MSGC-wafer production
4.2 Optical inspection
4.2.1 Requirements
The MSGCs have a size of about 30×30 cm2. The width of the anode strips (the smallest structure on the glass plates) is 10µm. With these boundaries, the following characteris-tics are needed for a control instrument:
• A high quality optical microscope with different magnifications (50×, 100×, 500×, 1000×) to attain a good overview and to examine small faults in the metal struc-tures.
4.2 Optical inspection 43
• A large object table (40×40 cm2).
• The object table should be movable by hand (joystick), by computer and by the coordinate instrument.
• A digital xy-coordinate measurement device with an accuracy of about 1µm.
• A computerized bookkeeping system to register and analyze the different defects.
• A clean atmosphere to avoid spoiling of glass plates.
4.2.2 Construction
The whole equipment is located in a clean room. To further minimize the spoiling the actual working place is located in a flow box1. Figure4.1shows the floor plan.
① a
① b
② ③ ④ ⑤ ⑥ ⑦ ⑧
⑨
⑩
1a Airlock, dirty part 6 Flow Box 1b Airlock, clean part 7 Computer
2 Bench 8 Electronic Rack
3 Coat Rack 9 Vacuum Cleaner
4 Microscope 10 N2bottle
5 Stone Table
Figure 4.1: Floor plan and content of the clean room, which houses the inspection microscope.
The clean room is assembled from standard wall and window elements normally used for small offices in industrial production halls. The airlock is separated by a bench into
1Type: Elgera Reiner Arbeitsplatz, Model K 1914
a dirty and a clean part. The clean room and the airlock are piped to a vacuum cleaner, which is located outside of the room to avoid respoiling of the air. Thus every object brought into the clean room can easily be cleaned in the airlock. To clean the dust off the glass plates a tube is connected to aN2bottle outside the room. Clean air is received from the air conditioning system resulting in a small overpressure in the room. The flow box around the microscope leads to further reduction of dust in the air. The flow box and the air conditioning system satisfy the US cleanliness class 100.
Due to the big magnification, already small vibrations of the scanning table can destroy the sharpness of the image, especially for photos, because of the long exposure time. Thus the whole microscope is mounted on a massive stone table (179 cm×75 cm×4 cm). To reduce the transmission of oscillations of the building onto the table, it rests on rubber cushion filled with sand.
The support frame for the microscope (see Figure4.2) and the two runners for the object table are fixed on an aluminium plate (75 cm × 75 cm×2 cm). The runners2 [43], ar-ranged perpendicular to each other, have an active length of 400 mm each. The carriages are moved by AC motors [44] via a spindle. Attached to the runners are rulers3to deter-mine the exact position of the table with an accuracy of±1µm. The exact position can be read on the display unit4[45] on the left side of the microscope.
TheNikonmicroscope5is composed of:
• An illuminator6 consisting of a 100 W halogen lamp, filters, polarizers, and aper-tures. It can be switched to brightfield and to darkfield object illumination. In the darkfield mode the object is illuminated from the side in such a way, that exaltations get visible (see Figure5.3for an example).
• A motorized revolving nosepiece carrying four lenses (magnifications: 5×, 10×, 50×, 100×) and an error marker.
• A binocular tube (ocular magnification: 10×) switchable to photo or video projec-tion.
• A motorized focus unit.
4.2.3 Controls
The lens revolver, the focus and the light intensity are controlled by the same unit on the left side of the table. Thus during normal operation one does not have to manipulate
2Type: FESTO DEGL-25-400-SP-KF
3Type: Heidenhain LS 405
4Type: Heidenhain ND 920
5Type: Nikon modular microscope system
6Type: Nikon Universal Epi-illuminator 10
4.2 Optical inspection 45
Figure 4.2:The scanning apparatus:
The movable object table lies underneath the microscope. Its position is displayed on the instru-ment at the top left. The unit below is used for controlling the light, focus, and lenses. On the right side on the stone table you can see the joystick for manually moving the object table.
anything above the object table, which minimizes dust falling onto the glass plates. Be-cause the depth of focus decreases with increasing magnification, the speed of the focus adjustment is adapted to the selected lens.
The moving of the object table can be controlled with the computer, by typing the desired position into the coordinate unit or with a joystick. The actual moving is always executed by the AC motors. The speed of the motors is controlled by two separated regulator units7[44] and it is proportional to a reference voltage (-5V – +5V). This reference voltage can be controlled either by the joystick, the coordinate instrument, or the computer.
Moving manually with the joystick is the default steering method which is selected when-ever the PC is switched off or the controlling program is not running. Three sensitivity ranges for slow, medium, and high speed can be selected. The two little buttons on the joystick are read out by the computer and used to start and stop the computerized object
7Type: Baldor BSC-1102
Figure 4.3:A typical anode break. The picture was taken with the 5×lens and the 10×projection lens.
table moving.
The computer8is connected to the position display unit via a serial interface to read back the actual coordinates. A digital I/O card9 [46] is used to select the steering mode (by joystick, by computer or by coordinate unit), to program the DAC (Digital to Analog Converter), and to read the status of the steering electronics back (see Section12). The steering electronics contain a two channel DAC used to control the speed of the table. In this way, the computer can be used for heading for a selected position or to move the table along a desired path.
To move the object table by the coordinate instrument, one has to select the corresponding mode in the steering program (see Section12) and additionally thedistance-to-gofunction on the coordinate unit. Right after one has entered coordinate, the table moves to the desired position.
The steering electronic generates and controls the reference voltage for the AC motor (see Section4.2.3). Analog switches are used to select its (the reference voltages) source, either the joystick, the DAC (= the computer), or the coordinate display unit. The manual steer-ing signal is directly fed from the joystick through the analog switches to the regulator units. A two channel eight bit DAC (PM 7528) is used to set the reference voltage. Since
8486 PC, Windows 3.1
9National Instruments PC-DIO-24
4.2 Optical inspection 47
the polarity is switched separately, the actual resolution is nine bits. The coordinate dis-play delivers eight 24 V digital signals indicating whether the coordinate lies in a certain window around the target position or not. The 24 V signals are transformed by optical couplers (6N137) to aCMOS compatible level, i.e. 0 – 5 V. Then they are used to select the appropriate voltage, the nearer the final position the smaller the voltage and thus the slower the speed. Every reference voltage can be adjusted separately by potentiometers on the printed circuit board. The states of the joystick buttons and the coordinate display are fed back to the computer.
The control program is written inLabVIEW[47]. It performs the following different tasks:
• Setting of the desired steering mode (see Section4.2.3).
• Reading back the coordinates.
• Maintaining error report files: bookkeeping of the different classes of errors (anode and cathode breaks and holes, coating errors,. . .).
• Displaying of the error distribution graphically (see Figure4.4).
• Semiautomated glass plates inspection (see Section12).
Figure 4.4:Error distribution online display
The program automatically moves the table along a meander like inspection path parallel to the anodes. Normally seven anodes and cathodes are inspected simultaneously. If an error is recognized, a button on the joystick is pressed to switch to manual steering
mode. Then the error can be identified and registered. After pressing the button for a second time, the computer switches back to the automatic moving. The table moves back to the last position on the track and continues moving along the inspection path.
This greatly facilitates the controlling task, because one does not have to remember the exact inspection position after switching back and forth the lenses. Figure4.4shows an example of an error distribution registered on-line.