Optical film thickness monitor

Measuring arrangement of the optical film thickness monitor A Modulated light source E Test glass changer Receiver Refl. F Light beam deflection... 

Measuring arrangement of the optical film Thickness monitor of HONEYWELL. 

A miniaturized spectrometer with CCD array for mounting on a printed circuit card inside the electronics has been developed for a film thickness monitor (18, 19). Light from the input fibre is focused by a 40 mm achromatic lens on to a blazed reflection grating with 300 lines/mm. A 50-element CCD array gives a spectral resolution of 10 nm. The spectrometer is 10 mm thick and 80 mm long. Spectrometers made by integrated optics have also been suggested (20). 

Li Y-J, Zhang Y, Zhou F (2008) Sequential monitoring of film thickness variations with surface plasmon resonance imaging and imaging ellipsometry constructed with a single optical system. Anal Chem 80 891-897... 

Figure 3.24 shows the examples of output data of an eddy current monitor, a friction monitor, an optical monitor, and a vibration monitor. In the eddy current monitor output, the first-layer clear point is shown in actual data and differential data. In the friction monitor output the, Cu clear point is also shown in actual data and differential data. In the optical monitor output, the residual oxide thickness output data using 100-nm waves and 60-nm waves are adopted. Shorter waves can give us the finer information of the film thickness. In the vibration monitor, the end point of STI is shown as an example. 

WTW and RTR control of thickness are improved by the use of end-point detection systems and advanced process control. End-point detection, whether mechanical or optical, monitor the state of the wafer surface (film thickness, reflectivity, etc.) or of the entire polishing system (friction, slurry by products, etc.) in an attempt to predict when the desired amount of material has been removed (i.e., the end of process). End-point detection is most successful in processes where a change in the films on the wafer surface leads to an abrupt change in the optical or mechanical properties of the wafer surface. For example, copper CMP end point is easy to detect by optical means due to the large difference in reflectivity of the copper film compared to the barrier films. In contrast, end-point detection for small amounts of ILD removal is difficult due to the lack of change in the wafer surface or the wafer-pad interface. 

Since this method captures film thickness information from various areas covered by the CCD camera, the thickness distribution of an area can be measured at extremely high speed. For example. Fig. 6-a shows a three-dimensional plot of the thickness distribution around the Fig. 5-a test pattern taken with the CMP Semi-lnSitu Monitor. Please compare this with Fig. 6-b which shows a three-dimensional plot of an 8-inch wafer film measurement taken with another simultaneous film thickness measurement system with different optical specifications. 

Table 1 shows the CMP Semi-InSitu Monitor s specifications and the results of a recent performance evaluation. Compared with conventional optical spectrometric film thickness measurement systems, the reproducibility of CMP Semi-InSitu Monitor measurements is slightly lower due to the presence of water on the wafer. Nevertheless, when measuring SiO single films at 3 E over repeated trials, the reproducible accuracy is within 4 nm, which is more than sufficient to effectively control CMP processing. 

In one form of comparison reflactometer, a similar double-reflectance geometry is used but the reference is translated while the intensity is monitored (19). when the film thicknesses are equal, two of the four optical paths that result when all possible combinations of front- and back-surface reflections are taken into account are identical and therefore add in phase. This gives rise to an intensity maximum. Accuracies are comparable to those attainable with comparison ellipsometers. 

In the gravimetric method, the adsorbent (usually in the form of powder) is placed into a bulb, which is mounted on a sensitive balance and the bulb is then evacuated. Next, the weight increase of the adsorbent solid as a function of the absorptive gas pressure is monitored at constant temperature. More recently, the quartz crystal microbalance (QCM) technique has been applied this is very sensitive to mass increases. Quartz is a piezoelectric material and the thin crystal can be excited to oscillate in a traverse shear mode at its resonance frequency when a.c. voltage is applied across the metal (usually gold) electrodes, which are layered on two faces of the crystal. When the mass on the crystal increases upon adsorption, its resonance frequency decreases. The increase in the mass is calculated from the reduction in resonance frequency. On the other hand, adsorption on single flat surfaces can also be measured by ellipsometry, which measures the film thickness of transparent films optically using the difference between light reflection from bare and adsorbed surfaces. 

An optical monitor and/or a quartz crystal monitor is used to control the film thickness and deposition rate. Quarter-wave films in the visible range can be deposited within 3-4 min. 

In evacuated coating plants, it is usual to mount the parts to be coated on rotating work holders permitting the uniform subsequent or simultaneous deposition of evaporated, sputtered or ion-plated deposits of any combination of metals or dielectric materials. The film thickness on the sample itself or on a test glass can be monitored optically by transmitted and/or reflected light using a photometer. Such instruments are commercially available with nearly all coating plant manufacturer. 

R. Richier, A. Fomier, E. Pelletier, Optical Monitoring of Thin Film Thickness, Chapter 3 in Thin Films for Optical Systems, F.R. Flory (Ed.), Marcel Dekker, New York, 1995... 

These are based on constructive and/or destructive interference effects of incident and reflected light. These antireflection coatings have bandwidths of a few nanometers for optimal operation. With appropriate material selection, their absorption can be minimized, and excellent durability can be achieved. They are, however, more complicated to deposit, requiring precise knowledge of the optical constants of the films, endpoint monitoring for determining the correct thickness of the film, and optimization of deposition conditions for denser defect-free films. For these reasons, they cost more than aluminum mirrors. 

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