Upload
others
View
14
Download
0
Embed Size (px)
Citation preview
i
Heat transfer between plasma and substrate surface during Pulsed Laser Deposition
Bachelor thesis applied physics
Rik Willemse (s0075094) & Stef Carelsen (s0099481)
i
Preface
This report is made for our BSc assignment.
We would like to thank the Inorganic Material Science research chair for the learning environment offered. Especially, Joska Broekmaat, Paul te Riele and Gert‐Jan Koster, for their great commitment, useful hints, practical insights and supervision during our assignment.
Uros Lekic and Timo Roestenberg from the Thermal Engineering research chair for their help with the Nd:YAG laser time response measurement and a short reintroduction into heat transfer.
Andries den Ouden for being our tutor and his suggestions.
Rik Willemse & Stef Carelsen
ii
Abstract
During Pulsed Laser Deposition the energy of the laser pulse is absorbed by the target, evaporating a small volume of target material. The laser beam converts this into a plasma plume which expands in the direction of the substrate.
The kinetic energy of the species is transferred into the substrate surface during deposition, increasing the surface temperature. The surface temperature drops to the initial value in a time τ. If the frequency of the PLD laser is higher then 1/τ, the average temperature of the substrate will increase and this affects the mobility of the species.
In order to measure this temperature increase a Resistance Temperature Detector (RTD) is mounted on the substrate, the thickness of the RTD is ~ 30nm. Because the RTD is made of a very little material the sensitivity and response time of this sensor is expected to be higher than that of commercially sold RTDs, because these have a protective sheet.
The RTD is taken into a Wheatstone bridge circuit to measure the resistance‐difference accurate. .
This RTD is calibrated and for both platinum and gold an expected, linear relation between temperature and resistance is found. The temperature coefficient of the platinum meander was within a 10% accuracy of the theoretical value. Where that of gold was 50% smaller then the theoretical value.
In order to check if the response time of the RTD were sufficient both a excimer and Nd:YAG laser were used to see the reaction on a sequence of pulses. During both experiments the meander was heated enough to fluidize. This caused the meander resistors to get interrupted, and no useful is recorded.
iii
Contents Chapter 1: Introduction ........................................................................................................................... 1
Chapter 2: Theoretical aspects
2.1 Pulsed Laser Deposition .......................................................................................................... 2
2.2 Heat transfer in substrate ................................................................................................... 3
2.3 Electrical conduction in metals ........................................................................................... 6
2.4 Resistance Temperature Detector ...................................................................................... 7
Chapter 3: Experimental aspects
3.1: Production of a Resistance Temperature Detector ............................................................... 8
3.2. Calibration ............................................................................................................................ 13
Chapter 4: Results
4.1 Calibration ............................................................................................................................. 15
4.2 Response time ....................................................................................................................... 15
Chapter 5: Discussion and Conclusion
5.1 Discussion .............................................................................................................................. 18
5.2 Conclusion ............................................................................................................................. 18
Chapter 6: Recommendations ............................................................................................................... 19
List of references ................................................................................................................................... 20
Appendix A ............................................................................................................................................ 21
Appendix B ............................................................................................................................................ 22
1
Chapter 1: Introduction Pulsed Laser Deposition (PLD) is a powerful technique for depositing high quality thin films of a broad range of materials. An other advantage is that hardly any process heat is transferred into the substrate, heat is only transferred by the deposited species. There is not much experimental research done on the amount of energy that is transferred. The exact amount of energy is hard to determine due to the expected low quantity and the short duration in which the energy is deposited on the surface. This assignment has the purpose to measure the time resolved of the temperature of the substrate surface during PLD. This temperature is of interest because it has influence on the mobility of particles on the surface. This mobility defines the structure and epitaxy of the crystal lattice of the deposited material.
Because of the short duration of the pulse, it is crucial to use a sensor with a short response time and low heat capacity. Because the plasma plume interacts during a few microseconds, the sensor has to have a response time in the same time interval.
2
Chapter 2: Theoretical aspects
2.1 Pulsed Laser Deposition Pulsed Laser Deposition (PLD) is a deposition technique for growing thin films, of which a lot of effects and phenomena during deposition are unknown. Figure 1 shows the PLD process schematically.
Figure 1: schematic overview of PLD [1]
The basic principle of PLD is based on a high power pulsed laser beam which is focused on a spinning target of the desired material. The energy of the laser pulse is absorbed by a small volume at the surface layer of the target. This energy causes the target material to evaporate instantaneous without any melt phase. This phase transition is called ablation.
