For real-time thin film thickness measurement, laser reflection interferometry (LRI) is also considered as a very useful technique [34]. The calculation of the interference pattern requires a numerical description of the reference beam and the object beam.
Shack-Hartmann wavefront sensor
Measurement of local slopes
The first step of the wavefront estimation process in a Shack-Hartmann wavefront sensor is to determine the locations of the focal points. The sayxcandyccan focal point position can also be considered the center of the corresponding irradiance distribution. If we consider Ii,j as the intensity of the (i,j)th pixel in the irradiance distribution, then we can write
The summation is taken over all the pixels of the detector array within an area-of-interest (AOI) that captures a given focal point.
Reconstruction of wavefront
Relationship between phase and slope in (b) the Southwell algorithm and in (c) the Pathak-Boruah algorithm. 3.4(a) shows nine adjacent grid points in the Southwell geometry represented by eight black dots and one red dot. 3.4(b), the average of the slope values in vertical or horizontal direction for two adjacent grid points is expressed in terms of the phase difference between the same two grid points.
The relationship between slope values and the phase at a given grid point (i,j) in the Pathak-Boruah algorithm is shown in Fig.
Limitations of the Shack-Hartmann wavefront sensor
Principle of Operation of Grating-Based Wavefront Area Sensor The spatial resolution of a conventional Shack-Hartmann wavefront sensor is.
Working principle of grating array based zonal wavefront sensor
Thus, by correctly determining the spatial frequency of the grating elements, for a plane incident wavefront we can obtain a regular set of focal points in the detector plane due to the focusing of the +1 order beam diffracted by each grating. However, if the two are not equal, a 2D array of focal points will be created where the separation between adjacent focal points in the x and y direction will be different. For a deflected incident wavefront, the focal points are displaced from their reference positions, and the displacements at the focal points can be determined from the data provided by the array of detectors.
Thus, if the reference wavefront is not perfect or aberrated, the grating array can be configured such that the array of reference focal points in the detector plane is a regular one.
Basic components of the GAWS setup
- Laser
- Ferroelectric liquid crystal spatial light modulator
- Photo detector array or camera
- Microcontroller Circuit and synchronization unit
The ferroelectric liquid crystal spatial light modulator (FLCSLM) is the other key component of the setup. The camera can be triggered via an external sync signal or run in an internally controlled coasting mode. In our experiments, during image acquisition, the display in the FLCSLM panel and the camera exposure must be synchronized.
This trigger signal then ensures that the camera captures the focal points resulting from a specific binary pattern displayed on the FLCSLM.
The experimental arrangement of a basic GAWS setup
Determination of calibration constant to estimate local slope
As we have noted, the camera in GAWS estimates the local slope of the test wavefront from the shift of the focal point. If the camera plane coincides with the objective focal plane and the wavefront AB or AC coincides with the FLCSLM plane, we can write However, in the current setup, there is more than one lens between the FLCSLM and the camera plane.
Thus, the slope in the X or Y direction can be obtained by multiplying the displacements of the focal points in the camera plane in the X or Y direction by Cs.
Correction of the reference beam
Demonstration experiments using the basic GAWS setup
The line plots of the applied and estimated phase profiles along a line passing through the center, for the grid array dimensions of 4×4, 6×6 and 8×8 are shown in Figs. In the case of a holographically imported phase profile, once the wavefront estimation is completed, it is possible to measure the accuracy of the reconstructed wavefront in terms of the difference between actual and estimated phase values. Here, we use the applied phase profile and the estimated phase profiles for different grid array dimensions to calculate the RMSE for each case.
Thus, it is observed that the RMS error values decrease with the increase in the dimension of the grid array.
Conclusion
In the first chapter, it has already been emphasized on the requirement for on-line monitoring of the growth and thickness of the thin film, especially by using a diagnostic tool which is independent of the properties of the substrate as well as the thin film in question. This chapter demonstrates a new method for in-situ on-line monitoring of thickness and surface profile of thin film using a grid array based zonal wavefront sensor (GAWS), which is independent of the properties of the substrate and the thin film. Therefore, this chapter begins with the description of the PLD system on which GAWS is implemented.
We also introduce the scheme that enables the simultaneous measurement of thickness and surface profiles during the growth of the film on the substrate as a function of time.
