined by the
necessity to provide the same surface properties for the sensor as those of the substrates most commonly used for laser processing. Due to the techniques mentioned, deposition of films with the desired resistance was possible. The evaporation was carried out in a vacuum of 3 x 1O- 6 mbar. A graphite crucible heated up to 1300 K was used and the temperature of the substrate was kept at 45@470 K. The deposition rate obtained at these process parameters was 1.5 nmjs. In the case of sputtering with Ar+ the vacuum in the chamber was 3 x 10-j mbar, the temperature of the substrate 45&470 K and sputtering current 0.6 A. A thin film of gold was deposited upon the nickel for further formation of the electrodes. Gold was chosen due to its high chemical stability and low resistivity. Gold films were deposited by evaporation in a vacuum of 3 x 10e6 mbar and the temperature of the tungsten crucible was 1200 K. The thickness of the films obtained was in the range 5&70 nm. The formation of the electrodes was achieved by photolithography with an etching agent, I 2 + KI (in ethanol), which did not attack the nickel sublayer. The chosen form of the Ni element was a meander, which provided a higher resistance of sensor and higher sensitivity. In our case, the sheet resistance of the working element was 0.5-l D/n and the chosen form factor was 1000. The nickel meander was obtained by photolithography with an FeCl, (40 g l- ‘) etching agent and 67% H,SO, (150 ml l- ‘). The sensor was finally protected by a SiO film on top (200 nm); this material, which can be evaporated easily in vacuum, is transparent for visible light, possesses dielectric strength and provides reliable protection of the sensor in a chemical environment. The electrodes were bounded with golden wires 100 nm in diameter. The sensors obtained were calibrated ‘on-line’ by the experimental set-up shown in Figure 1, The calibration was performed several times until equal values of the resistance were obtained for one and the same temperature during heating and cooling. The samples were heated up to 350°C under a constant heating rate; in this way thermocycling of the sensor was achieved. The calibrated sensors were tested in perpendicular laser irradiation (Figure 2). A copper bromide vapour laser was used for this purpose with power in the range of 0.8-l .6 W. This relatively new laser source is attractive for the technological processes which were shown in 1187
V Shanov et al; Sensor for temperature
measurement
furnace
Figure 1 Experimental
set-up for sensor calibration.
Figure 3. Picture of the developed
sensor.
previous works“.‘. Unfortunately there is a problem connected with the theoretical calculation and modelling of tcmperature for high repetition rate pulsed lasersh. All this nccessitates the experimental determination of the rise in surface temperature when irradiated with such a laser area. It should be stressed that temperature measured in this way is the average of the maximum and minimum values for each pulse. our
Results and discussion The sensor picture is shown in Figure 3 and its layout in Figure 4. The nickel layers obtained by evaporation have high resistance and increasing their thickness, within reasonable limits, cannot decrease their resistance (R, = I Q/n). Evaporated Ni layers have lower density and hence resistivity, compared to that deposited by sputtering. The choice of the deposition method made it possible to obtain different desired resistances for one and the same thickness of nickel films. When sputtering was used the resistance was reduced (R, = 0.5 s1;O). During the calibration of the sensors, the set-up was as shown in Figure 1. where different values of resistance were obtained at one and the same temperature for heating and cooling. The hysteresis formed disappeared with three cycles. This result was expected and could be due to the process of thermal annealing and recrystallization. The final calibration curve of a sensor after thermocycling is shown in Figure 5 and it allows the determination of temperature with an accuracy of k I’C. On the basis of values for R =,f‘(T) the temperature coefficient of resistance was calculated and was in the range of 1.1 x 10 ‘-2.7 x IO- ’ K ’ for different sensors. These results show that the sensors developed have high sensitivity and reproducibility in the temperature range used, which makes them applicable for temperature determination. The developed and calibrated sensors were tested in laser irradiation (3 mm diameter) perpendicular to their surfaces (Figure 2). The dependence of temperature on the laser power for a sensor on a silicon substrate is presented in Figure 6. These results show that the temperature can be measured in a laser-heated area. The sensors developed could be applied for the experimental esti-
Figure 4. (a) Plan view and (b) lateral view of Ni sensor
‘0°1 50
150
250
Temperature, Figure 5. Calibration
350
Of.2
curve of Ni sensor
220 , o
200.
2- 180. 3
160.
G f 2 substrate with Ni -sensor
140 120. .“_.
.
0.5 0.75
.
1
1.25
1.5
1.75
Laser power, W
Figure 2. Schematic 1188
of sensors testing in laser irradiation.
Figure 6. Surface irradiation.
temperature
as a function
of the laser power after 30
V Shanov et al: Sensor for temperature measurement mation of the validity of a surface temperature model when using a pulsed, visible laser. Such a theoretical model is in progress and will be published in due course.