The evaporated material is converted into a plasma plume, containing a collection of molecules, atoms, ions, electrons atom clusters and micron‐sized particulates [2]. This plume has a temperature of 10.000K and expands highly forward directed from the target surface. Depending on the background pressure, the particles slow down before they reach the substrate surface. The background pressure is an important PLD‐parameter as it regulates the expansion velocity of the plasma plume and at the end the collision impact with the substrate surface. The heat transfer from the plasma plume into the substrate surface is the region of interest in this assignment.
Research has been done on the energy and mass flows during PLD. The mass and energy flow for a pulsed laser generated plasma is given in figure 2.
3
Figure 2: Energy and mass flow of plasma plume [3]
From figure 2 it is concluded that 25 % of the laser pulse energy is converted into the kinetic energy of the species in the plasma. The thermal energy of the plasma is not mentioned in this balance. When the mean free path of the particles is larger than the distance between target and substrate, the initial thermal energy of the plume is converted in kinetic energy[3].
2.2 Heat transfer in substrate
2.2.1 Introduction The substrate temperature is an important parameter when growing thin films using PLD, because it defines the phase of the crystallization of the deposited material. Experiments of SiC deposition have shown that the frequency of the pulsed laser beam is also a parameter which determines the structural phase[4]. When the frequency was raised from 1 Hz to 5 Hz, the crystallization of a high‐temperature stable phase was detected. This indicates that an increase in laser pulse frequency is possibly equivalent to an increase of the substrate temperature.
2.2.2 Heat transfer induced by one laser pulse The temperature of the substrate surface is probably influenced by the kinetic energy of the particles of the plasma plume. These particles collide with the surface and will heat it. This interaction between the plasma and surface has been simulated by Xianfan Xu [5]. The result of the simulation is shown in figure 3, the temperature of the substrate surface is plotted as function of time.
4
Figure 3: Surface temperature vs. time[5]
In this case a quartz substrate was deposited with carbon particles with Em = 100 eV. The surface heats up in 1,5 µs and drops to less than 330 K after 10 µs. This indicates the necessity of a sensor with a fast response time in order to measure this quick raise in temperature.
Figure 3 shows the temperature change of the surface, but it is interesting to investigate the change of temperature inside the substrate over time too. Figure 4 shows the temperature profiles of the substrate as a function of depth.
Figure 4: temperature distribution over time [5]
The difference in fluctuation is remarkable in this figure: at the substrate surface there is an increase of 100K, while the substrate heats up a few Kelvin at a depth of 8 µm. The top nanometres are important, since the surface temperature influences the growth. In order to measure the raise of temperature, the sensor has to be positioned on top of the substrate, instead of inside the substrate.
2.2.3 Heat transfer induced by multiple laser pulses The above described simulations are only valid for one single laser pulse, while during PLD repetitive pulses are used. In short it takes a certain amount of time τ before the temperature of the surface is decline d asymptotically towards the initial surface temperature.
5
Figure 5: Surface temperature vs. time[4]
When a set of multiple pulses is used, two scenarios can occur: if the period of the laser pulse is greater then τ, the surface will remain at the same initial surface temperature, see figure 6. However if the period is smaller, the surface is heated again before it could cool down. This scenario is shown in figure 7.
Figure 6: Constant average surface temperature [4]
Figure 7: Increasing average surface temperature [4]
The steady state temperature of the substrate will increase depending on the deposition frequency to a higher temperature level Tss>Ts (see figure 7), or remains at the initial value Ts (figure 6). In the simulated surface temperature of figure 3, τ is approximately 10 µs. This period is dependent of the PLD set up, the choice of substrate and the deposited material.
6
2.3 Electrical conduction in metals Temperatures can be measured by recording the resistivity of a metal while changing the temperature. These variations are based on electron scattering processes. A solid metal consists of metal ions aligned in a crystal lattice and is bound by free electrons, which are able to move through the lattice. The free electrons contribute to the conducting behaviour of the metal. When there is no voltage applied on a metal, the average velocity of all free electrons in the metal is zero, because the random directions of the electrons cancel each other out.
When a voltage is applied, there will be an average drift velocity, i.e. a current flows through the metal. The cloud of free electrons moves through the lattice of the ions. Lattice vibrations, i.e. phonons, arise when T > 0 and the electrons are more likely to scatter on the vibrating ions causing the mean free path of the electrons to decrease. This will be perceivable as an increase in resistance of the metal.
