Substrate heating and temperature calibration

There are two types of methods for substrate heating; direct contact methods and indirect methods. Direct contact methods use a block of material that has good thermal conductance properties (for example copper) and is larger than the substrate; the substrate is then attached to the block and heated by conduction. An example of a direct heating method that is successful for many deposition configurations where only low temperatures are required (< 400 °C) is the resistive wire technique [238]. High temperatures are increasingly difficult to obtain with this technique because the resistive wires are prone to oxidise and burn out, and there are also problems with out-gassing. The resistive wire technique has been used with PLD [239], and the crystallinity of films grown using this heating method has been improved by post-annealing [62].

Radiation is used with indirect heating techniques, and filament bulb heating [238; 240] is an example of such a technique. Filament heating is very inefficient because most of the radiation misses the substrate and heats the chamber walls, increasing the rate of out-gassing of contaminants deposited on the chamber walls in previous depositions. The filament method can be improved by enclosing the bulb in a 'black box' and using the box as in a direct contact technique [238; 241-243] and infra-red (IR) radiation coupled into a piece of black quartz attached to a substrate has also been used in this way [65].

A far more efficient heat source is a laser [244], which allows substrates to be heated without the chamber heating up. Laser heating also allows more precise control over the heating power applied to the substrate, and the unloading process is faster because a substrate cools far quicker than a large heating block. The two difficulties with laser heating are measuring the substrate temperature without changing the substrate temperature due to heat sinking to the probe (discussed later), and spreading the power evenly over the substrate to produce a homogeneous temperature distribution. There are a few methods for overcoming the problem of spreading the power evenly, such as use of an intermediate quartz plate as in a direct contact technique [239], beam homogeniser pipes, and beam scanning [18; 22; 58; 60; 78; 237; 245]. The technique of substrate heating used for research discussed here was a raster scanned CO2 laser, and computer control of the delay time at

each scanning point allowed continuous updating of the substrate temperature distribution, meaning in-situ temperature homogeneity control was possible. In particular, control of the delay time at scanning points located around the edge of the substrate allowed compensation of the local drop in temperature (due to extra emission because of the higher surface area to volume ratio).

Temperature can be measured either directly or remotely [246], but in-situ direct contact temperature measurement of the substrate is not possible when a remote heating technique is used because the low thermal mass of the substrate would result in significant heat sinking to a direct contact probe, which would lead to an inhomogeneous temperature distribution. Direct contact temperature measurement also has problems with repeatability due to the difficulties of recreating a good contact. It is critical that the substrate temperature distribution is homogeneous, otherwise film qualities such as stoichiometry, crystal phase, crystal orientation and surface topography could vary at different points on the substrate, leading to films with poor overall optical quality and possibly resulting in cracking.

Indirect techniques of temperature measurement are difficult to calibrate and are generally more complex than direct contact methods. Two examples of remote temperature measurement that have been used with deposition techniques are pyrometry [246-248] and diffuse reflectance spectroscopy (DRS) [249-255]. Two pyrometers sensitive to different wavelength ranges of radiation can be used to detect the radiation emitted by the substrate, and comparison of the intensities measured by the pyrometers allows the temperature to be calculated without knowledge of the emissivity of the substrate (providing it is constant for both wavelength ranges). Two-colour pyrometry is less effective when the emissivity of the substrate is low and doesn't work when the emissivity varies as a function of wavelength and

temperature unless this variance is known precisely a priori. Care must also be taken when choosing the detection ranges of the pyrometers, so as to avoid the transparency windows of the substrate material in use.

DRS takes advantage of the temperature dependent absorption band-edge that some materials possess, making it ideal for use with semiconductors. A light source is directed at the substrate and the diffusely reflected light is collected so that the temperature can be calculated from the spectrum. The diffuse reflectance spectrum is used more commonly than the diffuse transmission spectrum because it is easier to probe and sample from the same side of the substrate. In deposition techniques, one side of the substrate is always unavailable for in-situ probing because the deposition plume would interfere with the measurement. An extra advantage with DRS is its ability to measure film thickness [255]. A disadvantage of remote temperature methods is their dependence upon port windows, which gradually get coated throughout depositions, making the use of absolute intensity measurements impractical.

The limitations of in-situ temperature measurement techniques make them unsuitable for use with the PLD setup used to conduct the research reported in this thesis. An in-situ technique is not strictly necessary with PLD anyway because the ideal temperature can usually be found empirically. The power output of the CO2 laser is constant from day to day, allowing

the substrate temperature to be calibrated to the output power setting of the CO2 laser. Once

such a calibration has been made, a temperature can be 'dialled in' by using the output power setting of the CO2 laser.

An approximate calibration of substrate temperature to CO2 laser output power was

provided by a simple experiment. Some small samples of high purity metal foil were balanced on the substrate as it was heated, and the CO2 laser power required to melt them

was recorded. The foil samples were cut to approximately 1 mm × 3 mm in size and only one was used at a time to minimise heat sinking. It was found that this experiment had good repeatability when a good contact had been made, however when a bad contact had been made the foil samples melted at far higher temperatures (we believe due to the affect of oxidation), so when it became obvious that a bad contact had been made, that result was abandoned. Figure 4.2.3 shows the result of the calibration experiment.

y = 2.7129x + 216.65 0 200 400 600 800 1000 1200 0 50 100 150 200 250 300 350 400

CO2 laser output setting (divisions)

Y A G s ubs tr at e te mp er at ur e inf er re d fr om me lt ing poi nt s (°C ) Al Mp = 660 °C Ag Mp = 962 °C Cu Mp = 1083 °C

Figure 4.2.3: The results of the carbon dioxide laser power and substrate temperature calibration experiment.

Each data point is the average of the results from four experiments, and the error bars are representative of the repeatability of the measurements. The temperature increase appears to be roughly linear relative to the CO2 laser output power setting, within the range examined.

The temperature of a grey-body material (a material with a constant emissivity) can be found from Stefan's law shown as equation 4.2.1.

4

E E S S

P =

ε σ

PE = power emitted, T = temperature, AS = surface area, εE = emissivity,

σS = Stefan-Boltzmann constant,.

Under the assumption that heated substrates in vacuum have minimal contact with the substrate holder and only lose heat via radiation, the incident CO2 laser power will match the

power emitted from the substrate and the temperature will be stable. We would therefore expect the temperature of a grey-body to scale as PCO21/4 (PCO2 = output power of CO2 laser),

but since garnet crystals are not grey-bodies and their emissivity is wavelength and temperature dependent it is difficult to predict how the temperature should change with increasing incident CO2 laser power.