ConcIusious
The possibility for the surface temperature measurement of a laser heated area by thin-film nickel sensor was demonstrated. The resistance of the sensor showed aImost linear dependence on temperature in the range 50-350°C and a relatively high temperature coefficient of resistance. These features make it attractive for temperature measurement in a laser irradiated area and reliable results for a silicon wafer heated by a copper bromide vapour laser were obtained. Such an approach allows improved control of the process and could be used to check the validity of a developed model for the theoretical estimation of temperature during laser processing.
This work has been sponsored by the Ministry of Education and Science, under Project 177-X, to whom the authors are very grateful. We would also Iike to thank I Dimitrov for technical assistance and manuscript preparation.
References
’ K Hesch, P Hess, H Oetzmann and C Schmidt, Appl Surface Sci 36,81 (1989).
“P Baeri, S Campisano, E Rimini and J Zhang, Appl Phys Lett 45(4), 398 ( 1984).
‘T Kodas, T Baum and P Comita, f Appl Phyf 61(S), 2749 (1987). 4V Shanov, B lvanov and C Popov, T&s Soid Films 207,7 1 ( 1992). ‘V Shanov. B lvanov and C PODOV, Processinrr Advanced Mater 3, 41 (1993).
‘B Haba, B Hussey and A Gupta, J Appl P&s 69,287l (1991)
1189
Pergamon
Sensor for temperature heated surfaces
measurement
45/number
12/pages 1187 to 1189/1994 Elsevier Science Ltd Printed in Great Britain 0042-207x/94$7.00+.00
of laser
V Shanov*, P Petkovt, B Ivanov*, C Popov* and C Vodenicharovt, Technological University *Departments of Semiconductors and t Physics, 8 Kliment Ohridski Str, 1756 Sofia, Bulgaria received for publication
of Sofia,
9 August 1993
A small area nickel-based sensor for surface temperature measurement has been developed on different insulating substrates. The dependence of resistance on temperature in the range 50-350°C was studied and the temperature coefficient of resistance was determined, which provided the high sensibility of the device. The sensor was successfully tested in laser irradiation and the promising results obtained made it applicable for surface temperature estimation with an accuracy of + 1°C during laser processing.
Introduction Surface temperature is one of the most important parameters for laser processing. Because the different optical and thermal properties of the substrates used are not known accurately in some cases, theoretical estimates of laser-induced temperature cannot always be made. Reliable measurements of surface temperature in a laser spot are essential for the understanding of laser-induced processes. Furthermore, if temperature is measured experimentally, the values obtained could be compared to theoretically calculated ones, in order to check the validity of a particular model; it shows the necessity for the development of a precise sensor able to measure the surface temperature caused by a laser beam. Because of the peculiarities of the laser processing, these sensors should possess some special features: (i) to be without transit-time effects; (ii) to have small dimensions; (iii) to possess strict data reproduction; and (iv) to be covered on top with material with optical and thermal properties, similar to those of the substrate, on which the process can be carried out. In the literature there is scant information for such sensors’. Another approach exists, connected with the development of thin-film thermocouples for surface temperature measurement 2.3, but production is more difficult from a technological point of view; this directed our efforts towards the development of a sensor able to measure surface temperature directly and precisely during laser processing. Experimental A nickel film was chosen for the working element of the sensor, due to its relatively high temperature coefficient of resistance. Furthermore, it is much cheaper than the precious metal platinum, previously used for this purpose. Nickel films (9&100 nm, target value 100 nm) were deposited by evaporation and sputtering on different substrates: monocrystalline silicon wafers with undoped polysilicon, alumina ceramics, Corning glass and glass ceramics. The variety of substrates used was determined by the
necessity to provide the same surface properties for the sensor as those of the substrates most commonly used for laser processing. Due to the techniques mentioned, deposition of films with the desired resistance was possible. The evaporation was carried out in a vacuum of 3 x 1O- 6 mbar. A graphite crucible heated up to 1300 K was used and the temperature of the substrate was kept at 45@470 K. The deposition rate obtained at these process parameters was 1.5 nmjs. In the case of sputtering with Ar+ the vacuum in the chamber was 3 x 10-j mbar, the temperature of the substrate 45&470 K and sputtering current 0.6 A. A thin film of gold was deposited upon the nickel for further formation of the electrodes. Gold was chosen due to its high chemical stability and low resistivity. Gold films were deposited by evaporation in a vacuum of 3 x 10e6 mbar and the temperature of the tungsten crucible was 1200 K. The thickness of the films obtained was in the range 5&70 nm. The formation of the electrodes was achieved by photolithography with an etching agent, I 2 + KI (in ethanol), which did not attack the nickel sublayer. The chosen form of the Ni element was a meander, which provided a higher resistance of sensor and higher sensitivity. In our case, the sheet resistance of the working element was 0.5-l D/n and the chosen form factor was 1000. The nickel meander was obtained by photolithography with an FeCl, (40 g l- ‘) etching agent and 67% H,SO, (150 ml l- ‘). The sensor was finally protected by a SiO film on top (200 nm); this material, which can be evaporated easily in vacuum, is transparent for visible light, possesses dielectric strength and provides reliable protection of the sensor in a chemical environment. The electrodes were bounded with golden wires 100 nm in diameter. The sensors obtained were calibrated ‘on-line’ by the experimental set-up shown in Figure 1, The calibration was performed several times until equal values of the resistance were obtained for one and the same temperature during heating and cooling. The samples were heated up to 350°C under a constant heating rate; in this way thermocycling of the sensor was achieved. The calibrated sensors were tested in perpendicular laser irradiation (Figure 2). A copper bromide vapour laser was used for this purpose with power in the range of 0.8-l .6 W. This relatively new laser source is attractive for the technological processes which were shown in 1187
V Shanov et al; Sensor for temperature
measurement
furnace
Figure 1 Experimental
set-up for sensor calibration.