In figure 8, the resistivity of gold is plotted as a function of the temperature. This graph is almost linear in the region of 300K – 600K, and therefore probably suited for the PLD experiment.
Figure 8: Resistivity of gold vs. time
The resistance of a metal can be described with the following first order approximation:
)](1[ 00 TTRR −+= α
Where R is the resistance, R0 the initial resistance, T0 and T, the initial and present temperature respectively. “α” is the temperature coefficient belonging to the used metal. For gold this coefficient is 3,4∙10‐3 / °C and for platinum it is 3,92 ∙ 10‐3 / °C [6]. This first order approximation is accurate enough and will be used in the next sessions.
7
2.4 Resistance Temperature Detector The heat transfer effect during PLD is described in the previous sections. Fo a successful experiment a sensor has to be created with a very fast response time. The period of the peaks could be 10 µs, so its response time should be a few µs. As shown in subsection 3, the resistivity of metals is dependent of the temperature and in most of the cases in a linear behaviour.
A temperature sensor which uses this property of metals is called a Resistance Temperature Detector. The resistance of the sensor is dependent of the temperature. To make a quick responding sensor, it is crucial to reduce the mass of the metal core of the sensor as much as possible. A lower mass reduces the heat capacity of the sensor and thus the response time. It was shown that the effect occurs at the surface of the substrate, so the sensor is build on top of the substrate, instead of inside the substrate.
8
Chapter 3: Experimental aspects
3.1 Production of a Resistance Temperature Detector Commercially build Resistance Temperature Detectors (RTD) are usually packaged with a protective plastic sheet. That sheet increases the total heat capacitance, which results in a decrease of the sensitivity and increase of the time response of the sensor. For this experiment a RTD is designed where the sensing element is directly exposed to the plasma, in this way the fast response time is maintained. The RTD is created from a thin, metal film to make it more sensitive for temperature changes over the wanted small time interval. Although its temperature coefficient of resistance is smaller than that of platinum, gold is used for this experiment because its easily made in large numbers with sputtering equipment. Appendix A describes how these gold films are deposited on the substrate. A few platinum meanders were made later on with pulsed laser deposition to see if the sensitivity would improve. The PLD process has been described previously in section 2.1. The thickness of the thin film is important for the RTD to have a good sensitivity and response time. If the sensor is relatively thick, the sensitivity of the sensor will decrease because the heat capacity of the meander will be larger. Also the resistance of the meander will be lower, so a higher current density is needed to measure the same voltage, see formula 1.1. (1.1) RIU ·= If R is decreased, I should be increased in order to measure the same U. Due to this higher current, self heating from power dissipation will increase and interfere with the energy collected during measurements. This dissipated heat can be calculated from:
(1.2) tRItIUQ ···· 2==
Which will give a temperature raise of:
(1.3) cm
QT·
=Δ
Where m is the mass and c the heat capacity of the material that is heated by Q. If the film is too thin the film cannot be treated as a bulk and the resistance will be larger, which will also increase self heating. To minimize this self‐heating a optimum between thickness (response time) and resistance (self‐heating) has to be found. The resistance of the sheet can be described by:
(1.4) tWL
ALR
··· ρρ
==
Where ρ is the electrical resistivity of the material and is ρAu = 22 nΩ∙ m[7] for gold and ρPt = 106 nΩ∙ m[7] for platinum. The other parameters are shown in figure 9. The resistivity of a 15 nm thick film of gold on a square substrate of 1 cm2 will be 1,6 Ω according to formula 1.4.
9
Figure 9: Sheet Resistance
In order to enlarge this resistance without the use of thinner films, the film is structured as shown in figure 2. This structure is called a meander and is actually a folded long, thin wire with a length L = 20 mm and width W = 0,22 mm.
Figure 10: Meander structure
By using formula 1.4 resistance versus thickness plots are made to determine a suitable thickness of both the gold and platinum meanders. These plots are shown in figure 11.
Figure 11: Resistance vs. Channel height of meander
As can be seen in the figure above 30 nm is a good thickness for gold: doubling its value will decrease the resistance with 20 Ω and the power dissipation due to a current of 1 mA will decrease just 34 µJ/s. Comparable values can be found for 50 nm thick platinum meanders.
