attenuator that controls the scanning mirror excursion so that the mirrors have the largest excursion without the light raster being vignetted by the microscope optics. These conditions are best satisfied by setting the attenuator on 1000 μm regardless of which microscope objective is used. This 1000 μm position is calibrated so that the raster size on the specimen will be 1000 μm x 1000 μm when the 8X objective is used in the microscope. By switching the attenuator to the 500 μm position, the magnification is doubled. The picture definition is degraded however because of the non-zero spot size. The preferred method for changing the magnification is to change microscope objectives. It is important to remember that the markings on the attenuator are referenced to the 8X objective, and if a 16X objective is substituted for the 8X objective, and the attenuator is set on the 1000 μm position, then the raster size on the specimen will be 500 μm x 500 μm. The attenuator is of course very useful for electronically magnifying a portion of the specimen and measuring particular portions. An additional item that must be noted is that the markings on the attenuator are valid for the normal, 1.2 kHz horizontal and 3 to 4 Hz vertical scanning rates because, as previously described, the horizontal scanning mirror is operated slightly beyond resonance so that decreasing the scanning rate changes the amplitude of the excursion of that mirror. 4.4 Optimizing Scanning Rates In general it may be best to use the fastest scanning rates that the scanning mirrors will allow because it is important that a picture of the specimen is produced in the shortest possible time. This reduces the effects of drift in the specimen or electronics and is particularly appropriate when the display is being photographed. The fast scanning rates also make it easier to view the display in real time, especially the color display because fast-decay phosphors are used. Many specimens have carrier storage effects which make it necessary to use slower scanning rates to maintain resolution. Situations where these storage problems occur are usually made apparent by blurring of vertical lines in the image. Solar cells are particularly susceptible to these storage problems. On certain specimens there is a strong dependence on the storage time and bias. Increases in reverse bias voltage on the specimen usually shorten this storage time. As mentioned in the section of this report dealing with the principle of operation of the scanner, the horizontal scanning mirror normally is operated beyond resonance at 1.2 kHz. It was also stated that reducing the horizontal scanning rates not only changes the magnification in the horizontal direction, but also reverses the picture in such a way that the left-hand side of the picture becomes the right-hand side and vice It must also be remembered that the 1.2 kHz is the frequency where picture linearity is obtained by virtue of 180 deg. out of phase operation between the mirror movement and the electron beam movement in the display. When the scanning rates are changed to slower ones, usually only the frequency multiplier knob on the horizontal generator is disturbed because of the rather fine tuning required to find the 180 deg. phase shift needed for picture linearity. Thus sweep rates of 1.2 kHz, 120 Hz, or 12 Hz are usually used. The vertical generator for the frame rate can be set to any frequency. 4.5 Getting the Picture Presented below is a list of the steps that can be taken to obtain a monocromatic picture of a particular specimen as quickly as possible. 1. Energize scanner with main power switch. 2. Set knobs on the MIXER, from the left-hand side of the unit, as follows: 3. 1. OFFSET - so that both offset indicator lights are extinguished j. Y-AXIS MIX 1, 2, and 3 - full counter-clockwise k. Y-AXIS MIX-Y- 2 o'clock 1. Z-AXIS MIX 1,2, and 3 - full counter-clockwise Set knobs on the PRE-AMPLIFIER, from the left-hand side of the unit, as follows: 4. h. i. Channel 2 controls - same as channel 1 controls Attenuator controls on rear of unit 12 o'clock Set brightness on display so that a dim raster can be seen. 5. Set Magnifier/Attenuator on 1000 μm 6. 7. 8. 9. Install 8X objective in microscope. Place specimen on microscope stage. Use figures 6 through 9 in this report as a guide and connect the specimen to the input of the pre-amplifier use channel 1 for those devices that require a positive supply, and channel 2 for those devices that require a negative supply. Connect the output of channel 1 of the pre-amplifier to the input of channel 1 on the mixer. Do the same for channel 2 of both units. 10. Set the movable slide in the microscope so that the laser is blocked and the eyepiece is activated, energize the lamp in the vertical illuminator, and focus the microscope onto the specimen. 11. 12. Reset the prism slide so that it allows the laser to impinge on See that the scanner is working by using the reflected light signal. light image of the specimen. Adjust the offset control to extinguish 13. Set the SUPPLY voltage control on the channel being used to the desired operating voltage for the specimen. 14. The METER switch can be set on SAM so that the actual voltage on 15. Adjust the OFFSET control on the pre-amplifier of the channel being used so that both offset lamps are extinguished. If both lamps cannot be extinguished, reduce the gain so that both lamps can be extinguished with the offset control. 16. Adjust the OFFSET control on the mixer for the channel being used. Follow the same procedure as above. It may be necessary to reduce the gain on the pre-amplifier further to avoid overloading the input stages of the mixer. 