Thin film fabrication using pulsed laser deposition system
Experimental arrangement for the pulsed laser deposition unit
The carousel is inserted into the deposition chamber through one of the chamber's 150 CF ports, as shown in Fig. The substrate holder is adjustable to fine-tune the distance between the target and the substrate. A compact full range pressure gauge (Pfeiffer, D 35614) is attached through one of the ports as marked in Fig 4.2, for monitoring.
Measurement of thin film surface profile using GAWS pressure inside the ablation chamber continuously during thin deposition.
Surface profile measurement of thin film using the GAWS
As the deposition begins, thin film grows on the substrate and the incident wave is now reflected by the thin film. Therefore, the shape of the reflected wavefront is now determined by the thickness profile of the deposited thin film. This reflected wavefront from the surface of the deposited thin film now serves as the test wavefront for the GAWS for which the reference wavefront is the reflected wave from the bare substrate.
Consequently, this scheme does not rely on any of the material properties of the thin film and the substrate as long as it is reflective.
Integration of the GAWS with the Pulsed Laser deposition unit
Accounting for vibrational noise due to PLD system
Any relative vibration of the integrated subsystem leads to unwanted beam fluctuations in the detector plane, which is reflected as inaccuracy or noise in the measured wavefront. This shows that the effect of relative vibrations due to the vacuum pump is uniform across the beam in all areas. But due to degassing, pressure will build up in the chamber resulting in contamination of the film as well as the target during deposition.
Integration of the GAWS with the Pulsed Laser deposition unit As the pump is switched on, the vibration of the setup increases.
Determination of phase difference in the substrate plane (or thin
4.7 (a) Applied phase profile, (b) estimated phase profile after 2 hours without applying the modified algorithm, (c) estimated phase profile after 2 hours obtained using the modified algorithm. Therefore, after estimation, the reflected wavefront must be divided by a factor of 2 to get the phase difference in the substrate plane. Besides as already mentioned in our setup, the substrate (in the PLD) and the FLC-SLM planes are made optically conjugate using the lenses L2andL3.
Since the wavefront sensor measures the phase profile as incident on the FLCSLM plane, therefore the actual beam phase profile on the substrate plane or sample plane is given as .
Scheme to measure the thickness and surface profile simultane-
This happens especially towards the later stage of deposition, as peripheral deposition flattens the surface of the thin film. The consequence of this characteristic of the zonal phase reconstruction algorithm is further shown in the figure. Att=t1, growth occurs only in the central part of the substrate, while there is almost no deposition towards the peripheral region of the substrate, as shown in Fig.
The phase reconstruction algorithm therefore provides correct height (l1) of the thin film with respect to the substrate, since the minimum of the reconstructed.
Demonstration experiment to monitor the surface and thickness profile
Deposition of Cu thin film on the stainless steel (SS) substrate . 66
Before deposition, the substrate is cleaned by common protocols; first sonicate in acetone for 30 minutes then rinse with acetone and dry naturally. It is first coarsely sanded with 800 dpi silicon carbide (SiC) abrasive paper for 90 minutes to remove deep scratches and oxide layers on its surface. It is then sanded with 1000 dpi paper for nearly 60 minutes until fine scratches visually disappear.
After obtaining the metallic luster on the surface of the target, it is glued to one of the target mounts with silver paste.
Results and discussion
Thus, if the estimated surface profiles are not converted into thickness profiles, the conventional algorithm cannot accurately estimate the thickness of the film. A close look at the two figures clearly shows the visual similarity confirming the validity of the present measurement. After completion of deposition and cooling, film is removed from the PLD chamber and the thickness of the film is measured separately by a stylus profilometer (Dektak 150 manufactured by Veeco) to compare the measurement by the GAWS setup.
To measure the thickness of the film from the surface of the substrate, a mask must be placed on the substrate during deposition to build a step between the film and the bare substrate (i.e., without deposition).
Conclusion
Therefore, for all such applications the sensitivity and dynamic range of the proposed scheme should be improved. Similarly, the dynamic range of a Shack-Hartmann wavefront sensor can be increased by decreasing the focal length of the lens. The longer distance between the lens and the camera increases the sensitivity of the sensor.
5.12 (ii) (a) is the reference focal point of the same representative area for a concave lens. We also proposed further changes in the wavefront detection scheme to improve the sensitivity and dynamic range of the wavefront sensor. Boruah, “Improved dynamic range of a grating array based wavefront area sensor using an area scan method,” Proc.