10
3.1.1 Substrate The dimensions and the structure of the meander are determined above, but the structure has to be deposited on a substrate. .Suitable substrates are glass, quartz and oxidized silicon because they are good electrical insulators. This is needed for the meander to not be short‐circuited via the underlying substrate. It has to be kept in mind that an electrical insulator is a bad thermal conductor and therefore little heat can be transferred from the meander by conduction.
3.1.2 Lithography The meander resistor is made of a thin gold or platinum film and is structured by lithography and consecutive etching. First a layer of photoresist material is spin coated on the surface of the thin film, next it is exposed to UV‐light through a mask. The exposed photoresist is rinsed off with a developer and only the unexposed pattern remains on top of the film. After etching the uncovered metal film and removing the remainder of the photoresist the meander is ready for measurements. A schematic overview of the lithography process can be seen in figure 12. Appendix B gives a detailed description of the UV lithography process.
Figure 12: Lithography
An important issue is the etch time taken to remove the metal film. If taken too short a conductive residual layer will remain and short‐circuit the meander. When it is sputter etched longer than 5 minutes, the substrate will warm up, causing the photoresist to harden onto the meander. This makes the removal of photoresist with acetone and ethanol impossible.
3.1.3 Contacts The meander is mounted onto a substrate holder as it is a very fragile structure. The substrate holder is made of a 1 mm thick copper plate, because copper is a good conductor which is needed for the calibration experiment. Two copper, coated wires are connected to the meander, in order to measure the resistance in the future. The connection between the wire and the meander is made with conductive silver paint, which is very fragile too and it is important that the copper wires are bend a little to maintain contact mechanically. Figure 13 is a photograph of the meander fixed onto the copper substrate.
11
Figure 13: meander resistor mounted on copper plate
3.1.4 Test A heat gun was used to test if the meander is sensitive to temperature. Therefore a currentsource and a voltage amplifier were used to measure the change in resistance while heating the substrate. A positive, expected change in voltage was seen on the oscilloscope but the voltage dropped during a second experiment. It turned out that inhomogeneous heating of the contacts gave a similar result even with the current source switched off.
Figure 14: sketch of set‐up
The voltage measured was caused by the Seebeck effect at the contact of the copper wire and the iron probes of the oscilloscope. When a metal is heated a Volta potential arises due to this thermocouple effect. If two different metals, with different Volta potentials, are heated, a potential difference between these metals is induced. Relatively to platinum these potentials are +1,92 mV/100K for iron and +0,77 mV/100K for copper. In this case the potential difference is ca. 1,15 mV/K [8] The system is symmetric If both contacts are in identical condition: no net effect is present. But the system becomes asymmetric if a temperature gradient between both is present: a voltage is measured on the oscilloscope. The effect is shown schematically in figure 15
Figure 15: the Seebeck effect [9]
This problem was solved by enlarging the length of the copper wire and keeping the contacts out of the range of the heat gun. After excluding the thermocouple effect at the contacts no detectable change in resistance was seen. The measurements were all done at small bias currents from 1 to 10 µA and the voltage had to be amplified a thousand times for the oscilloscope to visualize. The voltage at different currents were measured to determine if more a higher currents can be used without warming up the resistor. It
12
was found that until ~ 1 mA no change in resistance was found. From equation 1.2 it is known that the heat dissipated is 0,25 mJ/s, which should increase temperature with 244 K/s (equation 1.3) if no heat is transferred away from the meander. But even after using higher current densities, no changes in the resistance were measured. This small response is mainly due to the small absolute resistance change on top of the total resistance. In order to level out this offset resistance, a Wheatstone bridge was implemented. The Wheatstone bridge is described in the next subsection. With the Wheatstone bridge implemented the sensor responded properly when heated with a heat gun.
3.1.5 Wheatstone bridge[10] A Wheatstone bridge is a electrical circuit and is often used to measure small resistance changes. It acts as a balance, a difference in resistance will unbalance the bridge and the effect is magnified. Its circuit diagram is shown in figure 16.
Figure 16: circuit diagram of Wheatstone bridge
A Wheatstone bridge contains four resistors and is balanced when the ratio R2/R1 equals Rx/ R3 . If unbalanced, e.g. Rx changes, a voltage drop between points B and D will arise. This voltage drop over the “bridge”, denoted by Vg, can be monitored with a oscilloscope. A formula to convert the values of Vg into a Rx is derived below: Kirchhoff’s Voltage (2nd) Law states that the voltage drop over both legs, ADC and ABC, must be equal, i.e. VADC = VABC or IADC(R1 + R2) = IABC(R3 + Rx). The electrical potential in points A and D are equal to V2 and Vx respectively.