17. Rotate the Z-AXIS MIX 1(or 2 if channel 2 is used) clockwise to obtain a display of the photo response of the specimen. It may be necessary to increase the gain on one or both of the units. Remember that there are also attenuators on the rear of the preamplifier. 18. The photo-response image can be superimposed on the reflected If the color display is to be used, the OFFSET on each of the three 5. OPTICAL ALINEMENT Optical alinement should be performed to some extent when the scanner is moved, when a laser is changed, or when the scanner is subjected to excessive vibration. The following is a complete listing of the steps for alinement of a new instrument; many of these steps can be omitted for realinement purposes. *CAUTION: Care must be exercised to avoid having either the infrared laser radiation or the visible laser light reflected into the eye. 1. All All power to the scanner should be off. Set the mirrors, both the stationary ones and the scanning mirrors, to their approximate positions needed to cause the laser radiation from the infrared laser to be deflected down the camera tube of the microscope. Use figure 1 as a guide to the deflection sequence. 2. Energize only the vertical illuminator lamp in the microscope. Adjust the four screws at the base of the scanner which set the height of the scanner mainframe relative to the microscope and microscope stand. The screws should be adjusted so that the camera tube passes through the center of the hole in the top of the scanner frame. The screws must also be adjusted so that the light from the vertical illuminator comes to a point on the scanning mirror directly above the camera tube. The prism in the microscope must be set so that the light passes through the camera tube and not to the eyepieces. 3. Turn off the vertical illuminator lamp and energize the infrared laser. Using an IR phosphor card, guide the infrared laser radiation through the optical system and through the microscope by adjustment of the mirrors. Aim the radiation down the center of the camera tube. Slightly adjust the screws at the base of the scanner so that the radiation passes through the center of the microscope optics onto the stage. 4. Activate the visible laser and move the slide which changes the laser sources to the position that blocks the infrared laser. Adjust the position of the two mirrors on this slide so that the visible light beam follows the same path as the infrared light beam from the scanning mirrors and through the microscope. Do not disturb any other adjustments. By switching back and forth between the two lasers, make the spot from the two lasers hit the microscope stage at the same point. 5. Turn on the generators and the scanning mirror drivers. Set the magnifier so that the scanning mirrors have large enough excursions so that the raster formed on the microscope stage by either laser has some of its corners cut off. Adjust the screws at the base of the scanner so that all four corners of the raster are equally cut off that is, center the raster. Lock the adjustment screws at the base of the scanner. 6. Adjust the half-silvered mirror and lens in the reflected-light circuit so that the raster degenerates to a point on the active area of the photo diode. 7. Energize the remainder of the electronics and adjust the 1/2 plate to maximize the output from the reflected light circuit when the infrared laser is used. 6. ACCESSORIES 6.1 Radio Receiver A specimen's response to VHF and UHF frequencies can be conven- iently studied with the use of a radio recey). The lasers are self amplifier channel in the scanning system same time. The output from the radio receiver is fed into an unused channel in the mixer for display. 6.2 Laser Modulator As an extension to the radio receiver concept, a dc to 100 kHz laser modulator was added to the system. The modulator is capable of at least a 10% depth of modulation. The modulator permits the response of the specimen to be studied at frequencies up to 100 kHz; however, much longer scanning times must be used as the signal bandwidth is limited. A lock-in amplifier can be substituted for the radio receiver to detect the desired signal(10). The circuit for the modulator is given in Appendix E. 6.3 High Impedance Probe Certain applications in optical scanning require the use of a high-impedance probe to minimize disturbance of the scanned specimen. Such a probe is shown schematically in Appendix E. The probe features dc coupling and can pass signals riding on dc voltages when these dc voltages are as high as 20 V and as low as -20 V. The probe loads the circuit being measured with 3.1 pF at 1 MHz and 22 Mm at dc. The probe obtains its operating power from the mixer. 7. SPECIFICATIONS 1. Optical radiation sources a. 0.6328 μm He-Ne laser 3 mW power output at 0.6328 μm, polarized Lowest axial mode modulation frequency: 500 MHz External modulator: dc to 100 kHz at 10% depth of modulation b. 1.15 um He-Ne laser: 2 mW power output at 1.15 μm, polarized Lowest axial mode modulation frequency: 385 MHz 2. Optical power at the specimen (approximate) 3. with 4x objective, 0.12 mw with 40x objective, 0.10 mW Resolution (This is a measure of the system's ability to separate closely-spaced features and it varies both with the laser wavelength and microscope objective chosen. Appendix F describes in detail how the following resolution values were obtained and defined). λ = 0.6328 μm; 40 x objective; 2 μm spaced lines yield 30% resolution. |