Optical head

The instrument consists of an optical head with a purge gas over the lenses to reduce coating by the powder stream, an interface box, computer and... 

Figure 10.12 shows the optical head inserted in the process line. The head is installed directly into the line, preferably via a flexible coupling for vibration isolation. The interface box is a NEMA 4.9 rated explosion proof enclosure, weighing 50 lb, which can be bolted to a wall or floor within 20 ft of the head. 

To measure the film thickness in the specified area (test pattern) on a wafer, precise image pattern comparisons are required on the wafer surface and the optical head must be aligned exactly with the test pattern. 

The measurement unit is composed of an optical head to measure film thickness and an R-0 stage to position the optical head below each test pattern on the wafer. To facilitate quick, convenient use, the measurement unit is located adjacent to the polishing table in the CMP equipment. Other components include a control unit, a fluid circulation unit, and an operation unit and they Eue all located neEU the CMP equipment. (Fig. 9)... 

To reduce the size of the measurement unit so that it can be installed in a small space adjacent to the CMP polishing table, we developed a small optical head and used a compact R-0 stage containing both linear and rotation sections to move the head across the wafer. (Figs. 10 and 11)... 

Slurry can be rinsed out of the measurement area by DI water dispensed from nozzles around the optical head s window, so that a clear image can be input. (Fig. 12) The gap between the wafer surface and measurement head is completely filled with DI water to prevent image... 

The measurement unit is 310 mm in diameter and 160 mm high, mostly due to the size of the R-0 stage. Although this unit is already quite compact, its size can be even further reduced if a method is developed to sample two-dimensional fields rather than strictly defined test patterns or points. In such an event, the unit s dimensions will not exceed those of the optical head, i.e, 100 x 70 x 30 mm. 

We have developed the CMP Semi-InSitu Monitor and confirmed that it is capable of highly precise measurements. This system incorporates a compact measurement unit composed of a small optical head, wafer rinse nozzles and an R-6 stage. Since measurement is performed while the wafer is still held in the CMP top ring, it can be immediately returned to the table for further polishing if necessary. 

The typical scanned sample AFM scanning unit consists of the following parts a base, a scanner, and an optical head, in which a holder for the cantilever is mounted (Fig. 2.1). In addition, a probe tip/cantilever and sample, which will be mounted on a metallic sample puck, are required. Careful handling of the sensitive equipment (avoid shock, mechanical stress on the cables, temperatures above 40°C, high humidity for the scanner, etc.) is a prerequisite for this work. We recommend wearing... 

Fig. 2.1 Photographs of the essential components of a sample scanning AFM (a) scanner base, (b) cantilever holder, (c) optical head, and (d) scanner of a typical scanned sample AFM...

The sample (mounted on a metallic sample puck) is attached to the scanner, which will later position the sample in all three spatial directions. The optical head comprises the cantilever-tip assembly in a special holder, as well as the optics (laser and photodetector) of the beam-deflection detection scheme. The base contains electronic circuitry and is the interface between controller and actual force microscope. It also serves as physical holder for the scanner and may include a stepper motor, which is used for the coarse and fine approach between tip and sample (see below). 

Mounting of the optical head/cantilever holder assembly to the scanner... 

To mount the optical head onto the scanner/base assembly, there are two alternative options ... 

Fig. 2.5 Essential steps of mounting the optical head (a) Mounting of the optical head and securing of the springs (b) optical head mounted on the scanner (c) insertion of cantilever holder into optical head (d) fixation of cantilever holder...

Once the optical head, scanner, and base have been assembled and the cables of the scanner and the optical head have been attached, the base can be connected to the controller, following the specifications and instructions of the manufacturer. Care has to be taken that the controller is not running and is not sending high-voltage signals to the scanner. 

Fig. 2.6 (a) Insertion of cantilever holder (b) premounted cantilever holder (the holder has been fixed by tightening the corresponding screws, compare Fig. 2.5d) (c) mounting of cantilever holder/optical head assembly onto the scanner... 

To prepare the set up for an experiment, the sample should be mounted. For this purpose, we remove optical head, after driving the stepper motor upward to protect the tip and the sample from unintended contact. The sample (mounted to the sample puck see Sect. 2.2.2) is placed on the piezo scanner in center position. Finally, the optical head is placed again carefully on scanner (please ensure that the tip is far from the sample) and, with utmost care, is secured with the springs (for warning see above). 

The set up is now in principle ready to start an experiment. The coarse approach is carried out to position the tip close to (but not in contact with) the sample surface. Using an eyepiece, the cantilever can be viewed from the side (or alternatively one can use a top-view CCD camera for this purpose). With the eyepiece, we locate the reflection of the laser light on the cantilever and its reflection (mirror image) on the sample (red spots). Using the stepper motor, we lower the optical head until the spots are close however, we still want to clearly be able to detect a gap between the spots. In older scanner types, the optical head is lowered by using the stepper motor for one pod of a tripod, while the other two are lowered manually using the corresponding screws. In this case, it is essential that the head is lowered such that it stays leveled at all times. 

With a top-view CCD camera, we bring the camera in focus on the cantilever and then on the sample surface. Subsequently, we move the focal plane upward. Using the stepper motor, we lower the optical head until cantilever becomes sharp in focus. Following both approaches, the tip is now < several 100 pm away from the surface. 

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