(1.5) 221
22 RRR
VRIV s
ADC +==
(1.6) xx
sxABCx R
RRV
RIV+
==3
Vg is the difference in electric potential: Vg = Vx - V2.
(1.7) sx
xg V
RRR
RRR
V )(21
2
3 +−
+=
Where Vs is the source voltage. This can be rewritten into:
13
(1.8) 321
2
21
21 RRR
RVV
RRR
RVV
s
gx
s
g
⎟⎟⎠
⎞⎜⎜⎝
⎛+
+=⎟⎟⎠
⎞⎜⎜⎝
⎛+
+−
Which equals to
(1.9) )1(
3
bbR
Rx −=
with
(1.10) 21
2
RRR
VV
bs
g
++=
This gives the desired values of Rx for every voltage Vg. The potentiometer, R2, can be used to
balance the Wheatstone bridge initially. In order to get a optimal x
g
dRdV
, R1 and R3 have to be chosen
wisely. Therefore a script was written in which formula 1.7 is used to find good values of R3. If R3 =
330Ω and R1= 120 Ω a maximum slope of the Vg vs. Rx curve is found: x
g
dRdV
≈3,7 mV. From the value
As known from section 1.2.3, the temperature coefficients of resistance α, for platinum and gold are
3,4∙10‐3 and 3,92∙10‐3 Ω/K respectively. The expected value for dTdVg
≈1,45∙10‐5 V/K for platinum and
1,26∙ 10‐5 V/K for gold resistors.
3.1.6 Choice of depositionmaterial The kind of the target material used during the PLD experiment should be chosen wisely. The structure of the meander requires a non‐conducting material, otherwise the RTD will get short‐circuited by the deposited conducting layer. An example of a non‐conducting material is Al2O3, which could be deposited properly.
If the deposition effects the voltage over the RTD, which is possible due to the charged ions and electrons, it is possible to deposit a layer of 1‐2nm of non –conducting SiOx first. This layer will protect the bare meander and will make it possible to measure the resistance.
3.2 Calibration
3.2.1 Temperature response measurement In order to calibrate the temperature sensor, the voltage over the bridge is measured at different temperatures. The Wheatstone bridge is balanced by tuning the potentiometer, R2, until Vg ~ 0 V. Then, by placing the substrate holder on a hotplate, the meander is heated stepwise and the maximum value of Vg is measured. The temperature is measured by mounting a thermocouple on top of another substrate, imitating the meander its circumstances. The maximum value of the temperature is recorded. The setup is shown in figure 17.
14
Figure 17: Temperature response measurement setup
The values of Vg are now used to calculate Rx with formulas 1.9 and 1.10. To compare theory with the measured values, Rx versus temperature curves for both gold and platinum are made. These results are shown in figure 19 in section 4.1.
3.2.2 Response time measurement A response time measurement is preformed to see if the meander is able to determine the temperature quick enough. This is done by shooting a Nd:YAG laser onto the resistor through a filter, which weakens the beam. The substrate is positioned right after the lens, so the beam is not focussed yet and the spot size has a diameter of about 0,8 cm. The frequency of the laser, 10 Hz, and pulse duration, 7 ns, were fixed, so the only adjustable parameter left was the power. Even then, the power could only be chosen in some discrete values. The results are presented in section 4.2.
15
Chapter 4: Results
4.1 Calibration The experiment described in section 3.2.1 is done at a hotplate. Figure 18 shows the Rx versus T curves of the two meanders. From this figure can be concluded that the platinum meander responded nearly as expected: 0,91 V/K in stead of 0,99 V/K. Gold responded far from expected: 0,33 V/K instead of 0,64 V/K.
Figure 18: Rx vs. T curves for theoretical and measured values
4.2 Response time In the previous section the RTD is calibrated, an other test for this sensor is the response time. This experiment is performed with two different pulsed lasers: the Excimer and Nd‐YAG lasers.
4.2.1 Excimer laser The Excimer laser is a powerful laser, it is used in the PLD‐process where is it is used to ablate the target material. In this experiment the focussing lens is removed and a mask is placed to downscale the energy of the pulses, otherwise the meander will ablate immediately.
