Adjustment photo lithographic mask

In CRT, manufacturing exposure processes are needed to establish the multiple structured patterns at the inner surface of the panel. For this exposure a shadow mask is used to form a light pattern into the panel that corresponds with the particular layer. Usually, the functional shadow mask of the CRT is also used for the exposure process.
However, it is also possible to use a specific exposure mask, called an exposure master, to form nominal patterns on the panel interior. Such an exposure master could be a flat thin glass plate, (which is used in this particular case). It is necessary to manipulate the master to a predicted position within an accuracy zone of 10 micron.
FIT schematic setup FIT schematic setup detail camera and layers

On the pictures displayed above, the left one is depicted a sketch of the situation in general. The right picture is depicted a detail of the camera, the spotlight and the exposure light situation.
On the second picture on the right is a more detailed display of the setup and the layers.
The unique part of the camera system is the use of powerful light, like spotlights, and the exposure light. The camera system consists of cameras at dedicated positions on a rectangular pattern.
The light, coming from the spotlights, passes through the panel glass and through the already deposited layers.
The cameras are able to pick up the pattern information, in particular its position. The light, coming from the exposure light source, passes through the master and will project the master pattern onto the inside panel face.
Subsequently it is possible to match the projected pattern with the existing pattern. During measurement and manipulation the light coming from the exposure light source is filtered so that there will be no exposure effects in the sensitive layer.

Manipulator and control

A special designed 6D motorized manipulator (Physiek Instrument) is used to manipulate the master. Control algorithm The position of the pattern on the panel and the projected pattern on panel interior are specially designed and measured with 6 camera's that are fixed on the manipulator base.
The x and y position difference between the two patterns are calculated and by means of a mathematical model the difference position in 6D of both patterns is calculated.

A multivariable control algorithm is designed and the new position of the manipulator is calculated. The new 6D set points are sent to the manipulator controller and the manipulator is positioned to its new position.
Within 2 control actions (measure - control) the manipulator is at its position and a last measurement is done.
Simulation software, including process and camera noise, is designed and included in the overall software, to test and tune the control algorithm (as you can see on the left sketch).
The control software includes all the mathematical matrix calculations, which are created with LabVIEW(National Instruments ™).


Measurement and pattern

The pattern on the panel and on the master is a line pattern. Therefore special ‘markers’ at the end of the lines are designed and included. This design of the pattern contains a shorter line for every color (R,G,B, see image left top) and the master contains a larger line (left bottom).
pattern markers Master design with cameras 6 Camera's, positioned at the end of the lines (see Master design, blue dots), measure the position of the shorter lines on the panel and subsequently the longer line of the master. Special designed telecentric lenses are necessary to create a good and sharp image.
Above the panel, 6 special designed high power spotlight are mounted, which are built around high power leds and Fresnell lenses. This high power spotlights are necessary because of the 100 micron Aluminium layer that is applied on the phosphor lines on the inner side of the panel.
The vision system acquires 6 parallel images to prevent mismatches as a result of small vibrations of the environment (e.g. building vibrations).

The software searches for the marker of the short lines or long line and roughly calculates the position of this marker. Subsequently, the software detects all available lines and calculates the best fit using the least square algorithm.
The shift between the marker lines is the used to calculate the 6D position. As mentioned a mathematical model is created of the total setup.
Of course the whole system is calibrated so that the shift is calculated in microns. The final accuracy is 2 micron.
The vision library Promise from Philips Apptech Industrial Vision is used to measure the marker and the lines itself very accurate and reliable.

The bottom pictures are a few impressions of the setup.
The left picture is an overview of the setup, the centre pictures a view of the panel (screen) and top view of the master and at the right a picture of the spotlight.

FIT photo equipment_1 small Top view screen Top view master Spotlights