Figure 3. Picture of the developed
sensor.
previous works“.‘. Unfortunately there is a problem connected with the theoretical calculation and modelling of tcmperature for high repetition rate pulsed lasersh. All this nccessitates the experimental determination of the rise in surface temperature when irradiated with such a laser area. It should be stressed that temperature measured in this way is the average of the maximum and minimum values for each pulse. our
Results and discussion The sensor picture is shown in Figure 3 and its layout in Figure 4. The nickel layers obtained by evaporation have high resistance and increasing their thickness, within reasonable limits, cannot decrease their resistance (R, = I Q/n). Evaporated Ni layers have lower density and hence resistivity, compared to that deposited by sputtering. The choice of the deposition method made it possible to obtain different desired resistances for one and the same thickness of nickel films. When sputtering was used the resistance was reduced (R, = 0.5 s1;O). During the calibration of the sensors, the set-up was as shown in Figure 1. where different values of resistance were obtained at one and the same temperature for heating and cooling. The hysteresis formed disappeared with three cycles. This result was expected and could be due to the process of thermal annealing and recrystallization. The final calibration curve of a sensor after thermocycling is shown in Figure 5 and it allows the determination of temperature with an accuracy of k I’C. On the basis of values for R =,f‘(T) the temperature coefficient of resistance was calculated and was in the range of 1.1 x 10 ‘-2.7 x IO- ’ K ’ for different sensors. These results show that the sensors developed have high sensitivity and reproducibility in the temperature range used, which makes them applicable for temperature determination. The developed and calibrated sensors were tested in laser irradiation (3 mm diameter) perpendicular to their surfaces (Figure 2). The dependence of temperature on the laser power for a sensor on a silicon substrate is presented in Figure 6. These results show that the temperature can be measured in a laser-heated area. The sensors developed could be applied for the experimental esti-
Figure 4. (a) Plan view and (b) lateral view of Ni sensor
‘0°1 50
150
250
Temperature, Figure 5. Calibration
350
Of.2
curve of Ni sensor
220 , o
200.
2- 180. 3
160.
G f 2 substrate with Ni -sensor
140 120. .“_.
.
0.5 0.75
.
1
1.25
1.5
1.75
Laser power, W
Figure 2. Schematic 1188
of sensors testing in laser irradiation.
Figure 6. Surface irradiation.
temperature
as a function
of the laser power after 30
V Shanov et al: Sensor for temperature measurement mation of the validity of a surface temperature model when using a pulsed, visible laser. Such a theoretical model is in progress and will be published in due course.
ConcIusious
The possibility for the surface temperature measurement of a laser heated area by thin-film nickel sensor was demonstrated. The resistance of the sensor showed aImost linear dependence on temperature in the range 50-350°C and a relatively high temperature coefficient of resistance. These features make it attractive for temperature measurement in a laser irradiated area and reliable results for a silicon wafer heated by a copper bromide vapour laser were obtained. Such an approach allows improved control of the process and could be used to check the validity of a developed model for the theoretical estimation of temperature during laser processing.
This work has been sponsored by the Ministry of Education and Science, under Project 177-X, to whom the authors are very grateful. We would also Iike to thank I Dimitrov for technical assistance and manuscript preparation.
References
’ K Hesch, P Hess, H Oetzmann and C Schmidt, Appl Surface Sci 36,81 (1989).
“P Baeri, S Campisano, E Rimini and J Zhang, Appl Phys Lett 45(4), 398 ( 1984).
‘T Kodas, T Baum and P Comita, f Appl Phyf 61(S), 2749 (1987). 4V Shanov, B lvanov and C Popov, T&s Soid Films 207,7 1 ( 1992). ‘V Shanov. B lvanov and C PODOV, Processinrr Advanced Mater 3, 41 (1993).
‘B Haba, B Hussey and A Gupta, J Appl P&s 69,287l (1991)
1189