The energy of a single pulse was set at 11 mJ, which was the lowest possible setting. After a few pulses the resistor had a infinite resistance, the structure was interrupted . Figure 19 is a photograph of the structure after the experiment, figure 20 is a close‐up.
16
Figure 19: Photo of RTD after laser experiment
Figure 20: Close‐up of RTD
The resistor is clearly interrupted, so the temperature of the meander must have been higher than the melting point of platinum which is ~ 2041 K. The intensity of the beam is considered to be homogeneously. The spot size of the beam was 0,8 cm2, while the damaged area on the substrate is estimated to be 0,04 cm2 which is determined from figures 19 and 20. The energy offered to the substrate area is thus 1/20 of the total energy, equals to 0,55 mJ. The mass of the heated meander is estimated at 4µg. The raise in temperature is calculated with formula 1.3, the result is ∆T ≈ 1230 K.
4.2.2 NdYAG laser When the laser was set to give pulses of an energy of 1,5 mJ no change on the oscilloscope was noticeable. The laser was tuned one step (4 mJ) and the meander immediately vaporized. This happened for the golden meander as well as the platinum one. This was expected for the golden meander, because gold melts at 1300 K. The platinum meander though melted too, while its melting point is quite high: 2041,4 K. An explanation for that too happen is that the intensity distribution of the laser was not homogenous. Hotspots were also seen when the laser was first shot at thermal paper.
Hotspots may increase the temperature very locally, causing to melt just a minor part of the meander and thereby interrupt the meander. To test this hypothesis a picture of the broken platinum meander is taken and the surface of the hotspot is estimated to be about a rectangle from 2,1 x 1,2
17
mm, see figure 21. Within the hotspot the meander that is melted at three different places with a total length of about 1,2 x 3 = 3,6 mm and a surface of 3,6 x 0,22 = 0,78 mm2.
Figure 21: Platinum meander after it is melted with the Nd:YAG laser
18
Chapter 5: Discussion & conclusion
5.1 Discussion The platinum RTD acted as expected, in contrast with the gold meander. Gold responded half as sensitive as expected. The origin of this behaviour is not certain, it could depend on the titanium layer of 5 nm which is deposited to fix the gold layer on the substrate. Another cause could be found in the Wheatstone bridge, the optimal settings of the Wheatstone bridge are dependent on the initial resistance of the RTD. The settings of the bridge, which was optimized for platinum, were also used for the gold RTD, making the experiment less accurate. This could cause the error in the behaviour.
The experiments with the laser did not work out as expected. The first experiment with the excimer laser failed due to the energy of the laser pulses which was too high. The second laser experiment with the Nd:YAG laser was more promising, because lower energy values were available. The resolution of the energy values was too low, a energy of 3 mJ was wanted but only 1,5 mJ /pulse and 4,5 mJ were available. Unfortunately the RTD did not respond on the 1,5 mJ / pulse and was broken by the 4,5 mJ / pulse.
5.2 Conclusion Several conclusions can be made:
‐ During Pulsed Laser Deposition a raise in the substrate surface is experienced by other researchers.
‐ The RTD made of platinum acted as expected according to literature. The temperature dependence of this resistor followed the theoretical values within an error of 10%. However the RTD made of gold was less sensitive then expected, it was half as sensitive as the theory predicted .
‐ When the RTDs were tested with the Nd‐YAG and Excimer lasers, too much energy was offered. As a result the Platinum or Gold structure was interrupted due to the high local temperature. The estimated temperature was locally around the melting temperature of PT or Gold (2040K resp. 1340K).
19
Chapter 6: Recommendations The experiments gave some useful data, but some aspects of the experiments could be improved. It is expected that modifications will improve the results.
‐ It is recommended to test the response time of the RTD with a more suitable laser. In the experiments a laser was used where the energy of a single laser pulse was adjustable in discrete settings. These settings did not cover the domain of interest of our sensor. Setting A was too small to detect while the next setting was too high, even when optical filters were used. A laser where the energy can be regulated in small suitable steps is preferred.
‐ Next to the laser experiment, the response time could be tested in a different way. The concept of self heating is described in section 3.1 as a disadvantage. Self heating could also be used to heat up the RTD during a short time by directing a large current for a short time. This concept is used only before the use of the Wheatstone bridge, without any result. When using the Wheatstone bridge it might work, but it is unknown
‐ During the last test with a Pt‐meander, a lot of noise was detected by the scope. It was nearly impossible to detect the response of the RTD. It is therefore recommended to find the source of this noise and eliminate it. Possible sources are the contacts at the meander. The meander made a drop of about 30 cm and this could be the reason. The Wheatstone bridge was build on a (blue) electrical circuit board. The great advantage is the easy use of this tool, but maybe the contacts are poor and causes the noise. Also the environment of the experiment could be the origin. Our colleagues of CTW were experiencing a lot of noise too in this laboratory, maybe a coil of aluminum foil around the Wheatstone bridge could help. The wiring of our complete set‐up should be replaced by coax cables and the wires should be collected as early as possible. There are a lot of coils made by the wires in the current set‐up, these coils are acting like an antenna.
‐ There was not enough time to modulate the heat transfer through the substrate. The pulsed energy offered, made it hard to find the right differential equations and boundary conditions to describe the situation. It is recommended to spend time in this heat transfer theory, if available.
20
List of References
[1] Superconductivity Technology Center at ANL, http://superconductivity.et.anl.gov/Techniques/images/PLD_schem.jpg
[2] Materials science of thin films, Milton Ohring, second edition, p130
[3] Thin Solid Films , S. Amoruso et al., 453 –454 (2004) 562–572
[4] Control of polytype formation in silicon carbide heteroepitaxial films by pulsed‐laser deposition, Takeshi Kusumori et al, Applied Physics Letters, Vol 84‐Nr 8, P1272 – 1274].
[5] Perturbation of the substrate temperature by the impingement of laser ablated particles, Xianfan Xu, J. Appl. Phys. 77(12)
[6] HyperPhysics of Georgia State University, http://hyperphysics.phy‐astr.gsu.edu/hbase/electric/restmp.html
[7] Binas, Wolters‐Noordhoff Groningen. Tabel 8 (p18).
[8] Technical Faculty of the University of Kiel, http://www.tf.uni‐kiel.de/matwis/amat/elmat_en/kap_2/backbone/r2_3_3.html
[9] Fundamentals of Electrical Engineering and Electronics, T.R.Kuphaldt, http://www.vias.org/feee/dcsignal_06.html
[10] http://en.wikipedia.org/wiki/Wheatstone_bridge
21
Appendix A: Sputter deposition of gold This appendix enumerates the steps taken to deposit a thin film of gold on a substrate by sputtering.
1. Get the anode out of the sputtering machine (Perkin Elmer in MESA+).
2. Carefully paste the substrate on the anode with a conductive paste.
3. Load the anode into the sputter machine
4. Before deposition it is wise the clean the surface even better by sputter etching it for a few minutes.
5. The titanium target its surface might be oxidized. Therefore its etched by selecting sputter deposit on the machine while the anode with the substrate is in another position.
6. A thin film (5 nm) of titanium is deposited. Gold preferably binds with Titanium than to the substrate, and thereby the gold film should be fixed a little better now.
7. The gold film of the desired thickness is grown
8. The anode is unload from the machine.
9. The substrates are carefully removed and are put away properly.
10. The anode is cleaned and put back.
A manual should lay near the Perkin Elmer sputtering machine in MESA+. It describes how its operated and at the end the current growth rates of gold and titanium can be found. At the time when this report was written these were approximately 28 nm/s for gold and 5 nm/s for titanium (Pforward = 150 W).
22
Appendix B: Lithography After the deposition of a thin film gold or platinum on a substrate the meander‐structure is made by lithography. This appendix enumerates this process in detail.
1. Prepare the developer (Microposit 351 developer) by mixing with demiwater (developer : demiwater = 1:5). Also be sure that you prepared two goblets with only demiwater for rinsing off the developer later on.
2. Absorb the photoresist (Microposit S1813 photoresist) into the injector and attach a filter to the nozzle.
3. Put the substrate in the spin‐coat machine and apply the photoresist. Then spin‐coat for 30 seconds at 4000 round per minute. The photoresist film should be about 1,2 microns thick.
4. Softbake the substrate with photoresist on a hotplate for 10 minutes at 100 °C.
5. Put the mask in the mask‐aligner and expose the substrate to UV‐light for 8 seconds (alignment gap = 25 micron).
6. Now gently stir the substrate through the developer for about a minute. Then do the same in the two goblets of demiwater prepared in step 1. You should be able to see the structure now.
7. Sputter etch the substrate as long as necessary to etch off the thin film.
8. Ultrasonic rinse off the rest of the photoresist with acetone and ethanol (in that order).
The meander resistor of desired material is now ready to be used.